UNRAVELING THE AREA FORECAST DISCUSSION
A Glossary and Primer of Weather Terms used in National Weather Service Forecast Discussions
What exactly is the purpose of the Area Forecast Discussion
The Area Forecast Discussion (AFD) is generally issued before each regularly scheduled public forecast issuance (twice daily) by each National Weather Service Forecast Office. Intermediate AFDs may be issued as time permits and significant changes/updates have been made to the forecast. The main purpose of the AFD is to explain the technical reasoning behind the forecast issued/about to be issued by NWS field personnel. Its primary audience is other NWS offices with a goal of enhancing coordination amongst forecast offices. The AFD is also released to the general public for informational purposes. It is not a substitute for the forecast.
Numerical Model Terms:
The main numerical forecast models used by National Weather Service forecasters are:
Nested Grid Model (NGM): The NGM model is a grid-point model that forecasts meteorological variables (temperature, wind, humidity) at various grid points in the models' forecast domain (which consists of North America and parts of the Pacific and Atlantic Oceans). The model provides forecast output out to 48 hours, and is run twice daily after the 00Z and 12Z upper air data is in. NGM model data is also called RAFS data (for Regional Analysis and Forecast System).
ETA Model (ETA): The ETA model, like the NGM, is a grid-point model. The ETA is a relatively new model which provides forecast output to 60 hours and uses the eta coordinate system in the vertical, as opposed to pressure coordinates. The Eta model is run at much higher horizontal resolution that the NGM model, and is currently run at 32 km grid spacing over its entire domain. Many local offices run a version of the Eta model at much higher resolution than is available otherwise. These mesoscale forecasts are experimental in nature, but may be referred to from time to time in the AFD.
Global Spectral Model, Aviation Run (AVN), Medium Range Forecast (MRF) Model: The AVN, unlike the NGM and ETA, is a spectral model, which means that meteorological variables are represented as waves around the hemisphere, and not as values on grid points. The AVN is a hemispheric model, and is used in overseas aviation forecasting and pilot briefing. The AVN provides forecast data out to 72 hours and is run twice daily. The Medium Range Forecast (MRF) run of the Spectral model is run out to 240 hours (10 days) and is used in longer term weather prediction. The MRF is only run once daily, usually around 0600Z, with a model initialization of 0000Z.
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.
Model Output Statistics (MOS) Guidance: Output from a numerical model can be correlated with other data (such as surface observations and climatic data) in an equation to produce forecasts of temperature, probability of precipitation (POP), winds, cloud cover and heights, visibility, and others. These forecasts are known as Model Output Statistics, or MOS guidance, and are available for nearly 300 stations across the United States. NGM MOS guidance is referred to as FWC guidance, and AVN MOS guidance is called FAN guidance. There is no MOS guidance for the ETA model. Forecasters usually use this data as a "starting point", for a temperature or probability of precipitation forecast, and will adjust the numbers as he/she feels fit (i.e., FWC guidance may be predicting high temperatures that appear to be too cool based on expected conditions, so the forecaster will adjust them upward).
Another type of numerical model output is known as FOUS data (Forecast Output for the United States). This data is usually referred to as FRH data (ETA model) or FRHT data (NGM model) in forecast discussions. FOUS data output indicates what the model is forecasting, in six-hour increments, for values of different variables at a given station through 48 hours.
Forecasters compare the output (or solutions) of each model for consistency, and to search for model biases that can produce an incorrect forecast. When the model solutions begin to drift apart and become different from one another, it is said that the model solutions are diverging. It is then up to the forecaster to decide which model solution is the correct one to follow. Forecasters will sometimes use a compromise solution when two models are showing two different forecasts, and a third model has a solution "in-between" the two extremes or a compromise between the other two models.
Meteorological Terms used in Discussions:
The terms defined below are spelled out in full, with common contractions for those terms in parentheses.
Advection: (ADVCTN) Refers to the transport of something by the wind from one area into another. Moisture advection (wind bringing in higher values of moisture), and temperature advection (wind bringing in warmer or cooler air) are two examples. Cold Air Advection: (CAA) (See also Advection, Thermal Troughs) Cold air advection is simply the wind bringing colder air into an area. Associated with downward motion and subsidence. In addition, cold air advection can have an influence on atmospheric stability. Upper level (500 mb and above) cold air advection is a destabilizing mechanism, which can create an environment that would support convective type precipitation (i.e. thunderstorms). Low level (850 mb) cold air advection over the Great Lakes destabilizes the lower atmosphere, allowing lake effect rain and snow showers to develop provided the difference between the water temperature and the 850 mb temperature is large enough (13 degrees Celsius or more). These two examples are not meant to imply that cold air advection is a lifting mechanism. Rather, CAA helps produce an environment favorable for the development of these types of precipitation (both of which are convective type precipitation).
Amplification: (AMPLFCTN) Building, or sharpening, of an upper level high pressure ridge or low pressure trough.
Baroclinic: (BRCLNC) A region in the atmosphere where isotherms (lines of constant temperature) cross height contours at any angle, and is usually referred to as a baroclinic zone. These are areas of temperature advection, and thus are normally locations of vertical motion. (See also Advection, Warm Air Advection, Cold Air Advection).
Barotropic: (BRTRPC) Normally used in reference to a region of the atmosphere where isotherms (lines of constant temperature) run parallel to height contours on a constant pressure surface, thus there is no temperature advection. Technically, this is an equivalent barotropic situation. In a true barotropic atmosphere, constant pressure surfaces have a constant temperature, that is, the temperature everywhere on the surface is the same. Therefore, in a pure barotropic atmosphere, there would not be any isotherms on a constant pressure chart. (See also Advection).
Blocking Pattern or Blocking High: (BLCKG PTTN) A pattern which involves a strong upper level high pressure ridge. These ridges are quite strong and tend to move little, thus blocking the mean flow pattern. Storm systems must travel up and around these blocking ridges, thus, areas located underneath a blocking ridge can experience several days of dry weather. There are two main types of blocking patterns: Omega Blocks involve two upper level lows on either side of an upper level ridge (picture one upper low off the west coast, one off the east coast, and a ridge over the Midwest). Rex Blocks involve an upper level high directly north of an upper level low.
CAPE: Stands for Convective Available Potential Energy. CAPE is another type of stability index, and since it is a measure of energy, it has units of Joules/kilogram (J/kg). The higher the CAPE value, the more unstable the atmosphere is and the better able it is to support strong and severe thunderstorm activity. Values over 1000 J/kg are usually considered significant, but there is no "magic" value of CAPE which will allow thunderstorms to develop.
CIN: Stands for Convective Inhibition. CIN is simply the amount of negative energy the atmosphere must overcome to achieve a state of free convection, or thunderstorm development. Its units are also in Joules/kilogram.
Closed Low: (CLSD LO) A low pressure center having a closed circulation, which is used in reference to systems in the upper levels of the atmosphere. Closed lows that become cut off from the main flow pattern are called cut-off lows.
Condensation Pressure Deficit: (COND PRES DEF) (See also Isentropic Lift). 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.
Convergence/Confluence: (CNVGNC/CNFLNC) An area in the atmosphere where air flows together. In the lower levels of the atmosphere (generally below 550 mb), convergence implies rising motion, and subsequent clouds and precipitation if enough moisture is present. Above 550 mb, convergence implies downward motion, which results in clearing and drying of the atmosphere. (See the definition of Divergence/Difluence for an explanation of why the 550 mb level is important).
Confluence is height contours coming closer together on a constant pressure chart, which implies convergence. (See also Divergence/Difluence).
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.
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 Heating Boundary: (DFRNTL HTG BNDRY) A small scale "cold-frontal" type boundary that results from unequal surface heating. To understand how this comes about, consider a morning with varying cloud coverage between Lansing and Detroit. Lansing remains under cloud cover all morning, and by noon has a temperature of 58 degrees, while Detroit has sunshine during the morning and a temperature at noon of 70 degrees. This temperature (and resulting pressure) difference can result in the development of a small scale line of convergence, similar to a front. This can be a triggering mechanism for thunderstorms during the warm season.
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.
Divergence/Difluence: (DVGNC/DIFLNC) An area where air is moving away (diverging from a point), the opposite of convergence. Divergence in the low levels (below 550 mb) implies sinking motions, while divergence in the upper levels (above 550 mb) implies rising motion.
An atmospheric law known as Dines' Compensation Principle states that since air cannot be created or destroyed, there must be a level of non-divergence in the lower atmosphere (usually averages out to be at 550 mb). What this means is that divergence above this level of non-divergence has to be compensated by convergence in the lower levels (thus rising motion), and convergence above the level of non-divergence must be compensated by divergence in the lower levels (sinking motion). Thus, divergence and convergence occur simultaneously in the atmosphere.
Difluence is the spreading apart of height contours on a constant pressure surface and implies divergence. (See also Convergence/Confluence).
Diffluent Thickness Pattern: (DIFLNT THKNS PTTN) (See also Thickness). 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. See Figure 8.
Dynamics: Refers to atmospheric phenomena which cause upward vertical motion, most often attributed to positive vorticity advection, convergence of Q-Vectors, or jet streams. (See Positive Vorticity Advection, Q-Vectors, Jet Streak Quadrants).
Entrained or 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.
Equivalent Potential Temperature: (THETA-E) The equivalent potential temperature (or theta-e) of an air parcel is the temperature that parcel would have if it were raised from some reference level (850 mb is a popular one) until all of its moisture is condensed out, and then brought back down to the 1000 mb level. The theta-e of a parcel can be changed by adding or removing moisture or heat. Forecasters look for low level theta-e ridges, or an axis of high theta-e, which indicates areas of higher moisture and potential energy for a storm system to tap. Organized convection generally tends to develop within a theta-e ridge. Theta-e ridges are also important in identifying potential heavy snow areas within developing winter storms.
Frontogenesis: The initial formation of a frontal zone. The process of frontogenesis produces upward vertical motions around the front.
Height Fall Centers: Height changes (the 12-hour change in the height of a pressure surface at a station) are plotted on each new upper air chart, and height fall centers, or the area of maximum height falls, can be plotted (common practice on 500 mb charts). The movement of these centers can be used to forecast the short term (12 hours or less) movement of upper level low pressure centers, since upper lows tend to move along and to the left of the track of the height fall center. Height falls also imply low level convergence and thus rising motion. Height falls are given in meters or decameters (1 Dm = 10 m).
Helicity: 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. (See also Shear).
Isentropic Lift: (ISENT LFT) (See also Warm Air Advection) In the atmosphere, unsaturated air parcels (i.e., relative humidity less than 100 percent) are "bound" to surfaces called isentropic surfaces. Parcels move along these surfaces, which have ridges and troughs similar to constant pressure analyses. The main difference is that parcels move vertically through pressure surfaces, not along them as would be implied on a constant pressure analysis. Isentropic charts can "show" three-dimensional air motions, while areas of vertical motion must be inferred on constant pressure charts. Forecasters normally refer to isentropic lift as occurring when warm air is overriding cooler air in the lower levels (a process also called overrunning). Isentropic upslope motion refers to rising motions, and isentropic downslope means sinking motion. Isentropic lift and lift due to warm air advection are terms often used interchangeably.
Since isentropic surfaces are surfaces of potential temperature (THETA), they are labeled in degrees Kelvin, such as 302K or 312K. The surface chosen by a forecaster for analysis depends upon the season.
Jet Streak Quadrants: (LFQ and RRQ) A jet streak is a segment of the jet stream that contains higher wind speeds than the jet stream as a whole. Air flowing through these jet streaks is moving much faster than the jet streaks themselves. There are many of these jet streaks winding through the main upper level jet stream, and each of these jet streaks can be divided up into quadrants, two quadrants in the entrance region (where air enters the jet streak) and two in the exit region (where air flows out of the jet streak). These jet streaks have their own areas of convergence and divergence, and these areas are located within the four quadrants. Since the jet stream (and its jet streaks) is found at or above 300 mb, the quadrants where divergence occurs are where upward motion occurs (See the explanation of Dines' Compensation Principle in the definition of Divergence/Difluence). Divergence occurs in the Left Front Quadrant (LFQ) and Right Rear Quadrant (RRQ) of a jet streak. Rising motion due to this divergence is part of what is called a direct circulation, with, for example, rising motion under the divergent LFQ, and sinking motion under the convergent RFQ (right front quadrant). Vertical motion is thus enhanced in areas underneath the LFQ and RRQ.
Lifted Index: (LI) Like CAPE, Lifted Index is a measure of instability in the atmosphere. Negative values are generally considered unstable and conducive to thunderstorm development in the presence of a lifting mechanism.
Low Level Jet: (LLJ) A low level wind maximum usually found in the vicinity of the 850 mb level, important in transporting warm air and moisture northward from the Southern Plains and Gulf region, which in turn enhances instability and thunderstorm potential. Also is a region of upward motion since it is normally associated with warm air advection.
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. .
Negatively Tilted Trough: (NEG TILT TROF) A low pressure trough in the upper levels whose horizontal axis tilts westward with increasing latitude. Upward vertical velocities are usually enhanced when a trough becomes negatively tilted. Positively Tilted Trough: (POS TILT TROF) A trough of low pressure in the upper levels whose horizontal axis tilts eastward with increasing latitude.
Occlusion: (OCCLN) The weakening stage in the life cycle of a mature storm system. A storm system that is occluding is said to be "stacked" since the low pressure centers from the surface upward are right on top of one another. In a strengthening storm, the system tilts westward with height (surface low is farther east than its corresponding low at 500 mb).
Open Wave: A wave of low pressure that does not have a complete circulation around it; also called a short wave trough. See Figure 6. (See also Closed Low, Short Waves).
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.
Positive Vorticity Advection (PVA), Differential Positive Vorticity Advection (DPVA): (See Advection, Vorticity, Negative Vorticity Advection). Advection of higher values of vorticity into an area by the wind. In dynamic meteorology, one of the mechanisms resulting in upward motion is differential positive vorticity advection (DPVA), which simply is positive (cyclonic) vorticity advection increasing with height (can also be negative vorticity advection decreasing with height). PVA at 500 mb has been shown to correlate with differential positive vorticity advection about 85 percent of the time, thus, PVA at 500 mb can imply an area of rising motion. Negative Vorticity Advection: (NVA) Advection of lower values of vorticity into an area by the wind. In dynamic meteorology, one of the mechanisms of sinking motion is differential negative vorticity advection, which is an increase in negative (anti-cyclonic) vorticity advection with height (or decreasing positive vorticity advection with height). It has been shown that NVA at 500 mb correlates with differential negative vorticity advection about 85 percent of the time, thus, NVA at 500 mb implies an area of downward motion.
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. Negative Isothermal Vorticity Advection: (NIVA) Advection of lower values of vorticity into an area by the thermal wind on a map of vorticity and thickness contours. Implies downward motion. (See also Advection, Vorticity, Thickness, Positive Isothermal Vorticity Advection).
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.
Pressure Falls: (PRESFLS) An area where air pressure is falling. Contours of pressure falls (usually in inches or millibars per hour) can be used to predict where a surface low pressure center will move in the short term. Also used during the convective season, as concentrated areas of pressure falls can indicate areas of small scale convergence, important in determining where thunderstorms may develop.
Profilers: 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. An experimental wind profiler network is in place over the Midwest, and this data is frequently referred to in forecast discussions from neighboring states. Some wind profiler sites mentioned in the discussions include Blue River, WI (BLR), Wood Lake, MN (WDL), Slater, IA (SLA), and Neligh, NE (NLG).
Progressive Flow or Progressive Pattern: An upper level flow pattern in which storm systems move along at a fairly regular pace.
QPF: Stands for Quantitative Precipitation Forecast, or simply the amount of rain or snow forecast to fall.
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.
Shear: The change in wind speed and/or direction with distance. Shear can refer to horizontal shear or vertical shear.
Short Waves: (S/WV) (See also Vorticity Maximum) A wave or disturbance in the main atmospheric flow. Short waves (or short wave troughs) contain areas of rising and sinking motion, and can produce clouds and precipitation. Waves passing over slow moving or stationary fronts can also induce the development of low pressure centers (or waves) on the front. Vorticity maximums are also referred to as short waves. These waves move through the main flow pattern (called the long wave pattern). The wavelength of a short wave can vary from 1 to 40 degrees longitude, while long waves vary from 50 to 120 degrees longitude in wavelength.
Streamline Analysis: An analysis of wind direction.
Subsidence: (SBSDNC) Sinking motion, usually associated with clearing and drying.
Thermal Axis/Thermal Trough: A thermal axis (or Thermal Ridge) is an axis or an area of warmer temperatures. In low levels, it serves as a destabilizing mechanism important during thunderstorm season. A thermal trough is an axis of colder temperatures. In upper levels, it also serves as a destabilizing mechanism. In the cold season over the Great Lakes, thermal troughs in the lower levels (i.e. 850 mb) can enhance the temperature difference between the water and 850 mb, thus creating a more unstable low level environment, which is what is required to produce lake effect clouds and precipitation.
Thermal Pattern: Temperature, or isotherm, pattern on a weather chart.
Thermodynamics: Usually used in reference to atmospheric stability and the vertical temperature and moisture profile.
Thickness: The distance, in meters, between two pressure levels. This distance is directly proportional to the mean temperature of the layer. The colder the layer, the smaller the thickness. This is important in temperature forecasting, and in forecasting precipitation type since thickness values give an idea of what the low level vertical temperature profile will be. Important threshold thickness values for snowfall are:
Critical Thickness Values associated with snow
1000-500 mb 5400 m or less (referred to as the "540 line")
1000-850 mb 1300 m or less
850-700 mb 1540 m or less
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.
UVV's: Stands for Upward Vertical Velocities. Can't have much weather without them, since rising motion is what gives rise to most cloud cover and precipitation.
VVEL's: (VV) Stands for Vertical Velocities. Also mathematically referred to as Omega.
Vorticity: Simply put, the measure of rotation of an air parcel about a vertical axis. A parcel rotating clockwise is said to have negative vorticity, and a parcel rotating counterclockwise is said to have positive vorticity. There are two types of vorticity; shear vorticity, which arises from changes in wind speed over a horizontal distance, and curvature vorticity, which is due to turning of the wind flow.
Vorticity Maximum: (VORT MAX) An area of maximum positive vorticity. The terms vort max and short wave are often used interchangeably. Areas downwind of a vort max experience positive vorticity advection (and rising motion), while areas upwind of a vort max experience negative vorticity advection (and sinking motion).
Vorticity Lobes: (VORT LOBE) (See also Positive Vorticity Advection) An axis of higher vorticity values. Areas downwind of a vort lobe experience positive vorticity advection.
Warm Air Advection: (WAA) (See also Advection, Isentropic Lift) The wind bringing warmer air into an area. Associated with rising motion, and clouds and precipitation if enough moisture is available. Most commonly referred to as overrunning, since warm air rides over the top of denser low-level cool air. Lift due to warm air advection is also referred to as isentropic lift.
Water Vapor Loop: A loop of special infrared satellite images that show areas of mid and high level moisture, as well as areas of drying. Useful for locating short waves and jet streaks.
IR Loop: Infrared Satellite images which allow forecasters to see cloud top temperatures. Very useful for detecting clouds at night, which are generally colder than the ground.
Zonal Flow: Upper level flow that is essentially west to east and is usually quite fast. Can have many subtle (small) short wave troughs within it.
This glossary was adapted from one developed for FAA Flight Service personnel by John Boris of NWS APX.