آب و هواشناسی Climatology
آب و هواشناسی- اقلیم ایران - تغییر اقلیم - اوزون - مطالب جالب و مفید - سخن بزرگان

Meteorological Terms

The terms defined below are spelled out in full, with common contractions
for those terms in parentheses. 

Advection: (ADVCTN) Refers to the
transport of something by the wind from one area into another. Moisture
advection (wind bringing in higher values of moisture), and temperature
advection (wind bringing in warmer or cooler air) are two examples. Cold
Air Advection: (CAA)
(See also Advection, Thermal Troughs) Cold air
advection is simply the wind bringing colder air into an area. Associated with
downward motion and subsidence. In addition, cold air advection can have an
influence on atmospheric stability. Upper level (500 mb and above) cold air
advection is a destabilizing mechanism, which can create an environment that
would support convective type precipitation (i.e. thunderstorms). Low level
(850 mb) cold air advection over the Great Lakes destabilizes the lower
atmosphere, allowing lake effect rain and snow showers to develop provided the
difference between the water temperature and the 850 mb temperature is large
enough (13 degrees Celsius or more). These two examples are not meant to imply
that cold air advection is a lifting mechanism. Rather, CAA helps produce an
environment favorable for the development of these types of precipitation (both
of which are convective type precipitation).

Amplification: (AMPLFCTN) Building,
or sharpening, of an upper level high pressure ridge or low pressure trough.

Baroclinic: (BRCLNC) A region in the
atmosphere where isotherms (lines of constant temperature) cross height
contours at any angle, and is usually referred to as a baroclinic zone. These
are areas of temperature advection, and thus are normally locations of vertical
motion. (See also Advection, Warm Air Advection, Cold Air Advection).

Barotropic: (BRTRPC) Normally used
in reference to a region of the atmosphere where isotherms (lines of constant
temperature) run parallel to height contours on a constant pressure surface,
thus there is no temperature advection. Technically, this is an equivalent
barotropic situation. In a true barotropic atmosphere, constant pressure
surfaces have a constant temperature, that is, the temperature everywhere on
the surface is the same. Therefore, in a pure barotropic atmosphere, there
would not be any isotherms on a constant pressure chart. (See also Advection).

Blocking Pattern or Blocking High: (BLCKG PTTN)
A pattern which involves a strong upper level high pressure ridge.
These ridges are quite strong and tend to move little, thus blocking the mean
flow pattern. Storm systems must travel up and around these blocking ridges,
thus, areas located underneath a blocking ridge can experience several days of
dry weather. There are two main types of blocking patterns: Omega
Blocks
involve two upper level lows on either side of an upper level
ridge (picture one upper low off the west coast, one off the east coast, and a
ridge over the Midwest). Rex Blocks involve an upper level
high directly north of an upper level low.

CAPE: Stands for Convective
Available Potential Energy.
CAPE is another type of stability index, and since it is a measure of energy,
it has units of Joules/kilogram (J/kg). The higher the CAPE value, the more
unstable the atmosphere is and the better able it is to support strong and
severe thunderstorm activity. Values over 1000 J/kg are usually considered
significant, but there is no "magic" value of CAPE which will allow
thunderstorms to develop.

CIN:  Stands for Convective Inhibition. 
CIN is simply the amount of negative energy the atmosphere must overcome to
achieve a state of free convection, or thunderstorm development.  Its
units are also in Joules/kilogram.

Closed Low: (CLSD LO) A low pressure
center having a closed circulation, which is used in reference to systems in
the upper levels of the atmosphere. Closed lows that become cut off from the
main flow pattern are called cut-off lows.

Condensation Pressure Deficit: (COND PRES DEF) (See
also Isentropic Lift). On an isentropic chart (a layer of constant potential
temperature), condensation pressure deficit represents the amount of lift,
expressed in millibars, needed to saturate an air parcel. For example, an air
parcel at 850 mb has a condensation pressure deficit of 200 mb. This means that
this parcel needs to be lifted 200 mb (up to the 650 mb level) before it will
become saturated. It is one way of evaluating moisture content on an isentropic
chart.

Convergence/Confluence: (CNVGNC/CNFLNC) An
area in the atmosphere where air flows together. In the lower levels of the
atmosphere (generally below 550 mb), convergence implies rising motion, and
subsequent clouds and precipitation if enough moisture is present. Above 550
mb, convergence implies downward motion, which results in clearing and drying
of the atmosphere. (See the definition of Divergence/Difluence for an
explanation of why the 550 mb level is important).

Confluence is height contours coming closer together on a constant pressure
chart, which implies convergence. (See also Divergence/Difluence).

Deformation Zone: (DFRMTN ZN) An
area in the atmosphere where winds converge along one axis and diverge along
another. Deformation zones (or axis of deformation as they are sometimes
referred to) can produce clouds and precipitation. 

Dewpoint Pooling: (DWPT PLG) An area,
usually along a surface front or trough, where there is a "pool" of
higher dewpoints (or higher amounts of surface moisture). Since increasing low
level moisture increases atmospheric instability, an area of dewpoint or
moisture pooling tends to be more unstable than surrounding locations and can
be a prime area for the development of thunderstorms.

Differential Heating Boundary: (DFRNTL HTG
BNDRY)
A small scale "cold-frontal" type boundary that
results from unequal surface heating. To understand how this comes about,
consider a morning with varying cloud coverage between Lansing and Detroit.
Lansing remains under cloud cover all morning, and by noon has a temperature of
58 degrees, while Detroit has sunshine during the morning and
a temperature at noon of 70 degrees. This temperature (and resulting pressure)
difference can result in the development of a small scale line of convergence,
similar to a front. This can be a triggering mechanism for thunderstorms during
the warm season.

Diurnal Effects: A reference to an
effect that has its origins due to daytime heating, such as afternoon cumulus
cloud development or the formation of a lake/sea breeze. These phenomena
dissipate once the sun goes down and surface heating is lost.

Divergence/Difluence: (DVGNC/DIFLNC) An
area where air is moving away (diverging from a point), the opposite of
convergence. Divergence in the low levels (below 550 mb) implies sinking
motions, while divergence in the upper levels (above 550 mb) implies rising
motion.

An atmospheric law known as Dines' Compensation Principle states that since
air cannot be created or destroyed, there must be a level of non-divergence in
the lower atmosphere (usually averages out to be at 550 mb). What this means is
that divergence above this level of non-divergence has to be compensated by
convergence in the lower levels (thus rising motion), and convergence above the
level of non-divergence must be compensated by divergence in the lower levels
(sinking motion). Thus, divergence and convergence occur simultaneously in the
atmosphere.

Difluence is the spreading apart of height contours on a constant pressure
surface and implies divergence. (See also Convergence/Confluence).

Diffluent Thickness Pattern: (DIFLNT THKNS
PTTN)
(See also Thickness). Organized areas of thunderstorms tend to move
with the thickness pattern (the mean wind in a layer). An area of diffluent
thickness is an area where the thickness contours spread apart. Why this
actually occurs is not fully known, but it is usually found in an area of low
level warming and upper level cooling (processes that make the atmosphere more
unstable). Areas of convection will tend to move toward these areas of
diffluent thickness (including backwards), and can tip off forecasters when
trying to pinpoint potential areas of heavy rainfall. See Figure 8.

Dynamics: Refers to atmospheric
phenomena which cause upward vertical motion, most often attributed to positive
vorticity advection, convergence of Q-Vectors, or jet streams. (See Positive
Vorticity Advection, Q-Vectors, Jet Streak Quadrants).

Entrained or Entrainment: Refers to
the drawing in of moisture (or lack of moisture) into a system. Dry air
entrainment into the mid levels of a thunderstorm can enhance the potential for
damaging wind gusts. Moisture being entrained into a storm system can enhance
precipitation amounts.

Equivalent Potential Temperature: (THETA-E) The
equivalent potential temperature (or theta-e) of an air parcel
is the temperature that parcel would have if it were raised from some reference
level (850 mb is a popular one) until all of its moisture is condensed out, and
then brought back down to the 1000 mb level. The theta-e of a parcel can be
changed by adding or removing moisture or heat. Forecasters look for low level
theta-e ridges, or an axis of high theta-e, which indicates areas of higher
moisture and potential energy for a storm system to tap. Organized convection
generally tends to develop within a theta-e ridge. Theta-e ridges are also
important in identifying potential heavy snow areas within developing winter
storms. 

Frontogenesis:  The initial formation
of a frontal zone.  The process of frontogenesis produces upward vertical
motions around the front. 

Height Fall Centers: Height changes (the
12-hour change in the height of a pressure surface at a station) are plotted on
each new upper air chart, and height fall centers, or the area of maximum
height falls, can be plotted (common practice on 500 mb charts). The movement
of these centers can be used to forecast the short term (12 hours or less)
movement of upper level low pressure centers, since upper lows tend to move
along and to the left of the track of the height fall center. Height falls also
imply low level convergence and thus rising motion. Height falls are given in
meters or decameters (1 Dm = 10 m).

Helicity: A measure of low level wind
shear, normally within the lowest 3 km of the atmosphere, relative to the
movement of a thunderstorm (thus referred to as 0-3 km Storm Relative
Helicity). This gives forecasters an indication of an environment that is
favorable for supporting the development of thunderstorms with rotating
updrafts, a precursor to supercell thunderstorms (the most violent of severe
storms) and tornado development. Values of helicity greater than +150 are
considered significant; however, like CAPE values, there is no magic value of
(positive) helicity under which rotating thunderstorms will not develop.
Helicity is only an index to determining thunderstorm rotation potential. (See
also Shear).

Isentropic Lift: (ISENT LFT) (See
also Warm Air Advection) In the atmosphere, unsaturated air parcels (i.e.,
relative humidity less than 100 percent) are "bound" to surfaces
called isentropic surfaces. Parcels move along these surfaces, which have
ridges and troughs similar to constant pressure analyses. The main difference
is that parcels move vertically through pressure surfaces, not along them as
would be implied on a constant pressure analysis. Isentropic charts can
"show" three-dimensional air motions, while areas of vertical motion must
be inferred on constant pressure charts. Forecasters normally refer to
isentropic lift as occurring when warm air is overriding cooler air in the
lower levels (a process also called overrunning). Isentropic upslope motion
refers to rising motions, and isentropic downslope means sinking motion.
Isentropic lift and lift due to warm air advection are terms often used
interchangeably.

Since isentropic surfaces are surfaces of potential temperature (THETA),
they are labeled in degrees Kelvin, such as 302K or 312K. The surface chosen by
a forecaster for analysis depends upon the season.

Jet Streak Quadrants: (LFQ and RRQ) A jet
streak is a segment of the jet stream that contains higher wind speeds than the
jet stream as a whole. Air flowing through these jet streaks is moving much
faster than the jet streaks themselves. There are many of these jet streaks
winding through the main upper level jet stream, and each of these jet streaks
can be divided up into quadrants, two quadrants in the entrance region (where
air enters the jet streak) and two in the exit region (where air flows out of
the jet streak). These jet streaks have their own areas of convergence and
divergence, and these areas are located within the four quadrants. Since the
jet stream (and its jet streaks) is found at or above 300 mb, the quadrants
where divergence occurs are where upward motion occurs (See the explanation of
Dines' Compensation Principle in the definition of Divergence/Difluence). 
Divergence occurs in the Left Front Quadrant (LFQ) and Right Rear Quadrant
(RRQ) of a jet streak. Rising motion due to this divergence is part of what is
called a direct circulation, with, for example, rising motion
under the divergent LFQ, and sinking motion under the convergent RFQ (right
front quadrant). Vertical motion is thus enhanced in areas underneath the LFQ
and RRQ.

Lifted Index:  (LI)  Like CAPE,
Lifted Index is a measure of instability in the atmosphere.  Negative
values are generally considered unstable and conducive to thunderstorm
development in the presence of a lifting mechanism. 

Low Level Jet: (LLJ) A low level wind
maximum usually found in the vicinity of the 850 mb level, important in
transporting warm air and moisture northward from the Southern Plains and Gulf
region, which in turn enhances instability and thunderstorm potential. Also is
a region of upward motion since it is normally associated with warm air
advection.

Moisture Axis/Ridge: An area of higher
moisture values, usually in the form of a ridge of higher dewpoints at the
surface or 850 mb. Low level moisture axes enhance atmospheric instability,
which in turn promotes thunderstorm development. Existing storms can intensify
by moving into moisture axes. The concept is similar to dewpoint pooling. .

Negatively Tilted Trough: (NEG TILT
TROF) 
A low pressure trough in the upper levels whose horizontal
axis tilts westward with increasing latitude. Upward vertical velocities are
usually enhanced when a trough becomes negatively tilted.  Positively
Tilted Trough: (POS TILT TROF)
A trough of low pressure in the upper
levels whose horizontal axis tilts eastward with increasing latitude.

Occlusion: (OCCLN) The weakening
stage in the life cycle of a mature storm system. A storm system that is
occluding is said to be "stacked" since the low pressure centers from
the surface upward are right on top of one another. In a strengthening storm,
the system tilts westward with height (surface low is farther east than its
corresponding low at 500 mb).

Open Wave: A wave of low pressure that
does not have a complete circulation around it; also called a short wave
trough. See Figure 6. (See also Closed Low, Short Waves).

Phasing: When two separate short waves
come together to form one wave. Also, when upper and lower level features are
positioned so that each provides energy to the other, it is said that the
features are in phase with one another.

Positive Vorticity Advection (PVA), Differential
Positive Vorticity Advection (DPVA):
(See Advection, Vorticity,
Negative Vorticity Advection). Advection of higher values of vorticity into an
area by the wind. In dynamic meteorology, one of the mechanisms resulting in
upward motion is differential positive vorticity advection (DPVA), which
simply is positive (cyclonic) vorticity advection increasing with height (can
also be negative vorticity advection decreasing with height). PVA at 500 mb has
been shown to correlate with differential positive vorticity advection about 85
percent of the time, thus, PVA at 500 mb can imply an area of rising
motion.   Negative Vorticity Advection: (NVA) Advection
of lower values of vorticity into an area by the wind. In dynamic meteorology,
one of the mechanisms of sinking motion is differential negative vorticity
advection, which is an increase in negative (anti-cyclonic) vorticity advection
with height (or decreasing positive vorticity advection with height). It has
been shown that NVA at 500 mb correlates with differential negative vorticity
advection about 85 percent of the time, thus, NVA at 500 mb implies an area of
downward motion. 

Positive Isothermal Vorticity Advection:
(PIVA) 
Advection of higher values of vorticity by the thermal
wind on a map of vorticity and thickness contours. Implies an area of rising
motion. Negative Isothermal Vorticity Advection: (NIVA) Advection
of lower values of vorticity into an area by the thermal wind on a map of
vorticity and thickness contours. Implies downward motion. (See also Advection,
Vorticity, Thickness, Positive Isothermal Vorticity Advection).

Precipitable Water: (PW, PCPTBL WTR or H2O)
Total amount of water vapor in a layer of air, expressed in inches. Normally
taken between 1000 and 500 mb. Higher values of precipitable water indicate a
deep moisture layer, increasing the potential for heavy precipitation amounts.

Pressure Falls: (PRESFLS) An area
where air pressure is falling. Contours of pressure falls (usually in inches or
millibars per hour) can be used to predict where a surface low pressure center
will move in the short term. Also used during the convective season, as
concentrated areas of pressure falls can indicate areas of small scale
convergence, important in determining where thunderstorms may develop.

Profilers: A remote ground-based
sensing instrument that measures wind speed and direction at different levels
of the atmosphere. The new National Weather Service WSR-88D radar has this
capability, and its vertical wind profile is called a VAD Wind Profile (or
VWP). VAD stands for Velocity Azimuth
Display. An experimental wind profiler network is in place
over the Midwest, and this data is frequently referred to in forecast
discussions from neighboring states. Some wind profiler sites mentioned in the
discussions include Blue River, WI (BLR), Wood Lake, MN (WDL), Slater, IA
(SLA), and Neligh, NE (NLG).

Progressive Flow or Progressive Pattern: An
upper level flow pattern in which storm systems move along at a fairly regular
pace.

QPF:  Stands for Quantitative Precipitation
Forecast, or simply the amount of rain or snow forecast to fall.

Q-Vectors: (QVEC, DIVQ) A
mathematical entity (Q-vectors do not exist in the atmosphere) that allows
forecasters to better identify areas of vertical motion. Q-vectors essentially
show vertical motions arising from the combination of differential vorticity
advection (changes of advection with height) and temperature (thickness)
advection. Areas where Q-vectors converge implies upward motion and Q-vector
divergence (DIVQ) implies an area of sinking motion. Note: Negative Q-vector
divergence is the same mathematically as Q-vector convergence, so the statement
"...negative divq..." means Q-vector convergence. 

Shear: The change in wind speed and/or
direction with distance. Shear can refer to horizontal shear or vertical shear.

Short Waves: (S/WV) (See also Vorticity
Maximum) A wave or disturbance in the main atmospheric flow. Short waves (or
short wave troughs) contain areas of rising and sinking motion, and can produce
clouds and precipitation. Waves passing over slow moving or stationary fronts
can also induce the development of low pressure centers (or waves) on the
front. Vorticity maximums are also referred to as short waves. These waves move
through the main flow pattern (called the long wave pattern). The wavelength of
a short wave can vary from 1 to 40 degrees longitude, while long waves vary
from 50 to 120 degrees longitude in wavelength.

Streamline Analysis: An analysis of
wind direction.

Subsidence: (SBSDNC) Sinking motion,
usually associated with clearing and drying.

Thermal Axis/Thermal Trough: A
thermal axis (or Thermal Ridge) is an axis or an area of warmer
temperatures. In low levels, it serves as a destabilizing mechanism important
during thunderstorm season. A thermal trough is an axis of colder temperatures.
In upper levels, it also serves as a destabilizing mechanism. In the cold
season over the Great Lakes, thermal troughs in the lower levels (i.e. 850 mb)
can enhance the temperature difference between the water and 850 mb, thus
creating a more unstable low level environment, which is what is required to
produce lake effect clouds and precipitation.

Thermal Pattern: Temperature, or
isotherm, pattern on a weather chart.

Thermodynamics: Usually used in
reference to atmospheric stability and the vertical temperature and moisture
profile.

Thickness: The distance, in meters,
between two pressure levels. This distance is directly proportional to the mean
temperature of the layer. The colder the layer, the smaller the thickness. This
is important in temperature forecasting, and in forecasting precipitation type
since thickness values give an idea of what the low level vertical temperature
profile will be. Important threshold thickness values for snowfall are:

Critical Thickness Values associated with snow

1000-500 mb 5400 m or less (referred to as the "540 line")

1000-850 mb 1300 m or less

850-700 mb 1540 m or less

Thickness is also important because it gives rise to the concept of the
"thermal wind" (the thermal wind itself does not exist in the
atmosphere). On a map of thickness contours, the thermal wind "flows"
between the contours; the tighter the contour spacing, the "faster"
the thermal wind. This thermal wind is equivalent to the mean wind between the
layer it represents i.e., the 1000-500 mb thermal wind represents the mean wind
between 1000 and 500 mb, and it "flows" between contours of 1000-500
mb thickness. It is because the thermal wind represents mean wind flow that
organized convection (thunderstorm complexes) tends to move parallel to
thickness contours.

UVV's: Stands for Upward Vertical
Velocities. Can't have much weather without them, since rising
motion is what gives rise to most cloud cover and precipitation.

VVEL's: (VV) Stands for Vertical Velocities. 
Also mathematically referred to as Omega

Vorticity: Simply put, the measure of
rotation of an air parcel about a vertical axis. A parcel rotating clockwise is
said to have negative vorticity, and a parcel rotating counterclockwise is said
to have positive vorticity. There are two types of vorticity; shear vorticity,
which arises from changes in wind speed over a horizontal distance, and curvature
vorticity, which is due to turning of the wind flow.

Vorticity Maximum: (VORT MAX) An area
of maximum positive vorticity.  The terms vort max and short wave are
often used interchangeably. Areas downwind of a vort max experience positive
vorticity advection (and rising motion), while areas upwind of a vort max
experience negative vorticity advection (and sinking motion).

Vorticity Lobes: (VORT LOBE) (See also
Positive Vorticity Advection) An axis of higher vorticity values. Areas
downwind of a vort lobe experience positive vorticity advection.

Warm Air Advection: (WAA) (See also
Advection, Isentropic Lift) The wind bringing warmer air into an area.
Associated with rising motion, and clouds and precipitation if enough moisture
is available. Most commonly referred to as overrunning, since warm air rides
over the top of denser low-level cool air. Lift due to warm
air advection is also referred to as isentropic lift.

Water Vapor Loop: A loop of special
infrared satellite images that show areas of mid and high level moisture, as
well as areas of drying. Useful for locating short waves and jet
streaks.  

IR Loop:  Infrared Satellite images which allow
forecasters to see cloud top temperatures.  Very useful for detecting
clouds at night, which are generally colder than the ground.   

Zonal Flow: Upper level flow that is
essentially west to east and is usually quite fast. Can have many subtle
(small) short wave troughs within it.

 

 

 

 

State names are usually abbreviated. e.g. TX, OK, AR, GA,
NY




N, S, E, W, SE, SW, NE, NW, etc. - These
refer to compass directions.




H7, H5, H3, etc.- These refer to height
levels in the atmosphere. H7 is the 700 mb level, H5 is the 500 mb level. H3 is
the 300 mb level. And so forth.




MRF, ETA, NGM, AVN, ECMWF- These are all synoptic scale forecast models



88-D- In reference to the WSR-88D radar.
Stands for Weather Surveillance Radar, 1988 Doppler






AC- Altocumulus



ADIABATIC- A process that causes rising air
to cool and sinking air to warm




ADNLY- Additionally



ADV- Advisory



ADVECTION- The horizontal transport of air
(parallel to earth's surface)




AFD- Area Forecast Discussion



AGEOSTROPHIC- A wind that is not in geostrophic balance. An ageostrophic wind
will have vertical motions. Shortwaves and jet streaks cause
ageostrophic flow.




ALL QUADS- All Quadrants; in all directions



AMPLIFICATION: (AMPLFCTN)- Building, or
sharpening, of an upper level high pressure ridge or low
pressure trough




ANTICYCLONE- High pressure system



AP- Anomalous Propagation.
This is a false echo on radar data




APCH- Approach(ing)



ASL- Above Sea Level



ATTM- AT The Moment



AVN- Aviation Model. One of the short term synoptic
forecast models




AWIPS- Advanced Weather Interactive
Processing System. It is used at NWS offices to process weather data






BACKING WIND- A backing wind is a
wind direction that turns counterclockwise with height. An indication of cold air advection.




BAROCLINICITY- Baroclinicity is a cold air advection/warm air
advection
couplet that increases atmospheric instability. On
analysis and forecast charts it is the isotherms crossing the height contours.




BAROTROPIC- Barotropic is a
horizontally fairly homogeneous troposphere in which there are neither fronts
or any thermal advections.




BKN- Broken sky. 6/10 to 9/10 cloud coverage
observed from the surface.




BLACKBODY- A mass which absorbs and emits all
wavelengths of radiation.




BLIZZARD- Snow with winds greater than 35 mph
and visibility of 1/4 mile or less that lasts for several hours.




BLO- Below.



BLOCKING PATTERN- A situation in which the
upper tropospheric flow becomes highly meridional and stagnant. Two common
blocking patterns are the omega block and the rex block. Click here for more information on atmospheric
blocking.




BLUE NORTHER- A term used for a strong cold
front in the southern plains of the United States and drops temperatures very
quickly and has strong gust winds just behind the front.




BOMB- A bomb is a low
pressure system that decreases in central pressure by at least 24 millibars in
a 24 hour period.




BOUNDARY LAYER- The Planetary Boundary Layer (PBL) is the
layer of the troposphere closest to the earth's surface where friction is a
significant force and wind tends to be gusty (irregular flow).




BRIGHT BAND- A maximum in the radar
reflectivity caused by snow melting as it falls. Melting snow can have a
reflectivity similar to small hail. The bright band will
produce a ring around the radar since the snow melts aloft before it turns to
all rain before reaching the surface.




BUST- A situation when a certain type of
weather (often severe weather or winter weather) is expected but nothing
happens.






CAA- Cold Air Advection,
The movement of colder air horizontally toward a fixed point on the earth's
surface.




CAP- A cap is a temperature inversion
which prevents convection from
occurring.




CAT- A category. Usually refers to a category
of precipitation given by the forecast models. Also the Category of hurricane
intensity.




CB- Cumulonimbus.



CEILING- The height of the cloud base closest
to the earth's surface.




CELL- An individual thunderstorm (often
referred to as such on radar).




CELSIUS- A temperature scale using 0 as
freezing and 100 as boiling at standard sea-level atmospheric pressure
and temperature.




CG- Cloud to Ground lightning



CHC- Chance (usually in reference to precipitation probability)



CI- Cirrus



CIN or CIN/S or CINH- Convective Inhibition.
A cap which prevents
convection from occurring. A CIN of 50 or below is weak while 200 and above is
very strong.




CISK- Convective Instability of the
Second Kind
. This is the combination of an unstable atmosphere
with that of a large amount of latent heat
release.




CLASSIC EVENT- Having all the ingredients for
a specific type of weather (i.e. classic severe weather event, classic
Nor'easter)




CLOSED LOW- A low pressure center having a
closed circulation, which is used in reference to systems in the upper levels
of the atmosphere. Closed lows will have one or more height contours encircling
them.




CNVTN- Convection



COLD CORE HIGH- A dome of cold surface high pressure
that originates from the high latitudes.




COLD CORE LOW- A low pressure which has the
coldest temperatures located near its center. A cold core low is
deep and is often associated with the polar jet stream. Mid-latitude cyclones
are cold core lows.




COLD FRONT- A synoptic scale boundary between
cold and warm air. The cold air is displacing the warm air.




CONDENSATION PRESSURE DEFICIT (COND PRES DEF)-
On an isentropic chart (a layer of constant potential temperature),
condensation pressure deficit represents the amount of lift, expressed in
millibars, needed to saturate an air parcel. For example, an air parcel at 850
mb has a condensation pressure deficit of 200 mb. This means that this parcel
needs to be lifted 200 mb (up to the 650 mb level) before it will become saturated.
It is one way of evaluating moisture content on an isentropic chart.




CONFLUENCE- A 2-D process in which air
streams move toward each other at some angle.




CONQ- Converging Q vectors. Indicates air
will rise due to low level warm air advection
and/or upper level divergence




CONVECTIVE FEEDBACK- Convective feedback
deals with how a forecast model handles the vertical profile of changes in
temperature and moisture that are produced from convection.




CONVECTIVE INSTABILITY- Convective instability
is instability caused by very dry air advecting in the mid-levels of the
troposphere over a warm and moist lower troposphere. Dynamic lifting causes the
mid-levels to cool at a greater rate than the lower troposphere since air in
the lower troposphere cools at the WALR and air in the middle troposphere cools
at the DALR. CLICK HERE for an
in-depth essay on convective instability.




CONVERGENCE- CONV. A 1-D process in which
higher momentum air moves into lower momentum air.




CORIOLIS- An apparent force caused by the
earth's rotation. The Coriolis force is a
maximum at the poles and a minimum at the equator.




CU- An abbreviation for cumulus clouds



CU FIELD- Cumulus clouds covering a large spatial
area.




CUT-OFF LOW- Closed lows that become cut-off
from the main flow pattern are called cut-off lows. Click here for a cut-off
low example.




CWA- County Warning Area



CYCLONE- This is in reference to a low
pressure system.




CYCLOGENESIS- The intensification of a low
pressure system.






DEFORMATION ZONE (DFRMTN ZN) - An area in the
atmosphere where winds converge along one axis and diverge along another.
Deformation zones (or axis of deformation as they are sometimes referred to)
can produce clouds and precipitation.




DEEP- Extending a significant vertical
distance in the troposphere (i.e. deep moisture, deep uplift)




DEEPENING- Deepening is the
intensification of a low pressure system (in particular the lowering of central
surface pressure).




DERECHO- A derecho is a
widespread severe wind event resulting from persistent and violent outflow from
a MCS (Mesoscale Convective System). The derecho environment includes dry
mid-levels winds that are ingested into a squall line or a
segment of a squall line. The forward motion of the storm along with an intense
downdraft produced by negative buoyancy via evaporative cooling
brings down momentum from the middle levels of the storm to the surface.



A severe wind is one with wind speeds of 50 knots (58 miles per hour) at the
surface. In a derecho these severe winds encompass a distance of at least 400
km (250 miles) either out ahead of or along a squall line MCS. The length of
time the severe winds last can be particularly damaging. While a severe
thunderstorm may produce severe convective wind gusts
that last for several minutes at a point location, derecho wind can last 30
minutes or longer. Derechos can be tracked from radar and severe weather
reports while they are occurring since severe weather reports will be given in
sequence as the derecho traverses along.




DEWPOINT DEPRESSION (TDD)- The dewpoint depression
is the positive numerical difference between the temperature and the dewpoint.




DEWPOINT POOLING (DWPT PLG) - An area,
usually along a surface front or trough, where there
is a "pool" of higher dewpoints (or higher amounts of surface
moisture). Since increasing low level moisture increases atmospheric
instability, an area of dewpoint or moisture pooling tends to be more unstable
than surrounding locations and can be a prime area for the development of
thunderstorms.




DIFFERENTIAL ADVECTION- Differential advection
is the increasing or decreasing of advection with height. Vorticity advection
increasing with height is favorable for uplift as well as warm air advection
increasing when moving from aloft to the surface.




DIFFLUENCE- A 2-D process in which air
streams spread apart.




DIFFLUENT THICKNESS PATTERN (DIFLNT THKNS PTTN) -
Organized areas of thunderstorms tend to move with the thickness pattern (the
mean wind in a layer). An area of diffluent thickness is an area where the
thickness contours spread apart. Why this actually occurs is not fully known,
but it is usually found in an area of low level warming and upper level cooling
(processes that make the atmosphere more unstable). Areas of convection will
tend to move toward these areas of diffluent thickness (including backwards),
and can tip off forecasters when trying to pinpoint potential areas of heavy
rainfall.




DIGGING- A trough becoming more
amplified with time
. Often in reference to a trough building to the
South. When the winds are stronger on the upwind side of the trough the trough
will tend to dig.




DIRTY HIGH- High pressure usually brings clear
weather. A high pressure is termed "dirty" if it has a canopy of
clouds associated with it. These clouds can form due to orography, a saturated PBL (with low level fog and/or stratus building under a middle
level inversion), or
lifting mechanisms within the region of high pressure (WAA, moisture advection).




DISCONTINUITY- A rapid change of a
meteorological parameter over distance. A frontal boundary is an example of a
discontinuity.




DIURNAL EFFECTS- A reference to an effect
that has its origins due to daytime heating, such as afternoon cumulus cloud
development or the formation of a lake/sea breeze. These phenomena dissipate
once the sun goes down and surface heating is lost. Diurnal means a daily
effect.




DIVERGENCE- A 1-D process in which higher
momentum air moves away from lower momentum air.




DIVQ- Diverging Q vectors. Indicates air will
sink due to low level cold air advection
and/or upper level convergence




DLAD- Delayed



DOUBLE-BARRELLED LOW PRESSURE SYSTEM- Ideally
a low pressure system will have one distinct center where the surface pressure
and height aloft are lowest at one point. Often though the low pressure system
will be a broad area of lower pressure or it will have two distinct areas where
the surface pressure is lowest. The term double-barrelled low pressure system
means two side by side low pressure systems.




DOWN SLOPE- A wind that decreases in altitude
above sea level as it moves due to the air moving into a lower elevation. Downsloping air
tends to decrease in relative humidity
and it warms adiabatically.




DOWN STREAM / DOWN WIND- This is in the
direction something is traveling toward within a flow. For example, a boat on a
river current will move down stream.




DPNG- Deepening. Usually in reference to a
low pressure intensifying.




DRYLINE- A dryline is a
synoptic or large mesoscale boundary between warm/humid air and warm/dry air.




DRYLINE BULGE- A segment of a dryline
that advances forward due to strong upper level winds.




DRY PUNCH- This is in reference to a region
of drier air filtering toward a mid-latitude cyclone or sub-synoptic scale low
pressure area. It often has a fan shape with the small end of the fan
connecting to the cyclone.




DRY SLOT- Refers to an influx of dry air
(especially toward a mid-latitude cyclone)




DYNAMICS- Refers to upward forcing caused by
a mechanism that forces the air to rise or sink such as thermal advection, jet streak divergence and PDVA






EASTERLY WAVE- An inverted trough in
the tropics that moves with the tropical easterlies.




ECHO(ES)- This refers to radar returns such
as precipitation echoes.




EDDY- On any scale, this is a circulation
within a fluid or the air. The term is often used in reference to convective
eddies.




ENHANCED-V- An enhanced-V is a
V-shaped region over and downwind from strong to severe thunderstorms on
infrared imagery. The V signature is seen best on color enhanced infrared
satellite images. The cold cloud tops and anvil form a V-shape.
Think of how smoke fans out downwind from a chimney. The process is similar in
a strong to severe storms. An
intense updraft lifts
moisture high into the troposphere and strong upper level winds move the now
extremely cold moisture (ice crystals) downwind. The moisture fans out as it
moves downwind.




ENTRAINED / ENTRAINMENT- Refers to the
drawing in of moisture (or lack of moisture) into a system. Dry air entrainment
into the mid levels of a thunderstorm can enhance the potential for damaging wind gusts.
Moisture being entrained into a storm system can enhance precipitation amounts.




ENTRANCE REGION- Region of a trough or jet streak where
the winds enter the feature. This is the upstream region from the trough axis
or jet streak center.




ENUF- Enough



ETA- ETA forecast model- letters denote model
uses ETA coordinates. ETA is one of the primary synoptic forecast models used
by operational meteorologists




EQUIVALENT POTENTIAL TEMPERATURE- The
temperature of a parcel of air after all moisture and latent heat is
condensed out of an air parcel then descended to the 1000 mb level. Also known
as Theta-E




EXIT REGION- Region of a trough or jet streak where
the winds leave the feature. This is the downstream region
from the trough axis or jet streak center.




EXTRAPOLATE- Determining the value of a
meteorological value beyond the bounds of a data sample.




EXTRATROPICAL- A low pressure system that did
not develop in the tropics.
Extratropical cyclones have a baroclinic energy
regime. Extratropical can also be in reference to a tropical system that is
losing tropical characteristics.






FA- Forecast Area



FAHRENHEIT- A temperature scale using 32 as
freezing and 212 as boiling at standard sea-level atmospheric pressure and
temperature.




FCST- Forecast



FFA- Flash flood
advisory




FILLING- The weakening of a low pressure
system (in particular the increase of central surface pressure)




FIRE UP- A favorite phrase of forecasters. It
means storms are developing or will develop.




FLY IN THE OINTMENT- A favorite phrase of
some forecasters. Refers to a forecast problem or a potential forecast problem.
Could also be in reference to a forecasting problem that caused a
"busted" forecast.




FRCG- Forcing. This is in reference to dynamic lifting
such as from either low level convergence or upper level divergence.




FREEZING LEVEL- The pressure level above the
earth's surface where the temperature is freezing. If the entire troposphere is
below freezing then the freezing level will
be indicated as BG (Below Ground).




FREEZING FOG- Freezing fog is a fog
composed of supercooled water droplets. Freezing fog will occur when the
outside temperature is below freezing and the fog particles are still liquid.
Often freezing fog will occur with freezing drizzle. The supercooled droplets
freeze once they contact the earth's surface. Freezing fog is different from
ice fog. Ice fog occurs when
the fog particles turn to ice crystals and the outside temperature has to drop
well below freezing for the supercooled droplets to turn to ice crystals while
still in the air.




FREEZING RAIN (ZR)- Freezing rain is
liquid precipitation that freezes after reaching the earth's surface




FRICTION LAYER- This is another term for the boundary layer. This is the layer of the
troposphere closest to the earth's surface where friction is a significant
force and wind tends to be gusty (irregular flow).




FRONT- A transition zone between air masses



FRONTOGENESIS- The intensification of a front
(temperature gradient
is becoming more compact; isotherms closer together in the region the front is
developing.




FRONTOLYSIS- The weakening of a front (temperature gradient
is becoming less compact; isotherms are spreading apart in the region the front
is weakening.




FROPA- FROntal PAssage



FWC- Forecasted Weather Conditions; MOS
output from NGM model




FZRA / ZR- Freezing Rain





GALE- A wind speed ranging from 39 to 54 mph.



GFS- Global Forecast System. This is a
synoptic model that has output for several days into the future and was
previously known as MRF (Medium Range Forecast).




GRAVITY- Gravity is the
attraction of two masses to one another. Large masses have higher values of
gravitational accelerations than lighter masses.




GEOSTROPHIC- This is a balance between the pressure gradient
and Coriolis forces. A zonal wind with a
constant wind speed in the upper troposphere in the middle latitudes is an
example of a geostrophic wind.




GROUND CLUTTER- (a.k.a. Doppler Garbage).
These are false precipitation or velocity echoes on radar that are produced by
obstacles near the radar site.




GUIDANCE (GUID)- Forecast model data that
helps a forecaster make a forecast.




GUST- A brief and rapid increase in wind
speed. These most commonly occur when the overall wind speed is fairly high and
is most common near the earth's surface.




GUST FRONT- It is the leading edge of
thunderstorm outflow. Often cooler and gusty air follows behind the gust front.






HAZE (H)- Dust, salt and other particles that
restricts horizontal visibility.




HELICITY- Helicity is
streamwise vorticity available for ingestion into a thunderstorm. Higher values
are favorable for a rotating updraft (greater than 400). A measure of low level
wind shear,
normally within the lowest 3 km of the atmosphere, relative to the movement of
a thunderstorm (thus referred to as 0-3 km Storm Relative Helicity). This gives
forecasters an indication of an environment that is favorable for supporting
the development of thunderstorms with rotating updrafts, a precursor to supercell thunderstorms
(the most violent of severe storms) and tornado
development. Values of helicity greater than +150 are considered significant;
however, like CAPE values, there
is no magic value of (positive) helicity under which rotating thunderstorms
will not develop. Helicity is only an index to determining thunderstorm
rotation potential.




HI- High



HODOGRAPH- A graph that shows how the wind
speed and direction change with height.




HORIZONTAL VORTICITY- A rotation of air
caused by vertical speed or directional wind shear.




HP- High Precipitation supercell; It is a
supercell with a high moisture content.
Precipitation will wrap completely around the updraft region in an HP
supercell. They have a kidney bean shape on radar reflectivity.




HVY- Heavy



HYDROLAPSE- Rapid change in moisture
(particularly dewpoint) with height. A hydrolapse
separates warm and moist lower tropospheric air from mid-level dry air. CLICK HERE for an
example of a hydrolapse on a Skew-T.




HYDROMETEOR- Any precipitation particle that
is falling






ICE PELLETS- IP / PL, Same as sleet. Frozen
raindrops that strike the earth's surface.




INDCG- Indicating



INDIAN SUMMER- A warm spell in the fall or
early winter in which the temperatures are above normal and skies are clear for
several days in a row.




INFLOW- Wind speed, in knots, of the average PBL wind speed.



INSTABILITY (INSTBY)- Air that if lifted
adequately will rise on its own due to positive buoyancy. Instability is
needed for thunderstorm development.




INTERPOLATION- Drawing an isopleth between
known numerical values.




INVERSION (INV)- An inversion is a
temperature increase with height.




INVERTED TROF- This is a trough which bulges
to the north. Mid-latitude troughs have a north to south amplitude but in the
tropics the opposite is the case. Inverted troughs look like ridges but there
is lower pressure at their centers of curvature. Inverted troughs can occur
across areas of the tropics and sub-tropics. CLICK HERE for an
example of an inverted trough.




INVOF- IN the Vicinity OF



IP / PL- Ice Pellets



ISALLOBAR- A line of equal surface pressure
change.




ISENTROPIC LIFT / DESCENT- Isentropic motion
is lifting or sinking of air along constant potential temperature (theta)
surfaces. WAA (especially
over shallow frontal boundaries) leads to lift while CAA leads to
descent.




ISOBAR- A line of constant surface pressure.



ISODOP- A line of equal distance from a radar
site.




ISODROSOTHERM- A line of constant dewpoint temperature.



ISOHYET- A line of equal rainfall.



ISOHYPSE- A line of constant geopotential
height (height contour).




ISOPLETH- A line of a constant meteorological
value.




ISOSTER- A line of a constant density.



ISOTACH- A line of constant wind speed.



ISOTHERM- A line of constant temperature.





JET STREAK- A jet streak is a
relative maximum of windflow within the jet stream.




JET STREAM- The jet stream is a meandering belt of strong
upper level winds that separates milder mid-latitude air from cold polar air.







KELVIN- A ratio temperature scale using 0 as
the theoretical coldest temperature.




KNOT- A knot is a unit of
wind measurement found by taking the wind speed in miles per hour and dividing
it by 1.15.






LAKE EFFECT SNOW- Lake Effect Snow is
snow produced from lifting of moisture from a large lake such as the Great
Lakes.




LAPSE RATE- The rate of temperature decrease
with height. High lapse rates are indicative a strong cooling with height and
this can lead to instability.




LATENT HEAT- Energy released or absorbed that
changes the temperature of the surrounding environment but NOT the material
releasing or absorbing the latent heat. For
example, when ice melts, it cools the surrounding air but the ice cube/water
mixture stays the same temperature.




LEE TROUGH- Low pressure that develops just
east of the Rocky Mountains due to flow over the higher elevation region. Often
lee troughs become organized into mature cyclones as they develop eastward. The
lee wardside is the opposite of the windward side.




LEWP- Stands for Line Echo Wave Pattern. This
is an outward bulge along a squall line that is
often created from strong upper level winds intersection that segment of the
squall line. A LEWP can suggest strong surface winds will occur along the
LEWP's path.




LIFTED INDEX- The environmental temperature
at 500 millibars minus the 500 millibar parcel temperature on a Skew-T diagram.
Negative LI values are unstable.




LLJ- Low Level Jet. Strong Planetary Boundary Layer winds which
advect warmer temperatures and moisture into the forecast area.




LP- Low Precipitation Supercell. This is a supercell that
lacks abundant moisture. They occur around a dryline boundary
where moisture depth is greatly reduced. They tend to look much more severe in
the field than they do on radar.




LSR- Local Storm Report



LONGWAVE- The jet stream will have a series of waves. A
wave of higher heights is a longwave ridge and a wave of lower heights is a
longwave trough.




LUKG- Looking





MAV- MOS output from AVN forecast model



MCC- Mesoscale Convective Complex. A large
cluster of thunderstorms and rain. It is defined using satellite meteorology.
Must have cloud shield temperature of -32 C or less over 100,000 square
kilometers or more and a cloud shield temperature of -52 C or less over 50,000
square kilometers or more.




MCS- Mesoscale Convective System. A large
cluster of thunderstorms and rain. Can be a squall line, multi-cells or a
mesoscale convective complex.




MCV- Mesoscale Convective Vortex. It is an
MCC or MCC-like grouping of storms that shows a mesoscale rotation.




MET- MOS output from ETA forecast model



MEX- MOS output from extended GFS forecast
model




MICROBURST- A 4 kilometer diameter or less
downdraft of air that sinks due to strong negative buoyancy.




MID/UPPER LEVEL SUPPORT- This is in reference
to either positive differential vorticity
advection
or a jet streak creating
upper level divergence.
These processes result in a dynamic lifting of air.




MILLIBAR (MB)- A unit of pressure which is
equal to Pascals divided by 100.




MISG- Missing



MIXED BAG- Several precipitation types
falling at once or over a period of time.




MIXING DEPTH- The vertical distance the
process of convection mixes the air from the surface to aloft. The mixing depth
is often the same depth as the PBL. The mixing depth will increase with
solar warming of the surface and increased low level wind speed. Could also be
in reference to the depth of the "transition zone" between two air masses that are
horizontally differentially advecting
one over the other.




MIXING RATIO CHART- Used by forecasters to
determine moisture content on a constant pressure surface. Rawinsonde data is
used to produce this chart.




MOISTURE AXIS / RIDGE- : An area of higher
moisture values, usually in the form of a ridge of higher dewpoints at the
surface or 850 mb. Low level moisture axes enhance atmospheric instability, which
in turn promotes thunderstorm development. Existing storms can intensify by
moving into moisture axes. The concept is similar to dewpoint pooling.




MOS- Model Output Statistics. These are
numerical representations of expected weather such as forecasted temperatures
and precipitation chances.




MRF- Medium Range Forecast (now called GFS:
Global Forecast System); synoptic model that gives output for several days into
the future.




MSTR- Moisture



MXD- Mixed





NCEP- National Center for Environmental
Prediction. They are responsible for running and disseminating the forecast
models.




NEGATIVE TILTED TROUGH- A trough which tilts
from the northwest toward the southeast. This situation creates instability
since cold temperatures in the mid and upper levels advect over warmer air at
the surface. A negative tilt is a sign a trough is mature. CLICK HERE for an
example of a negatively tilted trough.




NEGATIVE VORTICITY ADVECTION (NVA)- The
advection of lower values of vorticity into the forecast area. NVA promotes
synoptic scale sinking air.




NEXRAD- Next Generation Radar. Reflectivity
and Doppler Radar used by NWS offices.




NGM- Nested Grid Model. A synoptic forecast
model for short term forecasting.




NOAA- National Oceanographic and Atmospheric
Administration




NOCTURNAL- A weather feature that occurs at
night (i.e. nocturnal inversion)




NORMAL- The average of a certain weather
parameter over several years (commonly 30 or more years). For example, the
normal yearly precipitation being 30 inches means that over a 30 year period
the average is 30 inches per year.




NOSIG- No significant change



NR- Near





OBS- Observation, a reporting station's
output




OCCLUDED FRONT- The lifting of one front by
another.




OMEGA FORCING- This is in reference to Q-G
theory (Quasi-Geostrophic theory). The omega equation states vertical motion is
a function of differential vorticity
advection
and thermal advection.
Positive omega forcing results from vorticity advection increasing with height
and/or low level warm air advection.




OROGRAPHIC- Air flowing from one elevation to
another due to forced lifting or sinking due to the slope of the land surface
(i.e. orographic uplift)




OPEN WAVE- A wave of low pressure that does
not have a complete circulation around it; also called a short wave trough.




OUTFLOW BOUNDARY- The outflow of air from
thunderstorm(s)
. They can act like miniature cold fronts.




OVERCAST (OVC)- Cloudy.





PACKAGE- A compilation of analysis and
forecast charts and their interpretation.




PBL- Planetary Boundary Layer. The lowest level
of the atmosphere where friction is an important force and vertical mixing is
common.




PCPN- Precipitation



PD- Period



PDS- Potentially Dangerous Situation, usually
in reference to the dangerous threats from a severe weather outbreak.




PG- Pressure gradient.
Tightening pressure gradient indicates stronger winds.




PHASING- When two separate short waves come
together to form one wave. Also, when upper and lower level features are
positioned so that each provides energy to the other, it is said that the
features are in phase with one another.




POP- Probability of Precipitation.
This is the chance the NWS gives to precipitation across the forecast region.
It may also refer to the POP a forecast model is predicting.




POSITIVE ISOTHERMAL VORTICITY ADVECTION (PIVA)-
Advection of higher values of vorticity by the thermal wind on a map of
vorticity and thickness contours. Implies an area of rising motion.




(DIFFERENTIAL) POSITIVE VORTICTY ADVECTION-
Advection of higher values of vorticity into an area and this vorticity
advection contributes to rising air.




POSITIVE TILTED TROUGH- A trough which tilts
from the northeast toward the southwest. Often a trough is positive tilted in
it's development stage.




POTENTIAL INSTABILITY- Also known as convective instability.
Instability caused by dry air advecting over warm and humid PBL air. Lapse rate of temperature
increases if lifting occurs since the low level air cools at the SALR while the
mid-level air cools at the DALR.




POTENTIAL TEMPERATURE- The temperature of an
air parcel after it is lowered or raised to the 1000 mb level.




PRECIPITATION (PCPN)- Solid or liquid water
falling from the air (i.e. snow, sleet, rain, hail, etc.)




PRECIPITABLE WATER (PW, PCPTBL WTR or H2O)-
Total amount of water vapor in a
layer of air, expressed in inches. Normally taken between 1000 and 500 mb.
Higher values of precipitable water
indicate a deep moisture layer, increasing the potential for heavy
precipitation amounts.




PRES- Pressure



PROFILER- A remote ground-based sensing
instrument that measures wind speed and direction at different levels of the
atmosphere. The new National Weather Service WSR-88D radar has this capability,
and its vertical wind profile is called a VAD Wind Profile (or VWP). VAD stands
for Velocity Azimuth Display.




PROG- Model forecasted output,
prognostication




PROGRESSIVE FLOW or PROGRESSIVE PATTERN- An
upper level flow pattern in which storm systems move along at a fairly regular
pace.




PULSE STORM- A pulse storm is a
storm with a strong updraft due to strong instability release that develops in
a weak shear environment. Pulse storms can produce brief severe weather. The
lack of wind shear results in the storm weakening quickly since the downdraft
cut-offs and chokes the updraft.




PUNCH (or PUSH)- A fast moving mass of air
(e.g. dry punch, cold punch)




PW- Precipitation Water,
the amount of liquid equivalent if all moisture in troposphere were condensed
onto the earth's surface.




PWAT(S)- Precipitable Water value(s)





QG Forcing- Upward vertical velocity
generated by low level convergence
or upper level divergence.
Example of phenomena that produce upward QG forcing include low level warm air advection
and moisture advection,
positive differential vorticity advection, and the right rear and left front
quadrant of a jet streak. If QG
forcing is in reference to sinking air, examples that cause this are low level
cold air and dry air advection, negative differential vorticity advection, and
the left rear and right front
quadrants of a jet streak
.




Q-VECTORS (QVEC, DIVQ)- A mathematical entity
(Q-vectors do not exist in the atmosphere) that allows forecasters to better
identify areas of vertical motion. Q-vectors essentially show vertical motions
arising from the combination of differential vorticity
advection
(changes of advection with height) and temperature
(thickness) advection. Areas where Q-vectors converge implies upward motion and
Q-vector divergence (DIVQ) implies an area of sinking motion. Note: Negative
Q-vector divergence is the same mathematically as Q-vector convergence, so the
statement "...negative divq..." means Q-vector convergence.




QPF- Quantitative Precipitation Forecast.



QUAD- Quadrant, 1/4th of a square or
rectangular area (i.e. Right rear quadrant, NE quadrant)




QUASI- Nearly (i.e. quasi-geostrophic,
quasi-stationary)






RA- Rain



RADIATIONAL COOLING- The earth's surface
cooling by emitting longwave radiation. The best radiational cooling occurs on
clear nights.




RELATIVE HUMIDITY (RH)- The amount of
moisture in the air as a ratio to the maximum amount of moisture that can be
evaporated into the air at a constant temperature. It can be expressed as the
mixing ratio divided by the saturation mixing ratio or the vapor pressure
divided by the saturation vapor pressure.




RETURN FLOW- Return flow is a
rapid increase of temperature and moisture. This is in common reference to
winds becomes southerly which allows Gulf moisture to be transported rapidly
northward. Return flow precedes the passage of a low pressure trough and cold
front.




RETROGRADE- To move backwards. It relates to
a ridge or trough moving from east to west. Typically in the mid-latitude,
troughs and ridges move west to east. Retrograde motion is the opposite
movement of normal.




RH- Relative Humidity



RIDGE- A wave of higher heights. The opposite
of a trough. Ridging tends to be associated with sinking air.




RING OF FIRE- This refers to thunderstorms
and rain across the edges of a high pressure ridge. If a ridge occurs across
the central US in summer, the ring of fire will extend across the west coast,
up into Canada and across the east coast. Underneath the main core of the ridge
weather will be stable.




Rapid Update Cycle (RUC)- A short range model
that projects a 12 hour forecast. The model is run each hour, with its initial
conditions updated by the inclusion of the latest surface observations
available. Its primary usage is for aviation and severe weather forecasting.






SATURATED- Air with 100% relative humidity



SBCAPE / MUCAPE- SBCAPE stands for surface
based CAPE and is the
value of CAPE relative to a parcel of air rising from the lower planetary boundary layer (the surface).
MUCAPE stand for "most unstable CAPE". MUCAPE is the highest CAPE
found by raising a parcel of air from every level in the lower to mid
troposphere.




SCA- Small Craft Advisory



SELS- Severe Local Storms



SEV, SVR- Severe



SFC- Surface



SHEAR (WIND SHEAR)- Significant change in wind speed or direction with
height
. For severe weather this is most relevant if it occurs in the
lower troposphere.




SHEAR AXIS- A linear feature (axis) which
denotes a directional or speed change in wind.




SHORT-FUSE WARNING- A warning by the NWS for
an event that poses an immediate danger and will occur over a relatively short
amount of time (i.e. Tornado warning, Severe Thunderstorm warning)




SHORTWAVE- sometimes abbreviated S/WV: This
term will come up many upon many times. This is a short amplitude trough
embedded within the large scale flow. A baroclinic shortwave occurs when the
isotherms cross the height contours at a sharp angle. Shortwaves are best
defined on the 700 and 500 millibar charts. Shortwaves range in size from
mesoscale to synoptic scale. They are responsible for producing instability and dynamic precipitation.
They create instability by cooling the mid-levels of the atmosphere, generating
vorticity and creating upper level fronts. CLICK HERE for an
example of a shortwave and more explanation.




SLEET- Ice Pellets, rain
that freezes before reaching earth's surface




SN- Snow



SOUNDING- Profile of temperature, dewpoint
and wind with height measured by a weather balloon




SPC- Storm Prediction Center, Norman OK



SPIN UP- An increase in the rotation rate
over time (i.e. spin up of vorticity, tornado spin up)




SPS- Special Statement



SST- Sea Surface Temperature



STACKED- A pressure system which shows up in
the same general location on every height analysis or forecast chart. This is
an indication low pressure is weakening (filling) or a pressure system is
stalling.




STATIONARY FRONT- A front which moves very
slowly or not at all over time.




STORM RELATIVE- Measurement made in the
storm's reference. For example, if the wind is from the south at 20 knots and
the storm is moving into the wind at 10 knots, the storm will experience a 30
knot headwind.




STREAMLINES- Arrows showing wind speed and
direction. The head of the arrow points toward where the wind is blowing and
the length of the arrow is proportional to the wind speed. Sometimes shows wind
direction and trajectory only.




STRAIGHT-LINE WIND- A strong wind from a
storm in which the wind direction is fairly constant. A severe convective wind
gust from a thunderstorm is a straight-line wind
while the winds of a tornado are not.





STRATIFORM- Stable clouds. Stratiform clouds
lack convective development and occur in broad layers. The lifting that forms
stratiform clouds is dynamic (slow rising air).




STREAMWISE VORTICITY- The amount of
horizontal vorticity parallel to storm inflow.




SUBSIDENCE- Sinking air caused by broad scale
sinking or dynamic sinking (i.e. High pressure subsidence, Mid-level
subsidence, orographic subsidence)




SUB-SYNOPTIC LOW- A relative small area of
surface low pressure. They are also referred to as mesolows.




SUPERCELL- A storm with a strong, tilted and
rotating updraft due to good instability and
wind shear in the troposphere. Most of the strong tornadoes and large hail occur with supercells.




SUPERCOOLED- Liquid water having a
temperature that is below freezing




SXNS- Sections





TCU- Towering Cumulus



TDA- Today



TEMP- Temperature



THERMAL RIDGE or THERMAL TROUGH- A ridge of warmer
temperatures or a trough of colder temperature. CLICK HERE for an
example and explanation.




THERMAL LOW- A surface low pressure caused by
intense surface heating. Thermal lows weaken with height since the source of
heating is at the surface. They commonly occur in desert regions in summer.
They often do not produce precipitation since the air is very dry at the
surface.




THERMODYNAMICS- In reference to the (in)stability and
other thermodynamic factors of the atmosphere. Important thermodynamic
information includes lapse rates, CAPE, changes in
temperature / moisture with height, and cap strength.




THETA-E ADVECTION- Movement of higher
temperature, higher moisture or a combination of higher moisture and
temperature toward forecast area. Generally referenced to low level
temperature/moisture advection. An increase of Theta-E in low
levels of atmosphere increases atmospheric instability.




THETA-E RIDGE- Region with a relatively
higher combination of temperature and dewpoint at the surface. In a
thunderstorm situation this region would be more unstable.




THICKNESS LINE- A line of constant
geopotential thickness. Thickness increases
by either increasing the temperature or increasing water vapor content
of the air.




THICKNESS (CRITICAL VALUES FOR SNOW)-
Thicknesses equal to or less than value given below are suggestive that
precipitation type will be snow. Forecaster MUST also look for warm and cold
biasing of thickness value, especially the 1000-500 mb thickness.
Consult the following website for information on thickness as well as thickness
biasing.



THICKNESS AND THICKNESS
BIASING




1000-500 mb 5400 m or less (referred to as the "540 line") is
suggestive of snow



1000-850 mb 1300 m or less is suggestive of snow



850-700 mb 1540 m or less is suggestive of snow



Thickness is also important because it gives rise to the concept of the
"thermal wind" (the thermal wind itself does not exist in the
atmosphere). On a map of thickness contours, the thermal wind "flows"
between the contours; the tighter the contour spacing, the "faster"
the thermal wind. This thermal wind is equivalent to the mean wind between the
layer it represents i.e., the 1000-500 mb thermal wind represents the mean wind
between 1000 and 500 mb, and it "flows" between contours of 1000-500 mb thickness.
It is because the thermal wind represents mean wind flow that organized
convection (thunderstorm complexes) tends to move parallel to thickness
contours.




TRACE- Less than 0.01 inches of precipitation



TRIGGER MECHANISM- A trigger mechanism
is any process that initiates precipitation or storm development. It is in
reference to a process that causes a precipitation or storm event and without
this process precipitation or storms would not have occurred. Common trigger
mechanism examples are lifting mechanisms, increase of low level moisture,
daytime heating, instability and wind shear. The
most common type of trigger mechanism that will be referenced are lifting
mechanisms such as fronts and other low level convergence
boundaries.




TRIPLE POINT- The intersection point between
two boundaries (dry line, outflow boundary,
cold front, etc.), often a focus for thunderstorm development. Triple point
also may refer to a point on the gust front of a supercell, where
the warm moist inflow, the rain-cooled outflow from the forward flank
downdraft, and the rear flank downdraft all intersect; this point is a favored
location for tornado development
(or redevelopment).


TROUGH (TROF)- Longwave of lower heights
aloft. A cold low level air mass will cause there to be troughing aloft. Rising
air over a broad region such as from a mid-latitude cyclone can also create troughing.


TS, TSRA- Thunderstorm

TUTT- Tropospheric Upper level Tilted Trough
or (Tropical Upper Tropospheric Trough). This is an upper level wave that
commonly develops in tropical environments along old frontal boundaries or in
association with an upper level low.


TVS- Tornado Vortex Signature

TWEAK- To change slightly. Often in reference
to slightly changing MOS output or any other form of model guidance.



UA- Upper Air



UVV- Upward Vertical Velocity. Thermodynamic UVV
results from the release of instability on the
sub-mesoscale (updrafts in thunderstorms). Dynamic UVV results
from broad synoptic lift caused by low level convergence (WAA) or upper
level divergence (Jet streak, PDVA). Synoptic
vertical velocities may be referred to as OMEGA forcing.




UNSTABLE- A situation in which air if lifted
adequately will rise on its own due to positive buoyancy. The optimum unstable
situation consists and warm and moist air near the surface and cold and dry air
in the middle and upper troposphere.


UPPER LEVEL DYNAMICS- Lifting due to a jet streak or positive vorticity advection.

UPPER RIDGE- Ridge of high pressure that is
evident on upper level charts (e.g 500 mb)


UPSLOPE- Upslope is a
movement of air from a lower to a higher elevation due to forced lifting of
land or dynamic uplift that occurs along a slope due to air density
differences.


UPSTREAM / UPWIND- The direction from which a
flow is coming from. A boat moving down a river will be moving downstream but came
from upstream
.


VAPOR DEFICIT- The number of millibars a
parcel of air must rise in order to achieve saturation. Air that has a low dewpoint depression
(near saturation), will have a low vapor deficit. Air with a low RH will have a
high vapor deficit.




VAPOR PRESSURE- The amount of pressure
(usually expressed in millibars) exerted from molecules of water vapor
independent from the pressure exerted from the air. The vapor pressure is a
trace to about 4% of the total air pressure at any one time.


VEERING WIND- A veering wind is a
wind that turns clockwise with height. It is associated with warm air advection.


VERTICALLY STACKED- In reference to a low
pressure trough that is at about the same location on each pressure surface
aloft. This is an indication the low is occluded and decaying. Strong and
maturing low pressure systems will tilt with height toward the colder air
aloft.



VIRGA- Virga is
precipitation that evaporates before reaching the earth's surface



VIRTUAL TEMPERATURE- The temperature of the
air plus latent heat release
to the air due to condensation. In air with moisture, the virtual temperature
is always greater than the actual temperature.


VIS- Visible satellite imagery

V-NOTCH- The term V-notch is used to refer to
the V-shape of a supercell on radar
imagery. Strong upper level winds move moisture downwind on each
side of the storm, producing a V shape. The storm represents a barrier to the
windflow thus the flow diverges around the edges of the storm and forces
precipitation to move downwind.


VORTICITY- Any rotation within a horizontal
or vertical windflow.


VORT MAX, VORT LOBE- Highest value of
vorticity. A region of maximum vorticity. A wind flow through a vort max will
produce divergence downwind from the
vort max.


WAA- Warm Air Advection.
Horizontal movement of warmer air into the forecast area.



WAD- Wind Advisory

WARM CORE HIGH- A dome of deep high pressure
that originates from low latitude areas.




WARM CORE LOW- A low pressure which is
deepest at the surface and gradually weakens in the vertical. Warm core lows
have warm temperatures near their center due to either surface solar warming.
large amounts of latent heat release
or subsidence.




WARNING- A particular region in which
threatening weather is already occurring or is imminent (i.e. Severe Thunderstorm Warning,
Tornado Warning, Winter Storm Warning)




WATCH- A region of the country in which
people should be on the lookout for threatening weather (i.e. Severe Thunderstorm Watch,
Tornado Watch, Winter Storm Watch)




WARM FRONT- A synoptic scale boundary between
warm and cold air. The warm air replaces the cooler air over time.




WASHES OUT- A diminishment of a
meteorological entity. Examples: The cold front washes out, Precipitation
washes out, Shortwave washes
out




WET BULB TEMPERATURE- Temperature after
cooling takes place due to evaporating moisture into the
air
(at constant pressure).




WET BULB ZERO TEMPERATURE- The wet bulb zero
is the wet bulb temperature (found by evaporating water and cooling unsaturated
air) equal to 0° C.




WET MICROBURST- Strong downdraft of 4 km in
diameter or less also consisting of heavy rain.


WIND SHEAR- A pronounced change in wind speed or
direction with height
in the vertical or the horizontal.


WINDWARD- The side that is facing the
on-coming wind


WK- Weak

WL- Will

WX- Weather

XPCTD- Expected

XTRM- Extreme

YDA- Yesterday

ZONAL FLOW- A zonal flow is a
flow of wind equal or nearly equal to the lines of latitude.




ZONE FORECAST- A forecast for a particular
region (often a forecast for certain cities, counties or parishes).


ZL- Freezing Drizzle

ZR / FZRA- Freezing Rain

Z-TIME- Z-time is time
relative to 0° longitude which runs through Greenwich, England. This time scale
uses a 24-hour clock and the time is the same for every point on the earth's
surface.



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