*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 Centre (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 favourable, the upward motion can
contribute to thunderstorm development ahead of a shortwave.
Slight Risk (of 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 Centre. A national forecast
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., south-westerly 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
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
Staccato Lightning - A CG lightning discharge which appears
as a single very bright, short-duration stroke, often with
Steering Winds (or Steering Currents)
- A prevailing synoptic
scale flow which governs the movement of smaller features embedded
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
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
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
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
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.