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Area Forecast Discussion Common Terminology Glossary 

Many of these definitions have been simplified somewhat, but this glossary may help users who have little or no meteorology background to better interpret the reasoning behind the current forecast as expressed by the forecaster in his/her Area Forecast Discussion.

ACCAS - Altocumulus Castellanus, small puffy mid level clouds indicative of instability aloft that may mix down to  the surface and lead to afternoon thunderstorms.

ACSL - Altocumulus Standing Lenticular, flying saucer shaped clouds over mountains indicative of strong winds aloft.

ADIABATIC - Describes the heating/cooling of an air parcel due to increasing/decreasing pressure as it sinks/rises in the atmosphere.

ADVECTION - The transfer of a weather element by air movement. Simplified it means cooler air  (Cold Advection CAA) or warmer air (Warm Advection WAA) moving into a region.

BACKING - Winds are shifting counterclockwise.

BAROCLINIC BAND - an area of clouds and/or precipitation caused by a difference in temperature between two air masses. A front is an example of this.

CAA - Cold Air Advection.  This will often stabilize an air mass, and can cause windy conditions.

CAP - "THE MORNING SOUNDING INDICATES A CAP AT 600MB", an inversion aloft that serves to put a lid on instability, making thunderstorms less likely, unless MIXING breaks the cap.

CAPE - Convective Available Potential Energy, a scientific measure of energy available for thunderstorms. No CAPE, no thunderstorms. Lots of CAPE means a strong thunderstorm potential. 

CB - Cumulonimbus clouds, thunderstorms.

CONVECTION - Usually means thunderstorms or showers created by daytime heating.

CONVECTIVE TEMPERATURE - The temperature that daytime heating needs to cause at the surface in order for convection to occur.

CONVERGENCE - Air molecules come together. The convergent area becomes too crowded with air molecules and they must go up or down depending on what level of the atmosphere convergence occurs at. Low level convergence causes lift. Upper level convergence causes sinking in the atmosphere.

CU - Cumulus clouds.

CWA - County Warning Area, NWS Boise's CWA includes most of Southwest Idaho and Southeast Oregon.

DEFORMATION - An area of stretching energy aloft creating clouds or precipitation.

DIFFLUENCE - Usually used in context of upper level flow. Diffluent flow splits apart and causes air from below to rise up and produce lift in the atmosphere.

DIVERGENCE - Opposite of Convergence. Air molecules become too sparse in a divergent area and need to be replenished by having other air molecules rise up from below or sink from above. Upper level divergence causes air from below to rise into the divergent area, causing lift. Low level divergence causes air from above to sink into the divergent area, stabilizing the atmosphere.

DYNAMICS - The forces and motions at play in a weather system. Strong dynamics usually means plenty of energy in the atmosphere to produce precipitation.

FA - Forecast Area. Same as CWA.

HEIGHTS - See 500MB.

INVERSION - A layer of warm air over a layer of cooler air. This warm layer acts as a lid on the atmosphere, preventing interaction between the air above and below the inversion.

LAPSE RATE - The change in temperature as you increase in altitude. A strong lapse rate means an unstable atmosphere.

LIFTED INDEX (LI) - A scientific measure of instability in the atmosphere. A "LI" above zero is stable. A below zero "LI" implies an unstable atmosphere.

MARINE PUSH - An intrusion of low level moist air from the Pacific. Fairly rare in our CWA.

MESOSCALE - A small scale weather system, covering a portion of a state or region, often caused by local terrain.

MIXING - The process of mixing air from aloft down to the surface and vice versa. Can cause gusty conditions as stronger winds aloft mix down to the surface. Usually caused by daytime heating of the surface by the sun which leads to rising and sinking air currents.

500MB - The 500 millibar pressure surface. This is a common level for analyzing upper level weather features. It generally occurs around 18,000 feet above sea level, higher over warm air masses and lower over cold air masses.  The term HEIGHTS ("HEIGHTS RISING AS RIDGE BUILDS") usually refers to the height of the 500MB pressure level. For perspective 1000MB is near sea level, 850MB is around 5000 feet, 700MB is around 9000 feet and 250MB is around 30,000 feet, where the jet stream is found.

NEGATIVELY TILTED - Usually used in referring to an upper level trough. The base of the trough moves out ahead of the rest of the trough. This is a good pattern for severe thunderstorms.

NVA - Negative Vorticity Advection; A kind of transfer of vorticity that under certain conditions causes sinking of air in the atmosphere and stabilization (see VORTICITY).

OMEGA BLOCK - A huge, slow moving upper level ridge shaped like a Greek letter Omega. They are very hard to get rid of and usually block weather systems from moving through, sometimes for weeks.

OROGRAPHIC - Pertains to mountains and how they influence the weather.

POP - Probability of Precipitation.

PRECIPITABLE WATER - The amount of precipitation an air mass could produce if there was lift available to squeeze all the water out of it. High precipitable water means a moist air mass.

PVA - Positive Vorticity Advection; A kind of transfer of vorticity that under certain conditions will cause lift in the atmosphere.

QPF/QPS - Quantitative Precipitation Forecast. "LOW QPF SHOWERS" means showers with not much measurable precipitation.

REX BLOCK - A split flow pattern aloft featuring a ridge to the north over an upper level low pressure to the south.

RIDGE - High pressure aloft, or high pressure at the surface. Dry air masses are found under an upper level ridge. Generally fair weather is found in a surface high pressure ridge.

RAOB - Radiosonde Observation. The balloon sounding of the atmosphere taken at 12Z (morning) and 00Z (late afternoon) every day.

SHEAR - The difference in wind speed and/or direction as altitude increases. As it pertains to thunderstorms, high shear can mean stronger and possibly severe thunderstorms.

SHORT WAVE - A weak and often fast moving upper level trough or ridge. 


SPLIT FLOW - An upper level flow pattern where energy splits between a northern track and a southern track.

SYNOPTIC - Large scale weather systems. Upper level ridges and troughs, large highs and lows.

TCU - Towering Cumulus clouds. If they keep growing they become Cumulonimbus (CB).

THERMAL TROUGH - A surface low pressure trough caused by surface heating which produces rising air and thus lower pressure. Usually an axis of warmest temperatures (THERMAL AXIS). Also called a THERMAL RIDGE which is technically more correct since thermally the temperatures are highest here. THERMALLY INDUCED PRESSURE TROUGH is the proper label, but seldom used because it is too cumbersome to type out.

THETA-E - Equivalent potential temperature. A scientific way of combining moisture and heat which are some of the ingredients needed for convection to occur.

TROUGH - Opposite of a RIDGE. A region of low pressure. Storms form just ahead of an upper level trough. Cooler air masses are generally found under an upper level trough. Fronts are found in a surface pressure trough.

VEERING - Winds are shifting clockwise. Opposite of BACKING.

VORTICITY - A scientific term for measuring the turning or spin around a point in space. Look at a satellite loop and you will see a lot of turning and spinning in the cloud masses. This is vorticity, and certain relationships between vorticity and atmospheric flow produce lift in the atmosphere.

VORT MAX/ VORT SPOKE/ VORT CENTER/ VORT LOBE - All "pieces" of vorticity that are often the cause of regions of lift and thus clouds and precipitation.

WAA - Warm Air Advection.  WAA into an atmospheric layer not only warms the temperature, but can cause upward motion for clouds and precipitation.

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