Contents
Quick Alpha Access: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
*Scud (or Fractus) - Small, ragged, low cloud fragments
that are unattached to a larger cloud base and often seen with and behind cold
fronts and thunderstorm
gust fronts. Such clouds generally are
associated with cool moist air, such as thunderstorm outflow.
SELS - SEvere Local Storms
Unit, former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).
*Severe Thunderstorm - A
thunderstorm which produces tornadoes,
hail 0.75 inches or more in diameter, or winds of 50 knots (58 mph) or more.
Structural wind damage may imply the occurrence of a severe thunderstorm. See
approaching (severe).
Shear - Variation in wind speed (speed shear) and/or direction (directional shear) over a short
distance. Shear usually refers to vertical wind shear, i.e., the change in wind
with height, but the term also is used in
Doppler radar to describe changes in
radial velocity over short
horizontal distances.
*Shelf Cloud - A low, horizontal
wedge-shaped arcus cloud, associated with a
thunderstorm gust front (or
occasionally with a cold front, even in the absence of thunderstorms). Unlike
the roll cloud, the shelf cloud is
attached to the base of the parent cloud above it (usually a thunderstorm).
Rising cloud motion often can be seen in the leading (outer) part of the shelf
cloud, while the underside often appears turbulent, boiling, and wind-torn.
Short-Fuse Warning - A
warning issued by the
NWS for a local weather hazard of relatively
short duration. Short-fuse warnings include tornado warnings, severe
thunderstorm warnings, and flash flood warnings. Tornado and severe
thunderstorm warnings typically are issued for periods of an hour or less,
flash flood warnings typically for three hours or less.
Shortwave (or Shortwave Trough) - A
disturbance in the mid or upper part of the atmosphere which induces upward
motion ahead of it. If other conditions are favorable, the upward motion can
contribute to thunderstorm development ahead of a shortwave.
Slight Risk (of
severe thunderstorms) -
Severe thunderstorms are expected to affect between 2 and 5 percent of the
area. A slight risk generally implies that severe weather events are expected
to be isolated. See high risk,
moderate risk,
convective outlook.
Sounding - A plot of the vertical profile of
temperature and dew point (and often
winds) above a fixed location (Fig. 6). Soundings
are used extensively in severe weather forecasting, e.g., to determine
instability, locate temperature
inversions, measure the strength of the
cap, obtain the
convective temperature,
etc.
SPC - Storm Prediction Center. A national forecast
center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing
short-term forecast guidance for severe convection, excessive rainfall (flash
flooding) and severe winter weather over the contiguous United States.
Speed Shear - The component of
wind shear which is due to a change in wind
speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet
increasing to 50 mph at 20,000 feet. Speed shear is an important factor in
severe weather development, especially in the middle and upper levels of the
atmosphere.
Spin-up - [Slang], a small-scale vortex
initiation, such as what may be seen when a gustnado, landspout, or
suction vortex forms.
Splitting Storm - A thunderstorm which
splits into two storms which follow diverging paths (a
left mover and a
right mover). The left mover typically
moves faster than the original storm, the right mover, slower. Of the two, the
left mover is most likely to weaken and dissipate (but on rare occasions can
become a very severe anticyclonic-rotating storm), while the right mover is the
one most likely to reach supercell
status.
*Squall Line - A solid or nearly solid line
or band of active thunderstorms.
Staccato Lightning - A
CG lightning discharge which appears as a
single very bright, short-duration stroke, often with considerable branching.
Steering Winds (or Steering Currents) -
A prevailing synoptic scale flow
which governs the movement of smaller features embedded within it.
Storm-relative - Measured relative to a
moving thunderstorm, usually referring to winds, wind shear, or
helicity.
Storm-scale - Referring to weather systems
with sizes on the order of individual thunderstorms. See
synoptic scale,
mesoscale.
*Straight-line Winds - Generally,
any wind that is not associated with rotation, used mainly to differentiate
them from tornadic winds.
Stratiform - Having extensive horizontal
development, as opposed to the more vertical development characteristic of
convection. Stratiform clouds cover
large areas but show relatively little vertical development. Stratiform
precipitation, in general, is relatively continuous and uniform in intensity
(i.e., steady rain versus rain showers).
Stratocumulus - Low-level clouds,
existing in a relatively flat layer but having individual elements. Elements
often are arranged in rows, bands, or waves. Stratocumulus often reveals the
depth of the moist air at low levels, while the speed of the cloud elements can
reveal the strength of the low-level
jet.
Stratus - A low, generally gray cloud layer
with a fairly uniform base. Stratus may appear in the form of ragged patches,
but otherwise does not exhibit individual cloud elements as do
cumulus and
stratocumulus clouds. Fog usually is
a surface-based form of stratus.
Striations - Grooves or channels in cloud
formations, arranged parallel to the flow of air and therefore depicting the
airflow relative to the parent cloud. Striations often reveal the presence of
rotation, as in the barber pole or
"corkscrew" effect often observed with the rotating
updraft of an
LP storm.
Subsidence - Sinking (downward) motion in
the atmosphere, usually over a broad area.
Sub-synoptic Low - Essentially the
same as mesolow.
Suction Vortex (sometimes Suction Spot)
- A small but very intense vortex within a tornado circulation. Several suction vortices
typically are present in a multiple-vortex tornado. Much of the
extreme damage associated with violent tornadoes (F4 and F5 on the
Fujita scale) is attributed to
suction vortices.
*Supercell - A thunderstorm with a persistent
rotating updraft. Supercells are rare, but
are responsible for a remarkably high percentage of severe weather events -
especially tornadoes, extremely large hail
and damaging straight-line
winds. They frequently travel to the right of the main environmental winds
(i.e., they are right movers). Radar
characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a
TVS. Visual characteristics often include a
rain-free base (with or without a
wall cloud),
tail cloud,
flanking line,
overshooting top, and
back-sheared anvil, all of which
normally are observed in or near the right rear or southwest part of the storm
(Fig. 7).
Storms exhibiting these characteristics often are called classic supercells;
however HP storms (Fig. 3) and
LP storms (Fig. 5) also are
supercell varieties.
Surface-based Convection -
Convection occurring within a
surface-based layer, i.e., a layer in which the lowest portion is based at or
very near the earth's surface. Compare with
elevated convection.
SWEAT Index - Severe Weather
ThrEAT index. A stability index developed by the Air Force which
incorporates instability,
wind shear, and wind speeds as follows:
SWEAT = (12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where
- Td 850 is the dew point temperature
at 850 mb,
- TT is the total-totals
index,
- f 850 is the 850-mb wind speed (in knots),
- f 500 is the 500-mb wind speed (in knots), and
- s is the sine of the angle between the wind directions at 500 mb and 850 mb
(thus representing the directional
shear in this layer).
SWEAT values of about 250-300 or more indicate a greater potential for
severe weather, but as with all stability indices, there are no magic numbers.
The SWEAT index has the advantage (and disadvantage) of using only
mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative
disuse with the advent of more detailed sounding analysis programs.
SWODY1, SWODY2 (sometimes pronounced swoe-dee) -
The day-1 and day-2 convective outlooks issued by SELS.
Synoptic Scale (or Large Scale) - Size
scale referring generally to weather systems with horizontal dimensions of
several hundred miles or more. Most high and low pressure areas seen on weather
maps are synoptic-scale systems. Compare with
mesoscale,
storm-scale.
*Tail Cloud - A horizontal, tail-shaped
cloud (not a funnel cloud) at low
levels extending from the precipitation cascade region of a
supercell toward the
wall cloud (i.e., it usually is
observed extending from the wall cloud toward the north or northeast). The base
of the tail cloud is about the same as that of the wall cloud. Cloud motion in
the tail cloud is away from the precipitation and toward the wall cloud, with
rapid upward motion often observed near the junction of the tail and wall
clouds. See Fig. 7,
supercell.
Compare with beaver tail, which is a
form of inflow band that normally attaches to the storm's main
updraft (not to the wall cloud) and has a
base at about the same level as the updraft base (not the wall cloud).
Tail-end Charlie - [Slang], the
thunderstorm at the southernmost end of a squall line or other line or band of
thunderstorms. Since low-level southerly inflow of warm, moist air into this
storm is relatively unimpeded, such a storm often has a higher probability of
strengthening to severe levels than the other storms in the line.
Thermodynamic Chart (or
Thermodynamic Diagram) - A chart containing contours of pressure, temperature,
moisture, and potential
temperature, all drawn relative to each other such that basic thermodynamic
laws are satisfied. Such a chart typically is used to plot atmospheric
soundings, and to estimate potential
changes in temperature, moisture, etc. if air were displaced vertically from a
given level. A thermodynamic chart thus is a useful tool in diagnosing
atmospheric instability. (See
Fig. 6,
sounding.)
Thermodynamics - In general, the
relationships between heat and other properties (such as temperature, pressure,
density, etc.) In forecast discussions, thermodynamics usually refers to the
distribution of temperature and moisture (both vertical and horizontal) as
related to the diagnosis of atmospheric instability.
Theta-e (or Equivalent Potential Temperature) -
The temperature a parcel of air would have if a) it was lifted until it became
saturated, b) all water vapor was condensed out, and c) it was returned
adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000
millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly
related to the amount of heat present in an air parcel. Thus, it is useful in
diagnosing atmospheric instability.
Theta-e Ridge - An axis of relatively
high values of theta-e. Severe weather and
excessive rainfall often occur near or just upstream from a theta-e ridge.
Tilt Sequence - Radar term indicating
that the radar antenna is scanning through a series of antenna elevations in
order to obtain a volume scan.
Tilted Storm or Tilted Updraft - A
thunderstorm or cloud tower which is not
purely vertical but instead exhibits a slanted or tilted character. It is a
sign of vertical wind shear, a favorable
condition for severe storm development.
*Tornado - A violently rotating column of air
in contact with the ground and extending from the base of a thunderstorm. A
condensation funnel does
not need to reach to the ground for a tornado to be present; a
debris cloud beneath a thunderstorm
is all that is needed to confirm the presence of a tornado, even in the total
absence of a condensation funnel.
Tornado Family - A series of
tornadoes produced by a single
supercell, resulting in damage path
segments along the same general line.
Total-Totals Index - A stability
index and severe weather forecast tool, equal to the temperature at 850 mb plus
the dew point at 850 mb, minus twice the
temperature at 500 mb. The total-totals index is the arithmetic sum of two
other indices: the Vertical Totals Index (temperature at 850 mb minus
temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus
temperature at 500 mb). As with all stability indices there are no magic
threshold values, but in general, values of less than 50 or greater than 55 are
considered weak and strong indicators, respectively, of potential severe storm
development.
Tower - (Short for
towering cumulus), a cloud
element showing appreciable upward vertical development.
Towering Cumulus - (Same as
congestus.) A large
cumulus cloud with great vertical
development, usually with a cauliflower-like appearance, but lacking the
characteristic anvil of a
Cb. (Often shortened to "towering
cu," and abbreviated TCU.)
Transverse Bands - Bands of clouds
oriented perpendicular to the flow in which they are embedded. They often are
seen best on satellite photographs. When observed at high levels (i.e., in
cirrus formations), they may indicate
severe or extreme turbulence. Transverse bands observed at low levels (called
transverse rolls or T rolls)
often indicate the presence of a temperature inversion (or cap) as well as
directional shear in the low- to
mid-level winds. These conditions often favor the development of strong to
severe thunderstorms.
Transverse Rolls - Elongated low-level
clouds, arranged in parallel bands and aligned parallel to the low-level winds
but perpendicular to the mid-level flow. Transverse rolls are one type of
transverse band, and often
indicate an environment favorable for the subsequent development of
supercells. Since they are aligned
parallel to the low-level inflow, they may point toward the region most likely
for later storm development.
T Rolls - [Slang], same as
transverse rolls.
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).
Tropopause - The upper boundary of the
troposphere, usually characterized by
an abrupt change in lapse rate from
positive (decreasing temperature with height) to neutral or negative
(temperature constant or increasing with height). See
Fig. 6,
sounding.
Troposphere - The layer of the atmosphere
from the earth's surface up to the tropopause, characterized by decreasing
temperature with height (except, perhaps, in thin layers - see
inversion, cap), vertical wind motion, appreciable water
vapor content, and sensible weather (clouds, rain, etc.).
Trough - An elongated area of relatively low
atmospheric pressure, usually not associated with a closed circulation, and
thus used to distinguish from a closed
low. The opposite of ridge.
Turkey Tower - [Slang], a narrow,
individual cloud tower that develops and
falls apart rapidly. The sudden development of turkey towers from small
cumulus clouds may signify the breaking of
a cap.
TVS - Tornadic Vortex
Signature. Doppler radar
signature in the radial velocity
field indicating intense, concentrated rotation - more so than a
mesocyclone. Like the mesocyclone,
specific criteria involving strength, vertical depth, and time continuity must
be met in order for a signature to become a TVS. Existence of a TVS strongly
increases the probability of tornado
occurrence, but does not guarantee it. A TVS is not a visually
observable feature.
UKMET - A medium-range numerical weather
prediction model operated by the United Kingdom
METeorological Agency.
Updraft - A small-scale current of rising air.
If the air is sufficiently moist, then the moisture condenses to become a
cumulus cloud or an individual
tower of a
towering cumulus or
Cb.
Updraft Base - Alternate term for a
rain-free base.
Upper Level System - A general term
for any large-scale or
mesoscale disturbance capable of
producing upward motion (lift) in the middle or upper parts of the atmosphere.
This term sometimes is used interchangeably with impulse or
shortwave.
Upslope Flow - Air that flows toward
higher terrain, and hence is forced to rise. The added lift often results in
widespread low cloudiness and stratiform precipitation if the air is
stable, or an increased chance of thunderstorm development if the air is
unstable.
Upstream - Toward the source of the flow, or
located in the area from which the flow is coming.
UVM (or UVV) - Upward Vertical
Motion (or Velocity).
VAD - Velocity Azimuth
Display. A radar display on which mean
radial velocity is plotted as a
function of azimuth. See VWP.
Vault - Same as BWER.
Veering Winds - Winds which shift in a
clockwise direction with time at a given location (e.g., from southerly
to westerly), or which change direction in a clockwise sense with height (e.g.,
southeasterly at the surface turning to southwesterly aloft). The latter
example is a form of directional
shear which is important for tornado
formation. Compare with backing
winds.
Vertically-stacked System - A
low-pressure system, usually a closed
low or cutoff low, which is not
tilted with height, i.e., located similarly at all levels of the atmosphere.
Such systems typically are weakening and are slow-moving, and are less likely
to produce severe weather than tilted systems. However,
cold pools aloft associated with
vertically-stacked systems may enhance instability enough to produce severe
weather.
VIL - Vertically-Integrated
Liquid water. A property computed by RADAP II and
WSR-88D units that takes into account the
three-dimensional reflectivity of an
echo. The maximum VIL of a storm is useful in determining its potential
severity, especially in terms of maximum hail size.
VIP - Video Integrator and
Processor, which contours radar reflectivity (in
dBZ) into six VIP levels:
- VIP 1 (Level 1, 18-30 dBZ) - Light precipitation
- VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
- VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
- VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
- VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
- VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail
possible.
*Virga - Streaks or wisps of precipitation
falling from a cloud but evaporating before reaching the ground. In certain
cases, shafts of virga may precede a microburst; see
dry microburst.
V Notch - A radar
reflectivity signature seen as a
V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often
is seen on supercells, and is thought to
be a sign of diverging flow around the main storm
updraft (and hence a very strong updraft).
This term should not be confused with inflow notch or with
enhanced V, although the latter is
believed to form by a similar process. See
Fig. 7,
supercell.
Volume Scan - A radar scanning strategy in
which sweeps are made at successive antenna elevations (i.e., a
tilt sequence), and then combined to
obtain the three-dimensional structure of the echoes. Volume scans are
necessary to determine thunderstorm type, and to detect features such as
WERs, BWERs,
and overhang.
Vorticity - A measure of the local rotation
in a fluid flow. In weather analysis and forecasting, it usually refers to the
vertical component of rotation (i.e., rotation about a vertical axis) and is
used most often in reference to synoptic scale or
mesoscale weather systems. By
convention, positive values indicate
cyclonic rotation.
Vort Max - (Slang; short for vorticity
maximum), a center, or maximum, in the
vorticity field of a fluid.
VWP - VAD Wind Profile. A
radar plot of horizontal winds, derived from VAD data, as a function of height above a
Doppler Radar. The display is
plotted with height as the vertical axis and time as the horizontal axis (a
so-called time-height display), which then depicts the change in wind with time
at various heights. This display is useful for observing local changes in
vertical wind shear, such as backing of
low-level winds, increases in speed
shear, and development or evolution of nearby
jet streams (including
low-level jets).
This product often is referred to erroneously as a
VAD.
*Wall Cloud - A localized, persistent, often
abrupt lowering from a rain-free
base. Wall clouds can range from a fraction of a mile up to nearly five
miles in diameter, and normally are found on the south or southwest (inflow)
side of the thunderstorm. When seen from within several miles, many wall clouds
exhibit rapid upward motion and cyclonic rotation. However, not
all wall clouds rotate. Rotating wall clouds usually develop before strong or
violent tornadoes, by anywhere from a few
minutes up to nearly an hour. Wall clouds should be monitored visually for
signs of persistent, sustained rotation and/or rapid vertical motion.
See Fig. 7,
supercell.
"Wall cloud" also is used occasionally in tropical meteorology to
describe the inner cloud wall surrounding the eye of a tropical cyclone, but
the proper term for this feature is eyewall.
Warm Advection - Transport of warm air
into an area by horizontal winds.
Low-level warm advection sometimes is referred to (erroneously) as
overrunning. Although the two terms
are not properly interchangeable, both imply the presence of lifting in low
levels.
Warning - A product issued by
NWS local offices indicating that a particular
weather hazard is either imminent or has been reported. A warning indicates the
need to take action to protect life and property. The type of hazard is
reflected in the type of warning (e.g., tornado warning, blizzard warning). See
short-fuse warning.
Watch - An NWS
product indicating that a particular hazard is possible, i.e., that conditions
are more favorable than usual for its occurrence. A watch is a recommendation
for planning, preparation, and increased awareness (i.e., to be alert for
changing weather, listen for further information, and think about what to do if
the danger materializes).
Watch Box (or Box) - [Slang], a severe thunderstorm or tornado
watch.
Waterspout - In general, a
tornado occurring over water.
Specifically, it normally refers to a small, relatively weak rotating column of
air over water beneath a Cb or
towering cumulus cloud.
Waterspouts are most common over tropical or subtropical waters.
The exact definition of waterspout is debatable. In most cases the term is
reserved for small vortices over water that are not associated with
storm-scale rotation (i.e., they are
the water-based equivalent of landspouts). But there is sufficient
justification for calling virtually any rotating column of air a waterspout if
it is in contact with a water surface.
Wedge (or Wedge Tornado) - [Slang], a large
tornado with a
condensation funnel that is at
least as wide (horizontally) at the ground as it is tall (vertically) from the
ground to cloud base.
The term "wedge" often is used somewhat loosely to describe any
large tornado. However, not every large tornado is a wedge. A true wedge
tornado, with a funnel at least as
wide at the ground as it is tall, is very rare.
Wedges often appear with violent tornadoes (F4 or F5 on the
Fujita Scale), but many documented
wedges have been rated lower. And some violent tornadoes may not appear as
wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only
as a series of suction vortices
without a central condensation funnel). Whether or not a tornado achieves
"wedge" status depends on several factors other than intensity - in
particular, the height of the environmental cloud base and the availability of
moisture below cloud base. Therefore, spotters should not estimate wind
speeds or F-scale ratings based on visual appearance alone. However, it
generally is safe to assume that most (if not all) wedges have the potential to
produce strong (F2/F3) or violent (F4/F5) damage.
WER - Weak Echo Region. Radar
term for a region of relatively weak (reflectivity at low levels on the inflow
side of a thunderstorm echo, topped by stronger reflectivity in the form of an
echo overhang directly above it (see
Fig. 2). The
WER is a sign of a strong updraft on the
inflow side of a storm, within which precipitation is held aloft. When the area
of low reflectivity extends upward into, and is surrounded by, the higher
reflectivity aloft, it becomes a BWER.
Wet Microburst - A
microburst accompanied by heavy
precipitation at the surface. A rain
foot may be a visible sign of a wet microburst. See
dry microburst.
Wind Shear - See
shear.
Wrapping Gust Front - A
gust front which wraps around a
mesocyclone, cutting off the inflow of
warm moist air to the mesocyclone circulation and resulting in an
occluded mesocyclone.
WSR-57, WSR-74 - NWS Weather Surveillance
Radar units, replaced by WSR-88D
units.
WSR-88D - Weather Surveillance
Radar - 1988 Doppler; NEXRAD unit.
Zonal Flow -
Large-scale atmospheric flow in
which the east-west component (i.e., latitudinal) is dominant. The accompanying
meridional (north-south) component often is weaker than normal. Compare with
meridional flow.
Bibliography
- American Meteorology Society, 1990: Glossary of Meteorology.
American Meteorological Society Press, Boston.
- Caracena, Fernando, Ronald L. Holle, and Charles A. Doswell III, 1989:
Microbursts - A Handbook for Visual Identification. NOAA, Environmental
Research Laboratories, National Severe Storms Laboratory.
- Doswell, Charles A. III, 1982: The Operational Meteorology of Convective
Weather. Volume I: Operational Mesoanalysis. NOAA Technical Memorandum NWS
NSSFC-5.
- Doswell, Charles A. III, 1985: The Operational Meteorology of Convective
Weather. Volume II: Storm Scale Analysis. NOAA Technical Memorandum ERL
ESG-15.
- Fujita, T. T., 1985: The Downburst - Microburst and Macroburst. SMRP
Research Paper No. 210, University of Chicago, 122 pp.
- Marshall, Tim, 19--: Storm Chase Manual. Published annually in
association with Storm Track. Contact: 1336 Brazos Blvd, Lewisville TX
75067.
- Marshall, Tim (Editor): Storm Track. Published bi-monthly by
Master Graphics, Lewisville TX. Contact the editor, 1336 Brazos Blvd,
Lewisville TX 75067, for subscription information.
- National Weather Service, 1982: Spotter's Guide for Identifying and
Reporting Severe Local Storms. Available from most National Weather Service
offices, or from the U. S. Dept. of Commerce, NOAA, Rockville MD 20852.
Figure 1, Bow
Echo
Figure 2, Weak
Echo Region, Bounded Weak Echo Region
Figure 3,
High-Precipitation Storm
Figure 4, Line
Echo Wave Pattern
Figure 5,
Low-Precipitation Storm
Figure 6,
Sounding
Figure 7,
Supercell
|
