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Weather 101

Glossary of Weather Terms, Abbreviations, and Acronyms

50/50 Low: The term "50-50 Low" refers to the development or appearance of a strong or intense Low-pressure area both at the surface and in upper levels of the atmosphere -- usually at 500 MB -- over or near the positions of 50 degrees North Latitude and 50 degrees West Longitude. Hence the name "50-50 Low". In the general sense the appearance of a 50-50 Low is favorable for the pattern to develop a significant East Coast snowstorm. That being said of course it is also true that having a 50-50 Low does NOT guarantee a SECS anymore that having a -NAO does.

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

AFTN - Afternoon

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. Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable.

Alberta Clipper
A fast moving area of low pressure which generally affects parts of the Upper Midwest, Great Lakes, Northeast and Mid-Atlantic regions of the US. Most clippers occur between December and February, but can also occur occasionally in the month of November. Alberta clippers take their name from the province from which they appear to descend from in Alberta, Canada. Most clippers produce minor amounts of snowfall, as they are usually moving very fast, and are not generally loaded with moisture to work with.

AMO: Atlantic Multidecadal Oscillation
The AMO is a mode of variability occurring in the North Atlantic Ocean and which has its principal expression in the sea surface temperature (SST) field.

AO: Arctic Oscillation Index
An index (which varies over time with no particular periodicity) of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37-45N.

Approaching (severe levels) - A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2" or larger but less than 3/4" in diameter.

ATTM - At This Time

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.

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. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.

BECMG - Becoming

BKN - Broken

BL - Boundary Layer

Blizzard
A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer...

Sustained wind or frequent gusts to 35 miles an hour or greater; and considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than ¼ mile).

Blowing Snow
Blowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.

Boundary Layer
In general, a layer of air adjacent to a bounding surface.

Bow Echo
A radar echo which is linear but bent outward in a bow shape. 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.

C - Celcius

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. The cap is an important ingredient 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.

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 an upper air 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.)

CC
Cloud-to-Cloud 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 towering cumulus cloud. A typical thunderstorm consists of several cells. The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.

CG
Cloud-to-Ground lightning flash.

CHC - Chance

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).

Cold Advection
Transport of cold air into a region by horizontal winds.

Cold Air Damming (CAD)
A situation that typically involves a high pressure system located poleward of a mountain range. This allows for the spin of the high pressure to bring cold air in from the poleward direction and funnel it down a narrow stretch of land in front of the mountains. This situation, combined with isentropic lifting, can lead to icy conditions in the Winter time at the surface where it remains below 32 degrees, while the mid-levels are above freezing and precipitation is present.

Convection
In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. Convective activity is used to describe thunderstorms or thunderstorm clusters.

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). "Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies.

CWA - County Warning Area

Deformation Bands
Stronger bands of heavy precipitation part of a much larger low pressure system, that can be accompanied by lightning and thunder, as in an intense Nor'Easter or tropical system.

Derecho
A widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Dew Point (or Dew-point Temperature)
A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

Dgex model
A medium range downscaled GFS with NAM extensions model that runs mid-range, for forecast hours 84-192.

Downburst
A strong downdraft that induces an outburst of damaging winds on or near the ground.

Drift Fence
[slang] term used by members of LWB to indicate incredible snow depth from major storms and blizzards.

Drifting Snow
Drifting snow is an uneven distribution of snowfall/snow depth caused by strong surface winds. Drifting snow may occur during or after a snowfall. Drifting snow is usually associated with blowing snow.

Dry Line
A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.

Dry Slot
A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs.

Dynamics
Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

Dynamic cooling
Cooling that results from decreasing pressure. Therefore, dynamic heating results from increasing pressure. Because the pressure gradient is much stronger in the vertical than in the horizontal, 'dynamic' changes in temperature due to expansion or compression are more likely to occur from vertical motion than from horizontal motion.

DZ - Drizzle

E - East

ECM or ECMWF
European Center for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's medium-range forecast model, which runs to 240 hours (10 days)

El Nino
The warm phase of ENSO that is typically characterized by Pacific sea surface temperatures up to +5 degrees above normal. It is the opposite of La Nina.


Enhanced Fujita Scale (or EF scale)
A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:

EF Scale:
EF 0 65-85 mph
EF 1 86-110 mph
EF 2 111-135 mph
EF 3 136-165 mph
EF 4 166-200 mph
EF 5 Over 200 mph

All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.

ENS Ensembles
Refers to the ensemble means for the GFS, ECMWF, or GGEM. These are smoothed down images that have their own teleconnection indices associated with them. The all have a blended mean guidance, as well as individual members. Used mostly for temperature anomalies and collaboration with the operational model guidance.

ENSO
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years. It is characterized by variations in the temperature of the surface of the tropical eastern Pacific Ocean—warming or cooling known as El Niño and La Niña respectively—and air surface pressure in the tropical western Pacific—the Southern Oscillation. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western Pacific.

EPO - Eastern Pacific Oscillation
A period of variability in the NH Eastern Pacific Ocean

F - Fahrenheit

FA - Forecast Area

Fallout Shelter
[slang] A place where one can go to seek refuge from the devastating effects of weather

FCST - Forecast

Fetch
When a stronger low pressure system is over a large source of water, it can draw moisture from it to augment rain or snowfall rates in inland areas.

Flash Flood
A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Also, at times a dam failure can cause a flash flood, depending on the type of dam and time period during which the break occurs.

Flood
The inundation of a normally dry area caused by an increased water level in an established watercourse, such as a river, stream, or drainage ditch, or ponding of water at or near the point where the rain fell.

Flood/Flash Flood Warning
Issued to inform the public that flooding is imminent or in progress.

Flood/Flash Flood Watch
Issued to inform the public and cooperating agencies that current and developing hydrometeorological conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent.

Fog
Fog is water droplets suspended in the air at the Earth's surface. Fog is often hazardous when the visibility is reduced to ¼ mile or less.

Front
A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

Frontogenesis
The formation or strengthening of an atmospheric front

FROPA - Frontal Passage

Freeze
A freeze is when the surface air temperature is expected to be 32°F or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops.

Freezing Rain or Drizzle
This occurs when rain or drizzle freezes on surfaces, such as the ground, trees, power lines, motor vehicles, streets, highways, etc. Small accumulations of ice can cause driving and walking difficulties while heavy accumulations produce extremely dangerous and damaging situations primarily by pulling down trees and utility lines.

Frost
Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects falls below 32°F. As with the term "freeze," this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost." Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s.

FT - Foot or Feet

Funnel Cloud
A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.


FWC - NGM MOS Guidance

GFS Model
Global Forecast System model; one of the operational forecast models run at NCEP with forecast output out to 384 hours (15 days).

GGEM Model
The Canadian forecast model, which runs for mid and long term forecasts

Gust Front
The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also gustnado or outflow boundary.

Heat Index
An index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels. It is a complicated formula, so the HPC has a conversion calculator to determine this apparent temperature, at the following link: http://www.hpc.ncep.noaa.gov/html/heatindex.shtml

Heavy Snow
This generally means...

snowfall accumulating to 4" or more in depth in 12 hours or less; or
snowfall accumulating to 6" or more in depth in 24 hours or less.

In forecasts, snowfall amounts are expressed as a range of values, e.g., "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...".

HECS
Historical East Coast storm (term generally over used, EX: Blizzard of 1996)

Helicity
A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).

High Wind
Sustained wind speeds of 40 mph or greater lasting for 1 hour or longer, or winds of 58 mph or greater for any duration.

Hook (or Hook Echo)
A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.

HPC - Hydrometeorological Prediction Center (for quantitative precip forecasts from NOAA)

HRRR model: High-Resolution Rapid Refresh
A 3-km resolution, hourly updated, cloud-resolving atmospheric model, initialized by DFI-fields from the 13km radar-enhanced Rapid Refresh, replacing the previous Rapid Update Cycle (RUC)

Humidity
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Hurricane
A severe tropical cyclone having winds greater than 64 knots (74 miles per hour; 119 kilometers per hour), originating in the equatorial regions of the Atlantic Ocean or Caribbean Sea or eastern regions of the Pacific Ocean, traveling north, northwest, or northeast from its point of origin, and usually involving heavy rains. It's strength is measured using the Saffir-Simpson Scale, as indicated in the chart below:

Ice Storm
An ice storm is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant accumulations of ice pull down trees and utility lines resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulations are usually accumulations of ¼" or greater.

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.

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.

Invest
A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets (e.g., microwave imagery) and/or running model guidance for the potential for possible further development

IR - Infrared

Isentropic Lifting - The process, especially in Winter, where due to the density differential of the air, warm air overrides cold air at the surface and enters the mid-levels of the atmosphere, often times causing icing (freezing rain/sleet) at the surface where cold air damming takes place. This process also can lead to thermodynamic thunderstorms in warmer temperatures.

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.

Isotach
A line connecting points of equal wind speed.

Isotherm
A line connecting points of equal temperature.

Jet Max (or Speed Max, or 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 (NJT, SJT)
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. NJT is the acronym for Northern jet, and SJT is the acronym for the Southern Jet.

JMA Model
The Japan Meteorological Agency Model that runs to 144 hours into the future

KMA Model
The Korean Meteorological Agency model guidance

Lake-effect Snow
Snow that is produced during cooler atmospheric conditions when cold winds move across long expanses of warmer lake water, providing energy and picking up water vapor, which freezes and is deposited on the leeward shores. The same effect over bodies of salt water is called ocean effect snow, sea effect snow, bay effect snow, or even sound effect snow in the most blisteringly extreme cases.

La Nina
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C. In the United States, an episode of La Niña is defined as a period of at least 5 months of La Niña conditions. The name La Niña originates from Spanish, meaning "the girl," analogous to El Niño meaning "the boy."

Lightning
An atmospheric discharge (spark) accompanied by thunder, which typically occurs during thunderstorms.

KT(S) - Knot(s)

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.

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.

Low-level Jet (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).

Macroburst
A convective downdraft with an affected outflow area of at least 2½ miles wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage of up to F3 intensity.

MB - Millibar (a unit of atmospheric pressure; 1 millibar = 0.02953 inches of mercury)

MECS
Major East Coast Storm

Medium Range
In forecasting, (generally) three to seven days in advance.

Mesoscale
Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles.

Microburst
A convective downdraft with an affected outflow area of less than 2½ miles wide and peak winds lasting less than 5 minutes. Microbursts may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.

Miller A Snow storm:
A type of snowstorm that has a low which originates in the Gulf of Mexico; it intensifies and races up the east coast, usually accompanied by a strong High pressure over SE Canana near Quebec. EX: Superstorm Blizzard of 1993. These tend to be more widespread in area BUT can be less favorable for historic snows in New England. However, they can clobber portions of the Mid-Atlantic.

Miller B Snow Storm:
A type of snowstorm that has a primary low over the Appalachians (usually in the form of an Alberta Clipper) while a new, secondary, and more powerful low spins up along the Gulf Stream waters off the coast of NC. It intensifies rapidly and deepens offshore, as the primary clipper-type low transfers energy to the secondary low bombing offshore. Depending on its track it can clobber areas of inland or coastal New England. These are less widespread in terms of area and a lot of times miss the Mid-Atlantic for big snows. EX: Blizzard of 2005.

MJO: Madden-Julian Oscillation
The largest element of the intraseasonal (30–90 days) variability in the tropical atmosphere. It is a large-scale coupling between atmospheric circulation and tropical deep convection. Rather than being a standing pattern (like ENSO) it is a traveling pattern, propagating Eastward at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans.

Model guidance
Computer generated data that is used for forecasting, but are only guidance and meteorologists then use this data to determine forecasts. Several types of models run in short range EX: NAM, RUC, HRRR, some are more effective in mid-range EX: ECM, Ukmet, Nogaps, Dgex, and some longer range EX: GFS, GGEM

N - North

NAM Model
North American Mesocale model (formerly known as the Eta model); one of the operational forecast models run at NCEP with forecast output out to 84 hours (3.5 days).

NAO: North Atlantic Oscillation
A climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity.

NCEP
National Centers for Environmental Prediction; the modernized version of NMC.

NE - Northeast

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.

NHC - The National Hurricane Center (for tropical cyclone tracking of current storms and potential storms from NOAA)

NM - Nautical Mile(s)

NOGAPS Model
Navy Operational Global Atmospheric Prediction System (120-hour numerical model)

Nor'Easter
A very large strom along the East Coast of the US and Atlantic Canada, so named because the storm travels to the Northeast from the South and the winds come from the Northeast, especially in the coastal areas of the Northeastern US and Atlantic Canada. The storm has characteristics very similar to a hurricane, and in Winter, can produce historic-level snows inland, and high Northeasterly winds.

Nowcasting
Defined as the period right before an impeding storm that is used primarily for observations with SFC observations, radar imagery and mesoscale analysis to compare to short range model guidance for verification.

NW - Northwest

NWS - National Weather Service

OBS - Observations

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.)

Outlook
An outlook is used to indicate that a hazardous weather or hydrologic event may develop. It is intended to provide information to those who need considerable lead time to prepare for the event.

OVC - Overcast

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.

PAC - Pacific

PDO
Pacific Decadal Oscillation - measurements are over a period of 20-30 years of general weather patterns. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs.

PNA: Pacific-North American teleconnection pattern
A climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.

POP - Probability of Precipitation

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.

PROG - Forecast

QPF - Quantitative Precipitation Forecast

RA - Rain

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.

RGEM
Short range run of the GGEM Canadian model, to 48 hours

RH - Relative Humidity

Ridge
An elongated area of relatively high atmospheric pressure; the opposite of trough.

RUC
Rapid Update Cycle, a numerical model run at NCEP that focuses on short-term (up to 12 hours) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

S - South

Saffir-Simpson hurricane scale: See Hurricanes

SAT - Satellite

SE - Southeast

SECS
Significant East Coast storm

Severe Local Storm
A convective storm that usually covers a relatively small geographic area, or moves in a narrow path, and is sufficiently intense to threaten life and/or property. Examples include severe thunderstorms with large hail, damaging wind, or tornadoes. Although cloud-to-ground lightning is not a criteria for severe local storms, it is acknowledged to be highly dangerous and a leading cause of deaths, injuries, and damage from thunderstorms. A thunderstorm need not be severe to generate frequent cloud-to-ground lightning. Additionally, excessive localized convective rains are not classified as severe storms but often are the product of severe local storms. Such rainfall may result in related phenomena (flash floods) that threaten life and property.

Severe Thunderstorm
A thunderstorm that produces a tornado, winds of at least 58 mph (50 knots), and/or hail at least ¾" in diameter. Structural wind damage may imply the occurrence of a severe thunderstorm. A thunderstorm wind equal to or greater than 40 mph (35 knots) and/or hail of at least ½" is defined as approaching severe.

Severe Thunderstorm Risk
The relative coverage and/or threat for severe thunderstorms in a specified area. The following describes the possible density/risk of severe thunderstorms in an outlook area...

*APPROACHING - A non-severe category that indicates an area of strong convection; used to highlight areas where strong thunderstorms are anticipated but not expected to become severe.
*SLIGHT risk - Severe thunderstorms are expected; the severe storms may not have a mesoscale organization or may be isolated in areal extent with between 2-5% aerial coverage.
*MODERATE risk - Severe thunderstorms are expected and are anticipated to be more organized on the mesoscale. They will be more numerous or widespread than in the SLIGHT category. The potential for personal injury and/or significant property damage is significantly enhanced with between 6 and 10 percent coverage. A moderate risk indicates the possibility of a significant severe weather episode.
*HIGH risk - Severe thunderstorms are expected and are anticipated to be widespread. A dangerous situation exists with the strong potential for killer tornadoes, devastating windstorms, and widespread property damage. This category generally is confined for use in anticipated tornado outbreaks with more than 10 percent coverage. A high risk is rare and implies the possibility of a major severe weather outbreak.

SFC - Surface

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.

Short Term Forecast
A product used to convey information regarding weather or hydrologic events in the next few hours.

Sleet
Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of ½" or more.

Snow Flurries
Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).

Snow Showers
A snow shower is a short duration of moderate snowfall. Some accumulation is possible.

Snow Squalls
Snow squalls are intense, but limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning (generally moderate to heavy snow showers). Snow accumulation may be significant.

Southern Oscillation
The Southern Oscillation is the atmospheric component of El Niño. This component is an oscillation in surface air pressure between the tropical eastern and the western Pacific Ocean waters.

SPC - The Storm Prediction Center (for mesoscale & convective threats, including severe weather from NOAA)

Squall Line
A solid or broken line of thunderstorms or squalls. The line may extend across several hundred miles.

SREF: Short range ensemble model
Basically the ensembles of the NAM model, runs to 87 hours.

SST
Sea Surface Temperatures

Straight-line Winds
Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

SW - Southwest

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 upper air sounding analysis programs.

S/W - Short Wave

Thunderstorm
A storm of heavy rain accompanied by lightning, thunder, wind, and sometimes hail. Thunderstorms occur when moist air near the ground becomes heated, especially in the summer, and rises, forming cumulonimbus clouds that produce precipitation. Electrical charges accumulate at the bases of the clouds until lightning is discharged. Air in the path of the lightning expands as a result of being heated, causing thunder. Thunderstorms can also be caused by temperature changes triggered by volcanic eruptions and forest fires, and they occur with much greater frequency at the equatorial regions than in polar regions.

Thundersnow
A rare phenomenon that occurs with the most dynamically intense areas of low pressure in Winter, where usually cloud to cloud lightning occurs during the heaviest deformation bands in a storm.

Teleconnections
In atmospheric science, refers to climate anomalies being related to each other at large distances (typically thousands of kilometers).

Tornado
A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena.

Tropical Cyclone
A violent storm originating over tropical or subtropical waters, characterized by violent rainstorms and high-velocity cyclonic winds.

Tropical Depression
A tropical cyclone having sustained surface winds less than 39 mi (63 km) per hour.

Tropical Storm
A cyclonic storm originating in the tropics and having winds ranging from 39 to 73 miles per hour (34 to 63 knots; 63 to 117 kilometers per hour).

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.

UKMET
A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency (72-hour numerical model of the atmosphere).

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.

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.


Urban and Small Stream Flooding
Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.

UTC - Universal Coordinated Time (same as Greenwich Mean Time)

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.

Virga
Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.

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.

W - West

WAA - Warm Air Advection

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. "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 overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

Warning
A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property.

Watch
A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so that those who need to set their plans in motion can do so.

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 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.

Wind Chill
The apparent temperature on exposed skin due to wind. The wind chill temperature is always lower than the air temperature, and the windchill is undefined at the higher temps (above 10 °C [50 °F]). Wind chill conversion calculator can be found at the following link: http://www.onlineconversion.com/windchill.htm

Winter Storm Advisories
Issued 36 hours in anticipation of a significant winter weather event (Watch), area specific as to the criteria for issuing such an advisory, and a warning within 12 hours of a storm if the threat becomes imminent. Criteria for most of the Mount Holly CWA is for 4 or more inches of snow expected and/or significant icing that could impact travel and property. However the Northern counties of the Lehigh Valley and Poconos, the criteria is for over 6" expected to warrant such issuance.

Winter Weather Advisory
Issued by the NWS when winter weather (snow and/or ice) are expected to impact an area in such a way that is less than that that meets the requisite for a Winter Storm Warning.

WPO - Western Pacific Oscillation
Pattern of variability over the NH Western Pacific Ocean, similar to the EPO

WV - Water Vapor

WX - Weather

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.