Glossary Of Cold Season 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 the winter time when talking about winter weather phenomena. While these terms are most often used in the cold season, some of them may appear at other times as well.


Alberta Clipper: A weak, usually moisture starved system that dives SE then E from western Canada through the great lakes and into New England. Sometimes, they will take a more southerly track and can occasionally help to spawn Nor’easters. More often, their effects on our weather are limited to clouds or light snow.

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.

Blizzard: A blizzard only occurs when a location experiences sustained winds or very frequent gusts over 35 mph in combination with heavy falling or blowing snow which reduces visibilities to 1/4 mile or less. Both of these criteria must be met for three consecutive hours for a storm to be officially a blizzard.

Blocking: When high pressure systems downstream (in the path of) of a storm act like a wall and block off the ability of a storm to move in its usual direction, the high pressure system is referred to as ‘blocking’ since it blocks the path of the low. Lots of blocking, especially near Greenland, usually results in longer storms that dump more precipitation.

Bombogenesis: A word thrown around a lot because of its scary sound, this is actually a technical meteorology term. In order to undergo bombogenesis, a storm’s central pressure must drop 24mb in 24 hours.

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.

CAD: Cold Air Damming. A phenomenon which occurs when weak northerly surface winds across Maine and New Hampshire hold shallow cold air in place resulting in freezing rain or sleet. CAD situations are often underestimated by the models and models almost always have a warm bias during these events.

Coastal Front: A phenomenon that occurs when warm air blowing in off the ocean hits a barrier in the form of cold dense air locked in over land. The coastal front often determines ptypes near the coast.

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.

Dynamic Cooling: Occurs when strengthening storms produce strong upward motion and ‘manufacture’ their own cold air. Look for dynamic cooling when there is lots of upward motion in areas with heavy precip on the NW flank of a storm.

Freezing Rain: Precip that falls as rain but freezes on contact with any solid surface.

Frontogenesis: The strengthening of the thermal gradient with height. Intense frontogenesis usually favors intense banding of precip.

Inverted trough: Wind shift axis extending to the NW of a low that brings locally heavy precip. See Norlun Trough.

Nor’easter: A low pressure system that moves North East off of the North East coast of the US while strengthening and bringing a prolonged period of North East winds to the area. Nor’easters can also move across land or do very odd things like little loops or stalls but these are pretty rare.

Norlun Trough: An axis of shifting winds on the NW side of a low. These monsters are wicked fickle and are very hard to forecast. 30 miles can make the difference between 1″ and 11″ and it’s hard to tell where exactly they’ll set up.

Ocean Effect: Precipitation or clouds formed by cold air traversing the warm ocean (all terms relative 🙂 ).

Ocean Enhanced: Enhanced banding within a large-scale precip shield that comes from cold air passing over the warmer ocean.

Omega Block: A type of blocking high whose shape resembles the greek letter omega.

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.

Sleet: Frozen icy pellets that make lots of noise when they fall. They form when snow melts in a warm layer at 700mb (roughly 10,000 feet) and then refreezes in lower level cold air. NOT HAIL.

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.

Vertical Velocity: The amount of upward motion in the atmosphere. Think of it as how fast air is moving up and down.

Wet Bulbing: A cooling effect seen when winds from any direction pass over wet air. This is often seen when moisture from a storm arrives in the area and temps drop a few degrees since winds pass over air with lots of moisture in it. This is a very important factor in determining precip type in early and late season storms.


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

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