Meterology

Blizzard Warning

A bulletin issued by NWS offices when the following conditions are imminent or expected to develop: (1) sustained winds or frequent wind gusts of 35 mph or more with, (2) considerable falling and/or blowing snow that reduces visibility to 1/4 mile or less. Cold temperatures, usually less than 20oF, may also be present but not always. See Blowing Snow Advisory. See also, Warning.

Advisory

A bulletin issued by NWS offices which alerts the public to a non-severe, yet significant weather event which is expected to remain below the warning criteria, but may still cause significant inconvenience. These events can produce life and property damage if caution is not exercised. See Air Stagnation or Pollution Advisory, Blowing Snow Advisory, Dense Fog Advisory, Freezing Rain or Drizzle Advisory, Heat Advisory, Lake Snow Advisory, Snow Advisory, Urban and Small Stream Flood Advisory, Wind Advisory, Wind Chill Advisory, Winter Weather Advisory.

Blowing Snow Advisory

A bulletin issued by the NWS when snow and high wind conditions are expected to cause significant inconvenience to travel by reducing surface visibility, possibly to 1/4 statute mile or less, either continuously or intermittently during the period of the alert. Blowing snow may be falling snow or snow that has already accumulated but is picked up and blown by strong winds. See Advisory.

Cutoff Low

A closed low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).

Accessory Cloud

A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.

Algorithm

A computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.

Convergence

A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development (when other factors, such as instability, are favorable). Compare with confluence.

Convective Outlook (sometimes called AC)

A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC. The terms approaching, slight risk, moderate risk, and high risk are used to describe severe thunderstorm potential. Local versions sometimes are prepared by local NWS offices.

Cooling Degree Day

A form of degree day used to estimate the requuired energy for cooling. One cooling degree day occurs for each degree the daily mean temperature is above 65oF. It is defined as (MT - 65oF) = CDD, where MT is the mean temperature oF. If MT is less than 65oF, then CDD = 0, and the difference would be heating degree days.

Condensation Funnel

A funnell-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.). Compare with debris cloud.

Collar Cloud

A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. See Fig. 7, supercell.

Anvil Dome

A large overshooting top or penetrating top.

Cap (or Capping Inversion)

A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur. See CIN and Fig 6, sounding.

Clear Slot

A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as abright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

Closed Low

A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see cutoff low).

Arcus

A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.

Bubble High

A mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air.

Bulk Richardson Number (or BRN)

A non-dimensional number relating vertical stability (CAPE) and vertical shear (generally, stability divided by shear). High values indicate unstable (high CAPE) and/or weakly-sheared environments; low values indicate weak instability (low CAPE) and/or strong vertical shear. Values less than 10 indicate the shear may be too strong given the weak buoyancy to develop sustained convective updrafts. Given strong enough forcing, rotating supercells could evolve. Values of 10 to 45 are considered the "sweet spot" and are associated with supercell development. Generally, values in the range of around 50 to 100 suggest pulse/multicellular storm development is most likely.

Confluence

A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

Bow Echo

A radar echo which is linear but bent outward in a bow shape (Fig. 1). Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.

Baroclinic Zone

A region in which a temperature gradient exists on a constant pressure surface and temperature advection occurs. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.

Cold Pool

A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

Comma Cloud

A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.

Cumuliform Anvil

A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus. A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, mushroom. See Fig. 9, cumulonimbus with anvil.

Comma Echo

A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo. See Fig. 1.

Approaching (severe levels)

A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter is approaching severe conditions but has not achieved severe conditions. See severe thunderstorm.

Coulomb

A unit of electrical charge which is equal to 1 ampere-second.

Ampere

A unit of electrical current; the precise definition being "that current flowing in each of two long parallel conductors one meter apart, which results in a force of exacty 2 x 10-7 newton per meter of length of each conductor."

Barotropic System

A weather system in which temperature and pressure surfaces are coincident. Temperature advection does not occur. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.

AVN

AViatioN model; one of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.

Andes Lightning

Also called Andes lights or Andes glow. A series of apparently quiet electrical discharges observed generally over all the mountainous regions of the world when under disturbed electrical conditions. It is often intense enough to be observed several hundred miles away. This phenomenon is essentially a corona discharge. The quietness of Andes lightning contrasts with the typical cracking noise of corona discharge probably because it is seldom heard from points sufficiently close to permit audibility.

Corona Discharge

Also called brush discharge or St. Elmo's fire, or corposant. A luminous, and often audible, electric discharge that is intermediate in nature between a spark discharge (with usually, its single discharge channel) and a point discharge (with its diffuse, quiescent, and non-luminous character). It occurs from objects, especially pointed ones, when the electric field strength near their surfaces attains a value near 1000 volts per cm. Aircraft flying through active electrical storms often develop corona discharge streamers from antennas and propellers, and even from the entire fuselage and wing structure. So-called precipitation static results. It is seen also during stormy weather, emanating from the yards and masts of ships at sea.

Black Lightning

Also called dark lightning. This is simply a photographic effect in which lightning discharges register as dark instead of light due to multiple exposures caused by successive members of a composite flash. This effect, which results from a characteristic of the exposure vs. illumination curve of a photographic emulsion, is sometimes called the Clayden effect after its English discoverer.

Ball Lightning

Also called globe lightning. A relatively rare form of lightning consisting of a reddish, luminous ball of the order of one foot in diameter, which may move rapidly along solid objects or remain floating in mid-air. Hissing noises emanate from such balls and they sometimes explode noisily, but may also disappear noiselessly.

Cloud Discharge

Also called intracloud discharge also called cloud flash. A lightning discharge occurring between a positive charge center and a negative charge center, both of which lie in the same cloud. The most frequent type of cloud discharge is one between a main upper positive charge and a lower main negative charge; but occasionally discharges occur between the lower negative charge center of a thundercloud and a secondary positive center lying slightly below the level of the main negative center. Cloud discharges consist only of stepped leaders and rare dart leaders, but never return streamers because of the relatively diffuse charge centers involved. In weather observing, this type of discharge is often mistaken for a cloud-to-cloud discharge , but the latter term should be restricted to true intercloud discharges, which are far less common than intracloud discharges. Cloud discharges tend to outnumber cloud-to-ground discharges in semiarid regions where the bases of thunderclouds may be several kilometers above the earth's surface. In general, the channel of a cloud discharge will be wholly surrounded by cloud. Hence the channel's luminosity typically produces a diffuse glow when seen from outside the cloud, and this widespread glow is called sheet lightning.

Beaded Lightning

Also called pearl lightning. This is a particular aspect of a normal lightning flash occasionally seen when the observer happen to view end-on a number of segments of the irregular channel and hence receives an impression of higher intensity at a series of locations along the channel.

Air Stagnation or Pollution Advisory

An alert issued by the NWS for affected locations When winds are very light at the surface and aloft and an inversion keeps air from mixing vertically, air pollution may increase such that it becomes a potential health hazard.

AP

Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.

Blue Jet

Blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m3. Blue jets are not aligned with the local magnetic field.

BWER

Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft (Fig. 2). This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. See Weak Echo Region.

CA

Cloud-to-Air lightning.

CC

Cloud-to-Cloud lightning.

CG

Cloud-to-Ground lightning.

Cell

Convection in the form of a single updraft, downdraft,or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of severalcells (see multi-cellular thunderstorm).

CAPE

Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel over the layer within which the latter is warmer than the former. (This area often is called positive area.) See also CIN and Fig. 6 sounding.

CIN

Convective INhibition. A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.) See CAPE and Fig. 6, sounding.

AC

Convective outlook issued by the SPC. Abbreviation for Anticipated Convection; the term originates from the header coding [ACUS1] of the transmitted product. See SWODY1, SWODY2.

Cb

Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth,flat, often fibrous anvil. Also known colloquially as a "thunderhead." See Fig. 9 Cumulonimbus.

Cumulus

Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus. See Fig. 10, cumulus clouds.

Convection

Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used spicifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection"; and "thunderstorm" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

Cirrus

High-level clouds (16,000 feet or more) composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms. See Fig. 8, cirrus clouds.

Boundary Layer

In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.

Chain Lightning

Lightning in a long ziazag or apparently broken line. Sometimes it is called beaded lightning.

Coastal Flooding

Prolonged strong onshore flow of wind and/or high astronomical tides causing a rise in sea level that floods coastal areas.

Cloud Tags

Ragged, detached cloud fragment fractus or scud.

Anticyclonic Rotation

Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.

Cloud Streets

Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.

Cumulus Congestus (or simply congestus)

Same as towering cumulus. See Fig. 11, cumulus congestus clouds.

BRN

See Bulk Richardson Number.

Apparent Temperature

See Heat Index.

Air Discharge

See cloud-to-air discharge.

Band Lightning

See ribbon lightning.

Classic Supercell

See supercell.

Chaff

Small strips of metal foil, usually dropped in large quantities from aircraft or balloons. Chaff typically produces a radar echo which closely resembles precipitation. Chaff drops once were conducted by the military in order to confuse enemy radar, but now are conducted mainly for radar testing and calibrarion purposes.

Convective Temperature

The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding. See Fig. 6.

Anvil

The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (see back-sheared anvil).

Cloud

To-Air Discharge - Also called air discharge. A form of lightning discharge, intermediate in nature between a cloud discharge and a cloud-to-ground discharge in which the multibranching lightning channel descending from a cloud base does not reach the ground but rather succeeds only in neutralizing the space charge distributed in the sub-cloud layer. Such discharges propagate almost entirely by the stepped-leader process, with a few dart leaders, but never exhibit the return streamer phenomenon peculiar to cloud-to-ground discharges for the reason that neither charge center involved in these cases is in a region of extremely high conductivity. Air discharges tend to be more frequent in semiarid regions where high cloud bases are common. They are also favored by surface conditions which produce heavy point discharge currents that build up a relatively large space charge in the sub-cloud layer.

Cloud

To-Cloud Discharge - A lightning discharge occurring between a positive charge center of one cloud and a negative charge center of a second cloud; an intercloud (not intracloud) discharge. In some parts of the literature this term is also applied to intracloud discharges, which are much more common than cloud-to-cloud discharges, but this usage is undesirable and the more common type should be called cloud discharges.

Advection

Transport of an atmospheric property by the wind. See cold advection, moisture advection, warm advection.

Cold Advection

Transport of cold air into a region by horizontal winds.

Backing Winds

Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.

Anvil Rollover

[Slang], a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, mushroom.

Anvil Crawler

[Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.

Beaver('s) Tail

[Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger.

Core Punch

[Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting.

Bear's Cage

[Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm.

Box (or Watch Box)

[Slang], a severe thunderstorm or tornado watch. See blue box, red box.

Blue Watch (or Blue Box)

[Slang], a severe thunderstorm watch.

Barber Pole

[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.

Bust

[Slang], an inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.

Anvil Zits

[Slang], frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

Cold

air Funnel - A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

Back

building Thunderstorm - A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.

ACCAS (usually pronounced ACK

kis) - AltoCumulus CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show Cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

Air

mass Thunderstorm - Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter. See popcorn convection.

The cap is an important ingrediant in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But, without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability

often before instability levels become large enough to support severe weather development.

Congestus (or Cumulus Congestus)

same as towering cumulus.

Since all thunderstorms are associated with some type of forcing mechanism, synoptic

scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.

Back

sheared Anvil - [Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential. (See Fig. 7, supercell.)

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems

systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.

Cloud

to-Ground Discharge - A lightning discharge occurring between a charge center (usually negative) in the cloud and a center of opposite charge at the ground. This type of discharge, which is to be contrasted with a cloud discharge or cloud-to-cloud discharge, consists not only of the stepped leader found in cloud discharges, but also involves the extremely intense return streamer which is its principal source of luminosity and charge transfer. A cloud-to-ground discharge is usually a composite flash, that is, composed of several distinct lightning strokes, all using essentially the same channel and following each other at intervals of a few hundredths of a second (See dart leader).

Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo

warm front.