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Impulse - See
upper level system.
Inflow Bands (or
Feeder Bands) - Bands of low clouds,
arranged parallel to the low-level winds and moving into or toward a
thunderstorm. They may indicate the strength of the inflow of moist air into
the storm, and, hence, its potential severity. Spotters should be especially
wary of inflow bands that are curved in a manner suggesting
cyclonic rotation; this
pattern may indicate the presence of a mesocyclone.
Inflow Jets - Local jets of air near the
ground flowing inward toward the base of a tornado.
Inflow Notch - A radar signature
characterized by an indentation in the reflectivity pattern on the inflow side
of the storm. The indentation often is V-shaped, but this term should not be
confused with V-notch.
Supercell thunderstorms often exhibit
inflow notches, usually in the right quadrant of a
classic supercell, but sometimes in the
eastern part of an HP storm or in the
rear part of a storm (rear inflow notch).
Inflow Stinger - A
beaver tail cloud with a stinger-like
shape.
Insolation - Incoming solar radiation. Solar
heating; sunshine.
Instability - The tendency for air parcels
to accelerate when they are displaced from their original position; especially,
the tendency to accelerate upward after being lifted. Instability is a
prerequisite for severe weather - the greater the instability, the greater the
potential for severe thunderstorms. See lifted index, and Fig. 6, sounding.
Inversion - Generally, a departure from the
usual increase or decrease in an atmospheric property with altitude.
Specifically it almost always refers to a temperature inversion, i.e., an
increase in temperature with height, or to the layer within which such an
increase occurs. An inversion is present in the lower part of a
cap. See Fig. 6,
sounding.
Isentropic Lift - Lifting of air that
is traveling along an upward-sloping
isentropic surface.
Isentropic lift often is referred to erroneously as
overrunning, but more accurately
describes the physical process by which the lifting occurs. Situations
involving isentropic lift often are characterized by widespread
stratiform clouds and precipitation,
but may include elevated
convection in the form of embedded thunderstorms.
Isentropic Surface - A
two-dimensional surface containing points of equal
potential temperature.
Isobar - A line connecting points of equal
pressure.
Isodrosotherm - A line connecting points
of equal dew point temperature.
Isohyet - A line connecting points of equal
precipitation amounts.
Isopleth - General term for a line connecting
points of equal value of some quantity. Isobars, isotherms, etc. all are examples of
isopleths.
Isotach - A line connecting points of equal
wind speed.
Isotherm - A line connecting points of equal
temperature.
Jet Max (or Speed Max,
Jet Streak) - a point or area of
relative maximum wind speeds within a jet
stream.
Jet Streak - A local wind speed maximum
within a jet stream.
Jet Stream - Relatively strong winds
concentrated in a narrow stream in the atmosphere, normally referring to
horizontal, high-altitude winds. The position and orientation of jet streams
vary from day to day. General weather patterns (hot/cold, wet/dry) are related
closely to the position, strength and orientation of the jet stream (or jet
streams). A jet stream at low levels is known as a
low-level jet.
Knuckles - [Slang], lumpy protrusions on the
edges, and sometimes the underside, of a thunderstorm
anvil. They usually appear on the upwind side
of a back-sheared anvil, and
indicate rapid expansion of the anvil due to the presence of a very strong
updraft. They are not
mammatus clouds. See also
cumuliform anvil,
anvil rollover.
Laminar - Smooth, non-turbulent. Often used to
describe cloud formations which appear to be shaped by a smooth flow of air
traveling in parallel layers or sheets.
Landspout - [Slang], a
tornado that does not arise from organized
storm-scale rotation and therefore is
not associated with a wall cloud
(visually) or a mesocyclone (on
radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no
more than a dust whirl), and
essentially are the land-based equivalents of
waterspouts.
Lapse Rate - The rate of change of an
atmospheric variable, usually temperature, with height. A steep lapse rate
implies a rapid decrease in temperature with height (a sign of
instability) and a steepening lapse
rate implies that destabilization is occurring. See
Fig. 6,
sounding.
Large-scale - See
synoptic-scale.
Left Front Quadrant (or Left Exit
Region) - The area downstream from and
to the left of an upper-level jet max (as
would be viewed looking along the direction of flow). Upward motion and
severe thunderstorm potential
sometimes are increased in this area relative to the wind speed maximum. See
also entrance region,
right rear quadrant.
Left Mover - A thunderstorm which moves to
the left relative to the steering
winds, and to other nearby thunderstorms; often the northern part of a
splitting storm. See also
right mover.
LEWP - Line Echo Wave Pattern. A bulge in a
thunderstorm line producing a wave-shaped "kink" in the line (Fig.
4). The potential for strong outflow and damaging
straight-line winds increases
near the bulge, which often resembles a bow
echo. Severe weather potential also is increased with storms near the crest
of a LEWP.
Lifted Index (or LI) - A common measure of
atmospheric instability. Its value is
obtained by computing the temperature that air near the ground would have if it
were lifted to some higher level (around 18,000 feet, usually) and comparing
that temperature to the actual temperature at that level. Negative values
indicate instability - the more negative, the more unstable the air is, and the
stronger the updrafts are likely to be
with any developing thunderstorms. However there are no "magic
numbers" or threshold LI values below which severe weather becomes
imminent. See Fig. 6, sounding.
Loaded Gun (Sounding) - [Slang], a
sounding characterized by extreme
instability but containing a
cap, such that explosive thunderstorm
development can be expected if the cap can be
weakened or the air below it heated sufficiently to overcome it. See
Fig. 6, sounding.
Longwave Trough - A
trough in the prevailing westerly flow
aloft which is characterized by large length and (usually) long duration.
Generally, there are no more than about five longwave troughs around the
Northern Hemisphere at any given time. Their position and intensity govern
general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks,
or months. Smaller disturbances (e.g., shortwave troughs) typically move more
rapidly through the broader flow of a longwave trough, producing weather
changes over shorter time periods (a day or less).
Low-level Jet (abbrev. LLJ) - A region of
relatively strong winds in the lower part of the atmosphere. Specifically, it
often refers to a southerly wind maximum in the
boundary layer, common over the
Plains states at night during the warm season (spring and summer).
The term also may be used to describe a narrow zone of strong winds
above the boundary layer, but in this sense the more proper term would
be low-level jet stream.
LP Storm (or LP Supercell) -
Low-Precipitation storm (or Low-Precipitation
supercell). A supercell thunderstorm
characterized by a relative lack of visible precipitation. Visually similar to
a classic supercell, except without the heavy precipitation core (Fig. 5). LP storms often exhibit a striking visual
appearance; the main tower often is
bell-shaped, with a corkscrew appearance suggesting rotation. They are capable
of producing tornadoes and very large
hail. Radar identification often is difficult relative to other types of
supercells, so visual reports are very important. LP storms almost always occur
on or near the dry line, and thus are
sometimes referred to as dry line
storms.
LSR - Local Storm Report. A
product issued by local NWS offices to inform
users of reports of severe and/or significant weather-related events.
Mammatus Clouds - Rounded, smooth,
sack-like protrusions hanging from the underside of a cloud (usually a
thunderstorm anvil). Mammatus clouds often
accompany severe
thunderstorms, but do not produce severe weather; they may accompany
non-severe storms as well. See Figs. 3 (HP storm),
5 (LP storm), and 7
(supercell).
MCC - Mesoscale Convective
Complex. A large MCS, generally round
or oval-shaped, which normally reaches peak intensity at night. The formal
definition includes specific minimum criteria for size, duration, and
eccentricity (i.e., "roundness"), based on the cloud shield as seen
on infrared satellite photographs:
- Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or
more (slightly smaller than the state of Ohio), and area of cloud top -52
degrees C or less: 50,000 square kilometers or more.
- Duration: Size criteria must be met for at least 6 hours.
- Eccentricity: Minor/major axis at least 0.7.
MCCs typically form during the afternoon and evening in the form of several
isolated thunderstorms, during which time the potential for severe weather is
greatest. During peak intensity, the primary threat shifts toward heavy rain
and flooding.
MCS - Mesoscale Convective
System. A complex of thunderstorms which becomes organized on a scale
larger than the individual thunderstorms, and normally persists for several
hours or more. MCSs may be round or linear in shape, and include systems such
as tropical cyclones, squall lines,
and MCCs (among others). MCS often is used to
describe a cluster of thunderstorms that does not satisfy the size, shape, or
duration criteria of an MCC.
Medium Range - In forecasting, (generally)
three to seven days in advance.
Meridional Flow -
Large-scale atmospheric flow in
which the north-south component (i.e., longitudinal, or along a meridian) is
pronounced. The accompanying zonal
(east-west) component often is weaker than normal. Compare with
zonal flow.
*Mesocyclone - A
storm-scale region of rotation,
typically around 2-6 miles in diameter and often found in the right rear flank
of a supercell (or often on the eastern,
or front, flank of an HP storm). The
circulation of a mesocyclone covers an area much larger than the
tornado that may develop within it.
Properly used, mesocyclone is a radar term; it is defined as a rotation
signature appearing on Doppler radar
that meets specific criteria for magnitude, vertical depth, and duration.
Therefore, a mesocyclone should not be considered a visually-observable
phenomenon (although visual evidence of rotation, such as curved
inflow bands, may imply the presence
of a mesocyclone).
Mesohigh - A
mesoscale high pressure area, usually
associated with MCSs or their remnants.
Mesolow (or
Sub-synoptic Low) - A
mesoscale low-pressure center. Severe
weather potential often increases in the area near and just ahead of a mesolow.
Mesolow should not be confused with mesocyclone, which is a
storm-scale phenomenon.
Mesonet - A regional network of observing
stations (usually surface stations) designed to diagnose
mesoscale weather features and their
associated processes.
Mesoscale - Size scale referring to weather
systems smaller than Squall lines, MCCs,
and MCSs are examples of mesoscale weather
systems.
*Microburst - A small, concentrated
downburst affecting an area less than 4
kilometers (about 2.5 miles) across. Most microbursts are rather short-lived (5
minutes or so), but on rare occasions they have been known to last up to 6
times that long.
Mid-level Cooling - Local cooling of
the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet),
which can lead to destabilization of the entire atmosphere if all other factors
are equal. Mid-level cooling can occur, for example, with the approach of a
mid-level cold pool.
Moderate Risk (of
severe thunderstorms) - Severe
thunderstorms are expected to affect between 5 and 10 percent of the area. A
moderate risk indicates the possibility of a significant severe weather
episode. See high risk,
slight risk,
convective outlook.
Moisture Advection - Transport of
moisture by horizontal winds.
Moisture Convergence - A measure
of the degree to which moist air is converging into a given area, taking into
account the effect of converging winds and
moisture advection. Areas of
persistent moisture convergence are favored regions for thunderstorm
development, if other factors (e.g., instability) are favorable.
Morning Glory - An elongated cloud band,
visually similar to a roll cloud,
usually appearing in the morning hours, when the atmosphere is relatively
stable. Morning glories result from perturbations related to gravitational
waves in a stable boundary layer.
They are similar to ripples on a water surface; several parallel morning
glories often can be seen propagating in the same direction.
MRF - Medium-Range Forecast model; one of the
operational forecast models run at NCEP. The
MRF is run once daily, with forecast output out to 240 hours (10 days).
Multi-cell(ular) Thunderstorm -
A thunderstorm consisting of two or more cells, of which most or all are often visible at
a given time as distinct domes or towers in
various stages of development.
Nearly all thunderstorms (including supercells) are multi-cellular, but the
term often is used to describe a storm which does not fit the definition of a
supercell.
*Multiple-vortex (or Multi-vortex)
Tornado - a tornado in which two or
more condensation funnels or
debris clouds are present at the same
time, often rotating about a common center or about each other. Multiple-vortex
tornadoes can be especially damaging. See
suction vortex.
Mushroom - [Slang], a thunderstorm with a
well-defined anvil rollover, and
thus having a visual appearance resembling a mushroom.
NCEP - National Centers for
Environmental Prediction; the modernized version of
NMC.
Negative-tilt Trough - An
upper level system which is
tilted to the west with increasing latitude (i.e., with an axis from southeast
to northwest). A negative-tilt trough often is a sign of a developing or
intensifying system.
NEXRAD - NEXt-Generation Weather
RADar. Technologically-advanced weather radar being deployed to replace
WSR-57 and WSR-74 units. NEXRAD is a high-resolution
Doppler radar with increased
emphasis on automation, including use of algorithms and automated volume scans.
NEXRAD units are known as WSR-88D.
NGM - Nested Grid Model; one
of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast
output out to 48 hours.
NMC - National Meteorological
Center, with headquarters near Washington D.C.; now known as
NCEP.
NOAA - National Oceanographic and
Atmospheric Administration.
Nocturnal - Related to nighttime, or
occurring at night.
Nowcast - A short-term weather forecast,
generally out to six hours or less.
NSSFC - National Severe
Storms Forecast Center, in Kansas City MO; now known as
SPC.
NSSL - National Severe Storms
Laboratory, in Norman OK. (Sometimes pronounced NES-sel.)
NWP - Numerical Weather
Prediction.
NWS - National Weather
Service.
Occluded Mesocyclone - A
mesocyclone in which air from the
rear-flank downdraft has
completely enveloped the circulation at low levels, cutting off the inflow of
warm unstable low-level air.
Orographic - Related to, or caused by,
physical geography (such as mountains or sloping terrain).
Orographic Lift - Lifting of air caused
by its passage up and over mountains or other sloping terrain.
Orphan Anvil - [Slang], an
anvil from a dissipated thunderstorm, below
which no other clouds remain.
Outflow Boundary - A
storm-scale or
mesoscale boundary separating
thunderstorm-cooled air (outflow) from the surrounding air; similar in effect
to a cold front, with passage marked by a wind shift and usually a drop in
temperature. Outflow boundaries may persist for 24 hours or more after the
thunderstorms that generated them dissipate, and may travel hundreds of miles
from their area of origin. New thunderstorms often develop along outflow
boundaries, especially near the point of intersection with another boundary
(cold front,
dry line, another outflow boundary, etc.;
see triple point).
Overhang - Radar term indicating a region of
high reflectivity at middle and upper
levels above an area of weak reflectivity at low levels. (The latter area is
known as a weak-echo region, or WER.) The
overhang is found on the inflow side of a thunderstorm (normally the south or
southeast side). See Fig. 2, BWER.
Overrunning - A weather pattern in which a
relatively warm air mass is in motion above another air mass of greater density
at the surface. Embedded thunderstorms sometimes develop in such a pattern;
severe thunderstorms (mainly
with large hail) can occur, but tornadoes
are unlikely.
Overrunning often is applied to the case of warm air riding up over a
retreating layer of colder air, as along the sloping surface of a warm
front. Such use of the term technically is
incorrect, but in general it refers to a pattern characterized by widespread
clouds and steady precipitation on the cool side of a front or other boundary.
*Overshooting Top (or Penetrating Top)
- A dome-like protrusion above a thunderstorm anvil, representing a very strong
updraft and hence a higher potential for
severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a
supercell. A short-lived overshooting
top, or one that forms and dissipates in cycles, may indicate the presence of a
pulse storm or a
cyclic storm. See
Figs. 3 (HP storm), 5 (LP
storm), and 7 (supercell).
PDS Watch - [Slang], a tornado
watch with
enhanced wording
(Particularly Dangerous Situation).
Pendant Echo - Radar signature generally
similar to a hook echo, except that the hook
shape is not as well defined.
Penetrating Top - Same as
overshooting top.
Popcorn Convection - [Slang],
Showers and thunderstorms that form on a scattered basis with little or no
apparent organization, usually during the afternoon in response to
diurnal heating. Individual thunderstorms
typically are of the type sometimes referred to as
air-mass thunderstorms: they
are small, short-lived, very rarely severe, and they almost always dissipate
near or just after sunset.
Positive Area - The area on a
sounding representing the layer in which
a lifted parcel would be warmer than the environment; thus, the area between
the environmental temperature profile and the path of the lifted parcel. See
Fig. 6, sounding. Positive area is a measure of the
energy available for convection; see
CAPE.
Positive CG - A CG flash that delivers positive charge to the
ground, as opposed to the more common negative charge. Positive CGs have been
found to occur more frequently in some severe thunderstorms. Their occurrence
is detectable by most lightning detection networks, but visually it is not
considered possible to distinguish between a positive CG and a negative CG.
(Some claim to have observed a relationship between
staccato lightning and positive
CGs, but this relationship is as yet unproven.)
Positive-tilt Trough - An
upper level system which is
tilted to the east with increasing latitude (i.e., from southwest to
northeast). A positive-tilt trough often is a sign of a weakening weather
system, and generally is less likely to result in severe weather than a
negative-tilt trough if all
other factors are equal.
Potential Temperature - The
temperature a parcel of dry air would have if brought adiabatically (i.e.,
without transfer of heat or mass) to a standard pressure level of 1000 mb.
PPINE - Plan Position Indicates No Echoes,
referring to the fact that a radar detects no precipitation within its range.
Profiler - An instrument designed to measure
horizontal winds directly above its location, and thus measure the vertical
wind profile. Profilers operate on the same principles as
Doppler radar.
Pseudo-Cold Front - A boundary
between a supercell's inflow region and
the rear-flank downdraft (or
RFD). It extends outward from the mesocyclone center, usually toward the
south or southwest (but occasionally bows outward to the east or southeast in
the case of an occluded
mesocyclone), and is characterized by advancing of the
downdraft air toward the inflow region.
It is a particular form of gust front.
See also pseudo-warm front.
Pseudo-Warm Front - A boundary
between a supercell's inflow region and
the forward-flank
downdraft (or FFD). It extends outward from at or near the
mesocyclone center, usually toward the
east or southeast, and normally is either nearly stationary or moves northward
or northeastward ahead of the mesocyclone. See
pseudo-cold front and
beaver tail.
Pulse Storm - A thunderstorm within which a
brief period (pulse) of strong updraft
occurs, during and immediately after which the storm produces a short episode
of severe weather. These storms generally are not
tornado producers, but often produce large
hail and/or damaging winds. See overshooting top,
cyclic storm.
PVA - Positive Vorticity
Advection. Advection of higher
values of vorticity into an area, which
often is associated with upward motion (lifting) of the air. PVA typically is
found in advance of disturbances aloft (i.e.,
shortwaves), and is a property which
often enhances the potential for thunderstorm development.
RADAP II - RAdar DAta
Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls
the tilt sequence and computes
several radar-derived quantities at regular intervals, including
VIL, storm tops, accumulated rainfall, etc.
Radial Velocity - Component of motion
toward or away from a given location. As "seen" by
Doppler radar, it is the component
of motion parallel to the radar beam. (The component of motion
perpendicular to the beam cannot be seen by the radar. Therefore, strong
winds blowing strictly from left to right or from right to left, relative to
the radar, can not be detected.)
Rain Foot - [Slang], a horizontal bulging
near the surface in a precipitation shaft, forming a foot-shaped prominence. It
is a visual indication of a wet
microburst.
*Rain-free Base - A dark, horizontal
cloud base with no visible precipitation beneath it. It typically marks the
location of the thunderstorm updraft.
Tornadoes may develop from
wall clouds attached to the rain-free
base, or from the rain-free base itself - especially when the rain-free base is
on the south or southwest side of the main precipitation area.
Note that the rain-free base may not actually be rain free; hail or large
rain drops may be falling. For this reason,
updraft base is more accurate. See
Figs. 3 (HP storm), 5 (LP
storm), and 7 (supercell).
Rear Flank Downdraft (or RFD) - A
region of dry air subsiding on the back side of, and wrapping around, a
mesocyclone. It often is visible as a
clear slot wrapping around the wall
cloud. Scattered large precipitation particles (rain and hail) at the interface
between the clear slot and wall cloud
may show up on radar as a hook or
pendant; thus the presence of a hook
or pendant may indicate the presence of an RFD. See Fig.
7, supercell.
Red Watch or Red Box - [Slang], a tornado
watch.
Reflectivity - Radar term referring to the
ability of a radar target to return energy; used to derive echo intensity, and
to estimate precipitation intensity and rainfall rates. See
dBZ, VIP.
Relative Humidity - A dimensionless
ratio, expressed in percent, of the amount of atmospheric moisture present
relative to the amount that would be present if the air were saturated. Since
the latter amount is dependent on temperature, relative humidity is a function
of both moisture content and temperature. As such, relative humidity by itself
does not directly indicate the actual amount of atmospheric moisture present.
See dew point.
Retrogression (or Retrograde Motion) -
Movement of a weather system in a direction opposite to that of the basic flow
in which it is embedded, usually referring to a
closed low or a
longwave trough which moves
westward.
Return Flow - South winds on the back
(west) side of an eastward-moving surface high pressure system. Return flow
over the central and eastern United States typically results in a return of
moist air from the Gulf of Mexico (or the Atlantic Ocean).
Right Entrance Region (or
Right Rear Quadrant) - The
area upstream from and to the right of an
upper-level jet max (as would be viewed
looking along the direction of flow). Upward motion and
severe thunderstorm potential
sometimes are increased in this area relative to the wind speed maximum. See
also exit region,
left front quadrant.
Ridge - An elongated area of relatively high
atmospheric pressure; the opposite of trough.
*Right Mover - A thunderstorm that moves
appreciably to the right relative to the main steering winds and to other
nearby thunderstorms. Right movers typically are associated with a high
potential for severe weather. (Supercells often are right movers.) See
left mover,
splitting storm.
Right Rear Quadrant - see
Right Entrance Region.
*Roll Cloud - A low, horizontal tube-shaped
arcus cloud associated with a thunderstorm
gust front (or sometimes with a cold
front). Roll clouds are relatively rare;
they are completely detached from the thunderstorm base or other cloud
features, thus differentiating them from the more familiar
shelf clouds. Roll clouds usually
appear to be "rolling" about a horizontal axis, but should not be
confused with funnel clouds.
*Rope (or Rope Funnel) - A narrow, often contorted
condensation funnel usually
associated with the decaying stage of a tornado. See
rope stage.
Rope Cloud - In satellite meteorology, a
narrow, rope-like band of clouds sometimes seen on satellite images along a
front or other boundary.
The term sometimes is used synonymously with rope or rope funnel.
Rope Stage - The dissipating stage of a
tornado, characterized by thinning and
shrinking of the condensation
funnel into a rope (or
rope funnel). Damage still is possible during
this stage.
RUC - Rapid Update Cycle, a
numerical model run at NCEP that focuses on
short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are
prepared every 3 hours for the contiguous United States.
Glossary (A-B)
Glossary (C-H)
Glossary (S-Z)
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