Each day, National Weather Service Forecast offices and the SPC will release a forecast discussion and at times a convective discussion, severe weather discussion, winter storm discussion or some other type of related discussion. Reading these discussions is an important part of the forecasting process. It is best to make your preliminary forecast, then read the discussions to see how well they go along with conclusions about the forecast you have already made by looking at the models, analysis charts, sat/radar, and so forth. It is not a good idea to go along with these discussions blindly. To be a good forecaster you must be critical of all analysis and conclusions drawn in these discussions. That is why you should examine the discussions AFTER coming to your own forecast decision. If the discussions have relevant information that you missed in your forecast process, incorporate that into your forecast to make a change in your forecast. It is also important to read several different discussions to expose yourself to the various opinions of the weather situation. These discussions have valuable information. Be sure to research statements made in the discussions and verify them for yourself. If a discussion talks about how well a particular sounding looks or the impressiveness of PVA, look at the charts and verify this for yourself. Use discussions as a GUIDE to your forecasting process. Below is a list of common terminology (alphabetical order) you will run across when reading these discussions. There are many abbreviations and weather terminology that will be explained. It is a good idea to read discussions for your forecasting region each day. You will learn a little more about forecasting from each one you read and will be able to read and interpret the discussions quicker over time. Many different forecasters write these discussions. Therefore, there will be a variety of abbreviations and terminology you will come across. Some will even be written with no abbreviations. The discussions are in all capital letters. Forecast discussions can be found at these three web sites: http://iwin.nws.noaa.gov/iwin/iwdspg1.html click a state, then click "forecast discussion" http://asp1.sbs.ohio-state.edu go to weather by state, then scroll down to state list, click a state, then scroll down to heading "area forecast discussions" SPC: http://www.spc.noaa.gov/products/outlook/ click graphic thumbnails (such as Day 1 Outlook) to see discussion State names are usually abbreviated. e.g. TX, OK, AR, GA, NY N, S, E, W, SE, SW, NE, NW, etc. - These refer to compass directions. H7, H5, H3, etc.- These refer to height levels in the atmosphere. H7 is the 700 mb level, H5 is the 500 mb level. H3 is the 300 mb level. And so forth. MRF, ETA, NGM, AVN, ECMWF- These are all synoptic scale forecast models 88-D- In reference to the WSR-88D radar. Stands for Weather Surveillance Radar, 1988 Doppler AC- Altocumulus ADIABATIC- A process that causes rising air to cool and sinking air to warm ADNLY- Additionally ADV- Advisory ADVECTION- The horizontal transport of air (parallel to earth's surface) AFD- Area Forecast Discussion AGEOSTROPHIC- A wind that is not in geostrophic balance. An ageostrophic wind will have vertical motions. Shortwaves and jet streaks cause ageostrophic flow. ALL QUADS- All Quadrants; in all directions AMPLIFICATION: (AMPLFCTN)- Building, or sharpening, of an upper level high pressure ridge or low pressure trough ANTICYCLONE- High pressure system AP- Anomalous Propagation. This is a false echo on radar data APCH- Approach(ing) ASL- Above Sea Level ATTM- AT The Moment AVN- Aviation Model. One of the short term synoptic forecast models AWIPS- Advanced Weather Interactive Processing System. It is used at NWS offices to process weather data BACKING WIND- A backing wind is a wind direction that turns counterclockwise with height. An indication of cold air advection. BAROCLINICITY- Baroclinicity is a cold air advection/warm air advection couplet that increases atmospheric instability. On analysis and forecast charts it is the isotherms crossing the height contours. BAROTROPIC- Barotropic is a horizontally fairly homogeneous troposphere in which there are neither fronts or any thermal advections. BKN- Broken sky. 6/10 to 9/10 cloud coverage observed from the surface. BLACKBODY- A mass which absorbs and emits all wavelengths of radiation. BLIZZARD- Snow with winds greater than 35 mph and visibility of 1/4 mile or less that lasts for several hours. BLO- Below. BLOCKING PATTERN- A situation in which the upper tropospheric flow becomes highly meridional and stagnant. Two common blocking patterns are the omega block and the rex block. Click here for more information on atmospheric blocking. BLUE NORTHER- A term used for a strong cold front in the southern plains of the United States and drops temperatures very quickly and has strong gust winds just behind the front. BOMB- A bomb is a low pressure system that decreases in central pressure by at least 24 millibars in a 24 hour period. BOUNDARY LAYER- The Planetary Boundary Layer (PBL) is the layer of the troposphere closest to the earth's surface where friction is a significant force and wind tends to be gusty (irregular flow). BRIGHT BAND- A maximum in the radar reflectivity caused by snow melting as it falls. Melting snow can have a reflectivity similar to small hail. The bright band will produce a ring around the radar since the snow melts aloft before it turns to all rain before reaching the surface. BUST- A situation when a certain type of weather (often severe weather or winter weather) is expected but nothing happens. CAA- Cold Air Advection, The movement of colder air horizontally toward a fixed point on the earth's surface. CAP- A cap is a temperature inversion which prevents convection from occurring. CAT- A category. Usually refers to a category of precipitation given by the forecast models. Also the Category of hurricane intensity. CB- Cumulonimbus. CEILING- The height of the cloud base closest to the earth's surface. CELL- An individual thunderstorm (often referred to as such on radar). CELSIUS- A temperature scale using 0 as freezing and 100 as boiling at standard sea-level atmospheric pressure and temperature. CG- Cloud to Ground lightning CHC- Chance (usually in reference to precipitation probability) CI- Cirrus CIN or CIN/S or CINH- Convective Inhibition. A cap which prevents convection from occurring. A CIN of 50 or below is weak while 200 and above is very strong. CISK- Convective Instability of the Second Kind. This is the combination of an unstable atmosphere with that of a large amount of latent heat release. CLASSIC EVENT- Having all the ingredients for a specific type of weather (i.e. classic severe weather event, classic Nor'easter) CLOSED LOW- A low pressure center having a closed circulation, which is used in reference to systems in the upper levels of the atmosphere. Closed lows will have one or more height contours encircling them. CNVTN- Convection COLD CORE HIGH- A dome of cold surface high pressure that originates from the high latitudes. COLD CORE LOW- A low pressure which has the coldest temperatures located near its center. A cold core low is deep and is often associated with the polar jet stream. Mid-latitude cyclones are cold core lows. COLD FRONT- A synoptic scale boundary between cold and warm air. The cold air is displacing the warm air. CONDENSATION PRESSURE DEFICIT (COND PRES DEF)- On an isentropic chart (a layer of constant potential temperature), condensation pressure deficit represents the amount of lift, expressed in millibars, needed to saturate an air parcel. For example, an air parcel at 850 mb has a condensation pressure deficit of 200 mb. This means that this parcel needs to be lifted 200 mb (up to the 650 mb level) before it will become saturated. It is one way of evaluating moisture content on an isentropic chart. CONFLUENCE- A 2-D process in which air streams move toward each other at some angle. CONQ- Converging Q vectors. Indicates air will rise due to low level warm air advection and/or upper level divergence CONVECTIVE FEEDBACK- Convective feedback deals with how a forecast model handles the vertical profile of changes in temperature and moisture that are produced from convection. CONVECTIVE INSTABILITY- Convective instability is instability caused by very dry air advecting in the mid-levels of the troposphere over a warm and moist lower troposphere. Dynamic lifting causes the mid-levels to cool at a greater rate than the lower troposphere since air in the lower troposphere cools at the WALR and air in the middle troposphere cools at the DALR. CLICK HERE for an in-depth essay on convective instability. CONVERGENCE- CONV. A 1-D process in which higher momentum air moves into lower momentum air. CORIOLIS- An apparent force caused by the earth's rotation. The Coriolis force is a maximum at the poles and a minimum at the equator. CU- An abbreviation for cumulus clouds CU FIELD- Cumulus clouds covering a large spatial area. CUT-OFF LOW- Closed lows that become cut-off from the main flow pattern are called cut-off lows. Click here for a cut-off low example. CWA- County Warning Area CYCLONE- This is in reference to a low pressure system. CYCLOGENESIS- The intensification of a low pressure system. DEFORMATION ZONE (DFRMTN ZN) - An area in the atmosphere where winds converge along one axis and diverge along another. Deformation zones (or axis of deformation as they are sometimes referred to) can produce clouds and precipitation. DEEP- Extending a significant vertical distance in the troposphere (i.e. deep moisture, deep uplift) DEEPENING- Deepening is the intensification of a low pressure system (in particular the lowering of central surface pressure). DERECHO- A derecho is a widespread severe wind event resulting from persistent and violent outflow from a MCS (Mesoscale Convective System). The derecho environment includes dry mid-levels winds that are ingested into a squall line or a segment of a squall line. The forward motion of the storm along with an intense downdraft produced by negative buoyancy via evaporative cooling brings down momentum from the middle levels of the storm to the surface. A severe wind is one with wind speeds of 50 knots (58 miles per hour) at the surface. In a derecho these severe winds encompass a distance of at least 400 km (250 miles) either out ahead of or along a squall line MCS. The length of time the severe winds last can be particularly damaging. While a severe thunderstorm may produce severe convective wind gusts that last for several minutes at a point location, derecho wind can last 30 minutes or longer. Derechos can be tracked from radar and severe weather reports while they are occurring since severe weather reports will be given in sequence as the derecho traverses along. DEWPOINT DEPRESSION (TDD)- The dewpoint depression is the positive numerical difference between the temperature and the dewpoint. DEWPOINT POOLING (DWPT PLG) - An area, usually along a surface front or trough, where there is a "pool" of higher dewpoints (or higher amounts of surface moisture). Since increasing low level moisture increases atmospheric instability, an area of dewpoint or moisture pooling tends to be more unstable than surrounding locations and can be a prime area for the development of thunderstorms. DIFFERENTIAL ADVECTION- Differential advection is the increasing or decreasing of advection with height. Vorticity advection increasing with height is favorable for uplift as well as warm air advection increasing when moving from aloft to the surface. DIFFLUENCE- A 2-D process in which air streams spread apart. DIFFLUENT THICKNESS PATTERN (DIFLNT THKNS PTTN) - Organized areas of thunderstorms tend to move with the thickness pattern (the mean wind in a layer). An area of diffluent thickness is an area where the thickness contours spread apart. Why this actually occurs is not fully known, but it is usually found in an area of low level warming and upper level cooling (processes that make the atmosphere more unstable). Areas of convection will tend to move toward these areas of diffluent thickness (including backwards), and can tip off forecasters when trying to pinpoint potential areas of heavy rainfall. DIGGING- A trough becoming more amplified with time. Often in reference to a trough building to the South. When the winds are stronger on the upwind side of the trough the trough will tend to dig. DIRTY HIGH- High pressure usually brings clear weather. A high pressure is termed "dirty" if it has a canopy of clouds associated with it. These clouds can form due to orography, a saturated PBL (with low level fog and/or stratus building under a middle level inversion), or lifting mechanisms within the region of high pressure (WAA, moisture advection). DISCONTINUITY- A rapid change of a meteorological parameter over distance. A frontal boundary is an example of a discontinuity. DIURNAL EFFECTS- A reference to an effect that has its origins due to daytime heating, such as afternoon cumulus cloud development or the formation of a lake/sea breeze. These phenomena dissipate once the sun goes down and surface heating is lost. Diurnal means a daily effect. DIVERGENCE- A 1-D process in which higher momentum air moves away from lower momentum air. DIVQ- Diverging Q vectors. Indicates air will sink due to low level cold air advection and/or upper level convergence DLAD- Delayed DOUBLE-BARRELLED LOW PRESSURE SYSTEM- Ideally a low pressure system will have one distinct center where the surface pressure and height aloft are lowest at one point. Often though the low pressure system will be a broad area of lower pressure or it will have two distinct areas where the surface pressure is lowest. The term double-barrelled low pressure system means two side by side low pressure systems. DOWN SLOPE- A wind that decreases in altitude above sea level as it moves due to the air moving into a lower elevation. Downsloping air tends to decrease in relative humidity and it warms adiabatically. DOWN STREAM / DOWN WIND- This is in the direction something is traveling toward within a flow. For example, a boat on a river current will move down stream. DPNG- Deepening. Usually in reference to a low pressure intensifying. DRYLINE- A dryline is a synoptic or large mesoscale boundary between warm/humid air and warm/dry air. DRYLINE BULGE- A segment of a dryline that advances forward due to strong upper level winds. DRY PUNCH- This is in reference to a region of drier air filtering toward a mid-latitude cyclone or sub-synoptic scale low pressure area. It often has a fan shape with the small end of the fan connecting to the cyclone. DRY SLOT- Refers to an influx of dry air (especially toward a mid-latitude cyclone) DYNAMICS- Refers to upward forcing caused by a mechanism that forces the air to rise or sink such as thermal advection, jet streak divergence and PDVA EASTERLY WAVE- An inverted trough in the tropics that moves with the tropical easterlies. ECHO(ES)- This refers to radar returns such as precipitation echoes. EDDY- On any scale, this is a circulation within a fluid or the air. The term is often used in reference to convective eddies. ENHANCED-V- An enhanced-V is a V-shaped region over and downwind from strong to severe thunderstorms on infrared imagery. The V signature is seen best on color enhanced infrared satellite images. The cold cloud tops and anvil form a V-shape. Think of how smoke fans out downwind from a chimney. The process is similar in a strong to severe storms. An intense updraft lifts moisture high into the troposphere and strong upper level winds move the now extremely cold moisture (ice crystals) downwind. The moisture fans out as it moves downwind. ENTRAINED / ENTRAINMENT- Refers to the drawing in of moisture (or lack of moisture) into a system. Dry air entrainment into the mid levels of a thunderstorm can enhance the potential for damaging wind gusts. Moisture being entrained into a storm system can enhance precipitation amounts. ENTRANCE REGION- Region of a trough or jet streak where the winds enter the feature. This is the upstream region from the trough axis or jet streak center. ENUF- Enough ETA- ETA forecast model- letters denote model uses ETA coordinates. ETA is one of the primary synoptic forecast models used by operational meteorologists EQUIVALENT POTENTIAL TEMPERATURE- The temperature of a parcel of air after all moisture and latent heat is condensed out of an air parcel then descended to the 1000 mb level. Also known as Theta-E EXIT REGION- Region of a trough or jet streak where the winds leave the feature. This is the downstream region from the trough axis or jet streak center. EXTRAPOLATE- Determining the value of a meteorological value beyond the bounds of a data sample. EXTRATROPICAL- A low pressure system that did not develop in the tropics. Extratropical cyclones have a baroclinic energy regime. Extratropical can also be in reference to a tropical system that is losing tropical characteristics. FA- Forecast Area FAHRENHEIT- A temperature scale using 32 as freezing and 212 as boiling at standard sea-level atmospheric pressure and temperature. FCST- Forecast FFA- Flash flood advisory FILLING- The weakening of a low pressure system (in particular the increase of central surface pressure) FIRE UP- A favorite phrase of forecasters. It means storms are developing or will develop. FLY IN THE OINTMENT- A favorite phrase of some forecasters. Refers to a forecast problem or a potential forecast problem. Could also be in reference to a forecasting problem that caused a "busted" forecast. FRCG- Forcing. This is in reference to dynamic lifting such as from either low level convergence or upper level divergence. FREEZING LEVEL- The pressure level above the earth's surface where the temperature is freezing. If the entire troposphere is below freezing then the freezing level will be indicated as BG (Below Ground). FREEZING FOG- Freezing fog is a fog composed of supercooled water droplets. Freezing fog will occur when the outside temperature is below freezing and the fog particles are still liquid. Often freezing fog will occur with freezing drizzle. The supercooled droplets freeze once they contact the earth's surface. Freezing fog is different from ice fog. Ice fog occurs when the fog particles turn to ice crystals and the outside temperature has to drop well below freezing for the supercooled droplets to turn to ice crystals while still in the air. FREEZING RAIN (ZR)- Freezing rain is liquid precipitation that freezes after reaching the earth's surface FRICTION LAYER- This is another term for the boundary layer. This is the layer of the troposphere closest to the earth's surface where friction is a significant force and wind tends to be gusty (irregular flow). FRONT- A transition zone between air masses FRONTOGENESIS- The intensification of a front (temperature gradient is becoming more compact; isotherms closer together in the region the front is developing. FRONTOLYSIS- The weakening of a front (temperature gradient is becoming less compact; isotherms are spreading apart in the region the front is weakening. FROPA- FROntal PAssage FWC- Forecasted Weather Conditions; MOS output from NGM model FZRA / ZR- Freezing Rain GALE- A wind speed ranging from 39 to 54 mph. GFS- Global Forecast System. This is a synoptic model that has output for several days into the future and was previously known as MRF (Medium Range Forecast). GRAVITY- Gravity is the attraction of two masses to one another. Large masses have higher values of gravitational accelerations than lighter masses. GEOSTROPHIC- This is a balance between the pressure gradient and Coriolis forces. A zonal wind with a constant wind speed in the upper troposphere in the middle latitudes is an example of a geostrophic wind. GROUND CLUTTER- (a.k.a. Doppler Garbage). These are false precipitation or velocity echoes on radar that are produced by obstacles near the radar site. GUIDANCE (GUID)- Forecast model data that helps a forecaster make a forecast. GUST- A brief and rapid increase in wind speed. These most commonly occur when the overall wind speed is fairly high and is most common near the earth's surface. GUST FRONT- It is the leading edge of thunderstorm outflow. Often cooler and gusty air follows behind the gust front. HAZE (H)- Dust, salt and other particles that restricts horizontal visibility. HELICITY- Helicity is streamwise vorticity available for ingestion into a thunderstorm. Higher values are favorable for a rotating updraft (greater than 400). A measure of low level wind shear, normally within the lowest 3 km of the atmosphere, relative to the movement of a thunderstorm (thus referred to as 0-3 km Storm Relative Helicity). This gives forecasters an indication of an environment that is favorable for supporting the development of thunderstorms with rotating updrafts, a precursor to supercell thunderstorms (the most violent of severe storms) and tornado development. Values of helicity greater than +150 are considered significant; however, like CAPE values, there is no magic value of (positive) helicity under which rotating thunderstorms will not develop. Helicity is only an index to determining thunderstorm rotation potential. HI- High HODOGRAPH- A graph that shows how the wind speed and direction change with height. HORIZONTAL VORTICITY- A rotation of air caused by vertical speed or directional wind shear. HP- High Precipitation supercell; It is a supercell with a high moisture content. Precipitation will wrap completely around the updraft region in an HP supercell. They have a kidney bean shape on radar reflectivity. HVY- Heavy HYDROLAPSE- Rapid change in moisture (particularly dewpoint) with height. A hydrolapse separates warm and moist lower tropospheric air from mid-level dry air. CLICK HERE for an example of a hydrolapse on a Skew-T. HYDROMETEOR- Any precipitation particle that is falling ICE PELLETS- IP / PL, Same as sleet. Frozen raindrops that strike the earth's surface. INDCG- Indicating INDIAN SUMMER- A warm spell in the fall or early winter in which the temperatures are above normal and skies are clear for several days in a row. INFLOW- Wind speed, in knots, of the average PBL wind speed. INSTABILITY (INSTBY)- Air that if lifted adequately will rise on its own due to positive buoyancy. Instability is needed for thunderstorm development. INTERPOLATION- Drawing an isopleth between known numerical values. INVERSION (INV)- An inversion is a temperature increase with height. INVERTED TROF- This is a trough which bulges to the north. Mid-latitude troughs have a north to south amplitude but in the tropics the opposite is the case. Inverted troughs look like ridges but there is lower pressure at their centers of curvature. Inverted troughs can occur across areas of the tropics and sub-tropics. CLICK HERE for an example of an inverted trough. INVOF- IN the Vicinity OF IP / PL- Ice Pellets ISALLOBAR- A line of equal surface pressure change. ISENTROPIC LIFT / DESCENT- Isentropic motion is lifting or sinking of air along constant potential temperature (theta) surfaces. WAA (especially over shallow frontal boundaries) leads to lift while CAA leads to descent. ISOBAR- A line of constant surface pressure. ISODOP- A line of equal distance from a radar site. ISODROSOTHERM- A line of constant dewpoint temperature. ISOHYET- A line of equal rainfall. ISOHYPSE- A line of constant geopotential height (height contour). ISOPLETH- A line of a constant meteorological value. ISOSTER- A line of a constant density. ISOTACH- A line of constant wind speed. ISOTHERM- A line of constant temperature. JET STREAK- A jet streak is a relative maximum of windflow within the jet stream. JET STREAM- The jet stream is a meandering belt of strong upper level winds that separates milder mid-latitude air from cold polar air. KELVIN- A ratio temperature scale using 0 as the theoretical coldest temperature. KNOT- A knot is a unit of wind measurement found by taking the wind speed in miles per hour and dividing it by 1.15. LAKE EFFECT SNOW- Lake Effect Snow is snow produced from lifting of moisture from a large lake such as the Great Lakes. LAPSE RATE- The rate of temperature decrease with height. High lapse rates are indicative a strong cooling with height and this can lead to instability. LATENT HEAT- Energy released or absorbed that changes the temperature of the surrounding environment but NOT the material releasing or absorbing the latent heat. For example, when ice melts, it cools the surrounding air but the ice cube/water mixture stays the same temperature. LEE TROUGH- Low pressure that develops just east of the Rocky Mountains due to flow over the higher elevation region. Often lee troughs become organized into mature cyclones as they develop eastward. The lee wardside is the opposite of the windward side. LEWP- Stands for Line Echo Wave Pattern. This is an outward bulge along a squall line that is often created from strong upper level winds intersection that segment of the squall line. A LEWP can suggest strong surface winds will occur along the LEWP's path. LIFTED INDEX- The environmental temperature at 500 millibars minus the 500 millibar parcel temperature on a Skew-T diagram. Negative LI values are unstable. LLJ- Low Level Jet. Strong Planetary Boundary Layer winds which advect warmer temperatures and moisture into the forecast area. LP- Low Precipitation Supercell. This is a supercell that lacks abundant moisture. They occur around a dryline boundary where moisture depth is greatly reduced. They tend to look much more severe in the field than they do on radar. LSR- Local Storm Report LONGWAVE- The jet stream will have a series of waves. A wave of higher heights is a longwave ridge and a wave of lower heights is a longwave trough. LUKG- Looking MAV- MOS output from AVN forecast model MCC- Mesoscale Convective Complex. A large cluster of thunderstorms and rain. It is defined using satellite meteorology. Must have cloud shield temperature of -32 C or less over 100,000 square kilometers or more and a cloud shield temperature of -52 C or less over 50,000 square kilometers or more. MCS- Mesoscale Convective System. A large cluster of thunderstorms and rain. Can be a squall line, multi-cells or a mesoscale convective complex. MCV- Mesoscale Convective Vortex. It is an MCC or MCC-like grouping of storms that shows a mesoscale rotation. MET- MOS output from ETA forecast model MEX- MOS output from extended GFS forecast model MICROBURST- A 4 kilometer diameter or less downdraft of air that sinks due to strong negative buoyancy. MID/UPPER LEVEL SUPPORT- This is in reference to either positive differential vorticity advection or a jet streak creating upper level divergence. These processes result in a dynamic lifting of air. MILLIBAR (MB)- A unit of pressure which is equal to Pascals divided by 100. MISG- Missing MIXED BAG- Several precipitation types falling at once or over a period of time. MIXING DEPTH- The vertical distance the process of convection mixes the air from the surface to aloft. The mixing depth is often the same depth as the PBL. The mixing depth will increase with solar warming of the surface and increased low level wind speed. Could also be in reference to the depth of the "transition zone" between two air masses that are horizontally differentially advecting one over the other. MIXING RATIO CHART- Used by forecasters to determine moisture content on a constant pressure surface. Rawinsonde data is used to produce this chart. MOISTURE AXIS / RIDGE- : An area of higher moisture values, usually in the form of a ridge of higher dewpoints at the surface or 850 mb. Low level moisture axes enhance atmospheric instability, which in turn promotes thunderstorm development. Existing storms can intensify by moving into moisture axes. The concept is similar to dewpoint pooling. MOS- Model Output Statistics. These are numerical representations of expected weather such as forecasted temperatures and precipitation chances. MRF- Medium Range Forecast (now called GFS: Global Forecast System); synoptic model that gives output for several days into the future. MSTR- Moisture MXD- Mixed NCEP- National Center for Environmental Prediction. They are responsible for running and disseminating the forecast models. NEGATIVE TILTED TROUGH- A trough which tilts from the northwest toward the southeast. This situation creates instability since cold temperatures in the mid and upper levels advect over warmer air at the surface. A negative tilt is a sign a trough is mature. CLICK HERE for an example of a negatively tilted trough. NEGATIVE VORTICITY ADVECTION (NVA)- The advection of lower values of vorticity into the forecast area. NVA promotes synoptic scale sinking air. NEXRAD- Next Generation Radar. Reflectivity and Doppler Radar used by NWS offices. NGM- Nested Grid Model. A synoptic forecast model for short term forecasting. NOAA- National Oceanographic and Atmospheric Administration NOCTURNAL- A weather feature that occurs at night (i.e. nocturnal inversion) NORMAL- The average of a certain weather parameter over several years (commonly 30 or more years). For example, the normal yearly precipitation being 30 inches means that over a 30 year period the average is 30 inches per year. NOSIG- No significant change NR- Near OBS- Observation, a reporting station's output OCCLUDED FRONT- The lifting of one front by another. OMEGA FORCING- This is in reference to Q-G theory (Quasi-Geostrophic theory). The omega equation states vertical motion is a function of differential vorticity advection and thermal advection. Positive omega forcing results from vorticity advection increasing with height and/or low level warm air advection. OROGRAPHIC- Air flowing from one elevation to another due to forced lifting or sinking due to the slope of the land surface (i.e. orographic uplift) OPEN WAVE- A wave of low pressure that does not have a complete circulation around it; also called a short wave trough. OUTFLOW BOUNDARY- The outflow of air from thunderstorm(s). They can act like miniature cold fronts. OVERCAST (OVC)- Cloudy. PACKAGE- A compilation of analysis and forecast charts and their interpretation. PBL- Planetary Boundary Layer. The lowest level of the atmosphere where friction is an important force and vertical mixing is common. PCPN- Precipitation PD- Period PDS- Potentially Dangerous Situation, usually in reference to the dangerous threats from a severe weather outbreak. PG- Pressure gradient. Tightening pressure gradient indicates stronger winds. PHASING- When two separate short waves come together to form one wave. Also, when upper and lower level features are positioned so that each provides energy to the other, it is said that the features are in phase with one another. POP- Probability of Precipitation. This is the chance the NWS gives to precipitation across the forecast region. It may also refer to the POP a forecast model is predicting. POSITIVE ISOTHERMAL VORTICITY ADVECTION (PIVA)- Advection of higher values of vorticity by the thermal wind on a map of vorticity and thickness contours. Implies an area of rising motion. (DIFFERENTIAL) POSITIVE VORTICTY ADVECTION- Advection of higher values of vorticity into an area and this vorticity advection contributes to rising air. POSITIVE TILTED TROUGH- A trough which tilts from the northeast toward the southwest. Often a trough is positive tilted in it's development stage. POTENTIAL INSTABILITY- Also known as convective instability. Instability caused by dry air advecting over warm and humid PBL air. Lapse rate of temperature increases if lifting occurs since the low level air cools at the SALR while the mid-level air cools at the DALR. POTENTIAL TEMPERATURE- The temperature of an air parcel after it is lowered or raised to the 1000 mb level. PRECIPITATION (PCPN)- Solid or liquid water falling from the air (i.e. snow, sleet, rain, hail, etc.) PRECIPITABLE WATER (PW, PCPTBL WTR or H2O)- Total amount of water vapor in a layer of air, expressed in inches. Normally taken between 1000 and 500 mb. Higher values of precipitable water indicate a deep moisture layer, increasing the potential for heavy precipitation amounts. PRES- Pressure PROFILER- A remote ground-based sensing instrument that measures wind speed and direction at different levels of the atmosphere. The new National Weather Service WSR-88D radar has this capability, and its vertical wind profile is called a VAD Wind Profile (or VWP). VAD stands for Velocity Azimuth Display. PROG- Model forecasted output, prognostication PROGRESSIVE FLOW or PROGRESSIVE PATTERN- An upper level flow pattern in which storm systems move along at a fairly regular pace. PULSE STORM- A pulse storm is a storm with a strong updraft due to strong instability release that develops in a weak shear environment. Pulse storms can produce brief severe weather. The lack of wind shear results in the storm weakening quickly since the downdraft cut-offs and chokes the updraft. PUNCH (or PUSH)- A fast moving mass of air (e.g. dry punch, cold punch) PW- Precipitation Water, the amount of liquid equivalent if all moisture in troposphere were condensed onto the earth's surface. PWAT(S)- Precipitable Water value(s) QG Forcing- Upward vertical velocity generated by low level convergence or upper level divergence. Example of phenomena that produce upward QG forcing include low level warm air advection and moisture advection, positive differential vorticity advection, and the right rear and left front quadrant of a jet streak. If QG forcing is in reference to sinking air, examples that cause this are low level cold air and dry air advection, negative differential vorticity advection, and the left rear and right front quadrants of a jet streak. Q-VECTORS (QVEC, DIVQ)- A mathematical entity (Q-vectors do not exist in the atmosphere) that allows forecasters to better identify areas of vertical motion. Q-vectors essentially show vertical motions arising from the combination of differential vorticity advection (changes of advection with height) and temperature (thickness) advection. Areas where Q-vectors converge implies upward motion and Q-vector divergence (DIVQ) implies an area of sinking motion. Note: Negative Q-vector divergence is the same mathematically as Q-vector convergence, so the statement "...negative divq..." means Q-vector convergence. QPF- Quantitative Precipitation Forecast. QUAD- Quadrant, 1/4th of a square or rectangular area (i.e. Right rear quadrant, NE quadrant) QUASI- Nearly (i.e. quasi-geostrophic, quasi-stationary) RA- Rain RADIATIONAL COOLING- The earth's surface cooling by emitting longwave radiation. The best radiational cooling occurs on clear nights. RELATIVE HUMIDITY (RH)- The amount of moisture in the air as a ratio to the maximum amount of moisture that can be evaporated into the air at a constant temperature. It can be expressed as the mixing ratio divided by the saturation mixing ratio or the vapor pressure divided by the saturation vapor pressure. RETURN FLOW- Return flow is a rapid increase of temperature and moisture. This is in common reference to winds becomes southerly which allows Gulf moisture to be transported rapidly northward. Return flow precedes the passage of a low pressure trough and cold front. RETROGRADE- To move backwards. It relates to a ridge or trough moving from east to west. Typically in the mid-latitude, troughs and ridges move west to east. Retrograde motion is the opposite movement of normal. RH- Relative Humidity RIDGE- A wave of higher heights. The opposite of a trough. Ridging tends to be associated with sinking air. RING OF FIRE- This refers to thunderstorms and rain across the edges of a high pressure ridge. If a ridge occurs across the central US in summer, the ring of fire will extend across the west coast, up into Canada and across the east coast. Underneath the main core of the ridge weather will be stable. Rapid Update Cycle (RUC)- A short range model that projects a 12 hour forecast. The model is run each hour, with its initial conditions updated by the inclusion of the latest surface observations available. Its primary usage is for aviation and severe weather forecasting. SATURATED- Air with 100% relative humidity SBCAPE / MUCAPE- SBCAPE stands for surface based CAPE and is the value of CAPE relative to a parcel of air rising from the lower planetary boundary layer (the surface). MUCAPE stand for "most unstable CAPE". MUCAPE is the highest CAPE found by raising a parcel of air from every level in the lower to mid troposphere. SCA- Small Craft Advisory SELS- Severe Local Storms SEV, SVR- Severe SFC- Surface SHEAR (WIND SHEAR)- Significant change in wind speed or direction with height. For severe weather this is most relevant if it occurs in the lower troposphere. SHEAR AXIS- A linear feature (axis) which denotes a directional or speed change in wind. SHORT-FUSE WARNING- A warning by the NWS for an event that poses an immediate danger and will occur over a relatively short amount of time (i.e. Tornado warning, Severe Thunderstorm warning) SHORTWAVE- sometimes abbreviated S/WV: This term will come up many upon many times. This is a short amplitude trough embedded within the large scale flow. A baroclinic shortwave occurs when the isotherms cross the height contours at a sharp angle. Shortwaves are best defined on the 700 and 500 millibar charts. Shortwaves range in size from mesoscale to synoptic scale. They are responsible for producing instability and dynamic precipitation. They create instability by cooling the mid-levels of the atmosphere, generating vorticity and creating upper level fronts. CLICK HERE for an example of a shortwave and more explanation. SLEET- Ice Pellets, rain that freezes before reaching earth's surface SN- Snow SOUNDING- Profile of temperature, dewpoint and wind with height measured by a weather balloon SPC- Storm Prediction Center, Norman OK SPIN UP- An increase in the rotation rate over time (i.e. spin up of vorticity, tornado spin up) SPS- Special Statement SST- Sea Surface Temperature STACKED- A pressure system which shows up in the same general location on every height analysis or forecast chart. This is an indication low pressure is weakening (filling) or a pressure system is stalling. STATIONARY FRONT- A front which moves very slowly or not at all over time. STORM RELATIVE- Measurement made in the storm's reference. For example, if the wind is from the south at 20 knots and the storm is moving into the wind at 10 knots, the storm will experience a 30 knot headwind. STREAMLINES- Arrows showing wind speed and direction. The head of the arrow points toward where the wind is blowing and the length of the arrow is proportional to the wind speed. Sometimes shows wind direction and trajectory only. STRAIGHT-LINE WIND- A strong wind from a storm in which the wind direction is fairly constant. A severe convective wind gust from a thunderstorm is a straight-line wind while the winds of a tornado are not. STRATIFORM- Stable clouds. Stratiform clouds lack convective development and occur in broad layers. The lifting that forms stratiform clouds is dynamic (slow rising air). STREAMWISE VORTICITY- The amount of horizontal vorticity parallel to storm inflow. SUBSIDENCE- Sinking air caused by broad scale sinking or dynamic sinking (i.e. High pressure subsidence, Mid-level subsidence, orographic subsidence) SUB-SYNOPTIC LOW- A relative small area of surface low pressure. They are also referred to as mesolows. SUPERCELL- A storm with a strong, tilted and rotating updraft due to good instability and wind shear in the troposphere. Most of the strong tornadoes and large hail occur with supercells. SUPERCOOLED- Liquid water having a temperature that is below freezing SXNS- Sections TCU- Towering Cumulus TDA- Today TEMP- Temperature THERMAL RIDGE or THERMAL TROUGH- A ridge of warmer temperatures or a trough of colder temperature. CLICK HERE for an example and explanation. THERMAL LOW- A surface low pressure caused by intense surface heating. Thermal lows weaken with height since the source of heating is at the surface. They commonly occur in desert regions in summer. They often do not produce precipitation since the air is very dry at the surface. THERMODYNAMICS- In reference to the (in)stability and other thermodynamic factors of the atmosphere. Important thermodynamic information includes lapse rates, CAPE, changes in temperature / moisture with height, and cap strength. THETA-E ADVECTION- Movement of higher temperature, higher moisture or a combination of higher moisture and temperature toward forecast area. Generally referenced to low level temperature/moisture advection. An increase of Theta-E in low levels of atmosphere increases atmospheric instability. THETA-E RIDGE- Region with a relatively higher combination of temperature and dewpoint at the surface. In a thunderstorm situation this region would be more unstable. THICKNESS LINE- A line of constant geopotential thickness. Thickness increases by either increasing the temperature or increasing water vapor content of the air. THICKNESS (CRITICAL VALUES FOR SNOW)- Thicknesses equal to or less than value given below are suggestive that precipitation type will be snow. Forecaster MUST also look for warm and cold biasing of thickness value, especially the 1000-500 mb thickness. Consult the following website for information on thickness as well as thickness biasing. THICKNESS AND THICKNESS BIASING 1000-500 mb 5400 m or less (referred to as the "540 line") is suggestive of snow 1000-850 mb 1300 m or less is suggestive of snow 850-700 mb 1540 m or less is suggestive of snow Thickness is also important because it gives rise to the concept of the "thermal wind" (the thermal wind itself does not exist in the atmosphere). On a map of thickness contours, the thermal wind "flows" between the contours; the tighter the contour spacing, the "faster" the thermal wind. This thermal wind is equivalent to the mean wind between the layer it represents i.e., the 1000-500 mb thermal wind represents the mean wind between 1000 and 500 mb, and it "flows" between contours of 1000-500 mb thickness. It is because the thermal wind represents mean wind flow that organized convection (thunderstorm complexes) tends to move parallel to thickness contours. TRACE- Less than 0.01 inches of precipitation TRIGGER MECHANISM- A trigger mechanism is any process that initiates precipitation or storm development. It is in reference to a process that causes a precipitation or storm event and without this process precipitation or storms would not have occurred. Common trigger mechanism examples are lifting mechanisms, increase of low level moisture, daytime heating, instability and wind shear. The most common type of trigger mechanism that will be referenced are lifting mechanisms such as fronts and other low level convergence boundaries. TRIPLE POINT- The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment). TROUGH (TROF)- Longwave of lower heights aloft. A cold low level air mass will cause there to be troughing aloft. Rising air over a broad region such as from a mid-latitude cyclone can also create troughing. TS, TSRA- Thunderstorm TUTT- Tropospheric Upper level Tilted Trough or (Tropical Upper Tropospheric Trough). This is an upper level wave that commonly develops in tropical environments along old frontal boundaries or in association with an upper level low. TVS- Tornado Vortex Signature TWEAK- To change slightly. Often in reference to slightly changing MOS output or any other form of model guidance. UA- Upper Air UVV- Upward Vertical Velocity. Thermodynamic UVV results from the release of instability on the sub-mesoscale (updrafts in thunderstorms). Dynamic UVV results from broad synoptic lift caused by low level convergence (WAA) or upper level divergence (Jet streak, PDVA). Synoptic vertical velocities may be referred to as OMEGA forcing. UNSTABLE- A situation in which air if lifted adequately will rise on its own due to positive buoyancy. The optimum unstable situation consists and warm and moist air near the surface and cold and dry air in the middle and upper troposphere. UPPER LEVEL DYNAMICS- Lifting due to a jet streak or positive vorticity advection. UPPER RIDGE- Ridge of high pressure that is evident on upper level charts (e.g 500 mb) UPSLOPE- Upslope is a movement of air from a lower to a higher elevation due to forced lifting of land or dynamic uplift that occurs along a slope due to air density differences. UPSTREAM / UPWIND- The direction from which a flow is coming from. A boat moving down a river will be moving downstream but came from upstream. VAPOR DEFICIT- The number of millibars a parcel of air must rise in order to achieve saturation. Air that has a low dewpoint depression (near saturation), will have a low vapor deficit. Air with a low RH will have a high vapor deficit. VAPOR PRESSURE- The amount of pressure (usually expressed in millibars) exerted from molecules of water vapor independent from the pressure exerted from the air. The vapor pressure is a trace to about 4% of the total air pressure at any one time. VEERING WIND- A veering wind is a wind that turns clockwise with height. It is associated with warm air advection. VERTICALLY STACKED- In reference to a low pressure trough that is at about the same location on each pressure surface aloft. This is an indication the low is occluded and decaying. Strong and maturing low pressure systems will tilt with height toward the colder air aloft. VIRGA- Virga is precipitation that evaporates before reaching the earth's surface VIRTUAL TEMPERATURE- The temperature of the air plus latent heat release to the air due to condensation. In air with moisture, the virtual temperature is always greater than the actual temperature. VIS- Visible satellite imagery V-NOTCH- The term V-notch is used to refer to the V-shape of a supercell on radar imagery. Strong upper level winds move moisture downwind on each side of the storm, producing a V shape. The storm represents a barrier to the windflow thus the flow diverges around the edges of the storm and forces precipitation to move downwind. VORTICITY- Any rotation within a horizontal or vertical windflow. VORT MAX, VORT LOBE- Highest value of vorticity. A region of maximum vorticity. A wind flow through a vort max will produce divergence downwind from the vort max. WAA- Warm Air Advection. Horizontal movement of warmer air into the forecast area. WAD- Wind Advisory WARM CORE HIGH- A dome of deep high pressure that originates from low latitude areas. WARM CORE LOW- A low pressure which is deepest at the surface and gradually weakens in the vertical. Warm core lows have warm temperatures near their center due to either surface solar warming. large amounts of latent heat release or subsidence. WARNING- A particular region in which threatening weather is already occurring or is imminent (i.e. Severe Thunderstorm Warning, Tornado Warning, Winter Storm Warning) WATCH- A region of the country in which people should be on the lookout for threatening weather (i.e. Severe Thunderstorm Watch, Tornado Watch, Winter Storm Watch) WARM FRONT- A synoptic scale boundary between warm and cold air. The warm air replaces the cooler air over time. WASHES OUT- A diminishment of a meteorological entity. Examples: The cold front washes out, Precipitation washes out, Shortwave washes out WET BULB TEMPERATURE- Temperature after cooling takes place due to evaporating moisture into the air (at constant pressure). WET BULB ZERO TEMPERATURE- The wet bulb zero is the wet bulb temperature (found by evaporating water and cooling unsaturated air) equal to 0° C. WET MICROBURST- Strong downdraft of 4 km in diameter or less also consisting of heavy rain. WIND SHEAR- A pronounced change in wind speed or direction with height in the vertical or the horizontal. WINDWARD- The side that is facing the on-coming wind WK- Weak WL- Will WX- Weather XPCTD- Expected XTRM- Extreme YDA- Yesterday ZONAL FLOW- A zonal flow is a flow of wind equal or nearly equal to the lines of latitude. ZONE FORECAST- A forecast for a particular region (often a forecast for certain cities, counties or parishes). ZL- Freezing Drizzle ZR / FZRA- Freezing Rain Z-TIME- Z-time is time relative to 0° longitude which runs through Greenwich, England. This time scale uses a 24-hour clock and the time is the same for every point on the earth's surface. A glossary of forecast discussion terms is available at the website given below: GLOSSARY |
