Glossary Of General Weather Terms

Hello everyone!

In an effort to break down the long list of terms you may see in my discussions or in the discussions of others, here is a list of the terms you will find used in both the winter and the summmer but are not partial to either one. For terms specific to each season (cold season and warm season) please use the menu to navigate to the other pages.


AFD: Area Forecast Discussion. A product issued twice a day by the National Weather Service that gives a technical summary of the weather to come. Useful for weather geeks, not so much for everyone else.

Baroclinic Zone: An area that features a strong temperature differential over a relatively small area. These baroclinic zones, sometimes called barolinic leaves, are where storms like to form and intensify.

CAA: Cold Air Avection. The process of advecting cold air into the region from Canada. Usually this takes place after fronts or storms and involves gusty NW winds and upslope snow showers.

Cutoff Low: A low pressure system that becomes detached from the jet stream and meanders in circles for a long time causing generally bad weather for nearby areas.

Cyclogenesis: Technically, this is just the formation of any type of low pressure system although it is more often used to describe the rapid deepening of a low.

Deepening: Refers to the process of a low pressure system strengthening.

Downsloping: The process of drying out the atmosphere by squeezing out moisture over mountains. When moist air gets pushed over mountains, the moisture gets squeezed out and areas downwind of the mountains are left with dry air. Opposite of upsloping.

Dry Slot: A wedge of dry air that can be found usually to the SE of a low and causes a break in precip. Dry slots can occasionally be found to the NW of a low as well.

ENS: Ensemble Members. Each model has ensemble members which are like mini models run with very similar algorithms to their parent model but each one has a small tweak in the input or algorithms which allows for a more comprehensive synopsis of possible outcomes.

FROPA: Frontal Passage. Used as shorthand especially by the NWS to convey the passage of any type of front.

Jet Streak: An small, but very strong area of winds embedded within the larger jet stream. Jet streaks are responsible for some severe thunderstorm formation.

Mesolow: A small, highly localized area of low pressure that often brings enhanced precipitation as well as locally high winds. They rarely form or cause significant impacts on their own, instead, they often happen within the larger context of a synoptic scale system.

Mesoscale: Small scale, localized. Usually referred to weather happening over a small but not tiny area. A sea breeze that impacts the whole coast would be a mesoscale event.

Mesoscale Discussion: Issued by the SPC (Storm Prediction Center) for localized events such as severe weather, small scale banding in blizzards, or flooding.

NWS: National Weather Service. The official issuer of watches/warnings/advisories etc. ALWAYS consult with the NWS forecast to make important decisions and to get the best possible forecast.

OTS: Out To Sea. An acronym used when a storm passes harmlessly offshore and brings no impacts to the region. Storms that take this type of track are also sometimes called fish storms.

Overrunning: The process of warm air riding up and over cold air ahead of the warm front. This often causes stratiform precipitation.

QPF: Quantitative Precipitation Forecast. This is a measure of the total amount of liquid that falls from the sky during any kind of precipitation event.

Upslope Snow/Rain Showers: Happen when moisture in the lower levels is forced to rise because of mountains. When the warm, moist air rises into colder air, precipitation occurs. Basically, it is precipitation caused by the mountains squeezing the moisture out of the air like a sponge.

WFO: Warning Forecast Office. Local NWS subsidiaries that provide forecasts for specific locations as well as issue warnings for their CWA (County warning area)

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Reliably hype-free weather info for Western Maine and New Hampshire from amateur forecaster Jack Sillin

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