5 Of Winter's Extreme Weather Patterns Meteorologists Watch For

S​now, ice and cold temperatures spread across the United States each winter, but sometimes those impacts reach extremes that are often driven by multiple large-scale weather patterns.

Here are five of the extreme patterns meteorologists watch for in computer model guidance to alert that winter weather is about to shift into higher gear.

1. Arctic cold building in the heart of winter in Alaska, Canada's Arctic region and even Siberia: The cold air in those source regions doesn't always pour south of the Canadian border, but when it does, the continental U.S. usually experiences its most extreme cold outbreaks of the winter.

The frigid air from those regions is typically dislodged when the jet stream builds northward over western North America. As a result, a southward plunge of the jet stream pushes an arctic cold front across the central and eastern U.S., ushering in the shivering – and, in many cases, subzero – temperatures.

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2. There's a weather pattern near Greenland that can favor East Coast winter storms: The North Atlantic Oscillation (NAO) is an index that forecasters track; it has both positive and negative phases that shift to various degrees over periods of days or weeks.

When the NAO goes strongly negative, the jet stream plunges southward over the eastern U.S. because of what's called atmospheric blocking, where the block often takes the form of a strong bubble of high pressure near Greenland.

It's only one factor, but that blocking can help intensifying low-pressure systems crawl up near or just off the Eastern Seaboard with snow, rain, wind and coastal flooding instead of scooting harmlessly out to sea.

One of the best recent examples was in March 2018 when the NAO slipped into a negative phase for about three weeks, triggering the formation of four nor'easters.​

3. Repetitive atmospheric rivers are one of the most extreme weather patterns for the West Coast: An atmospheric river is a narrow plume of moisture extending from the tropics or subtropics that Pacific storm systems can tap into as they plow inland. They form many times from fall through early spring, but their potency can vary from weak to exceptionally strong.

From late December 2022 to early March 2023, a series of storms tapping into atmospheric rivers brought widespread drought relief to the state, but they also contributed to destructive flooding and incredible snowfall. The atmospheric river-fed storms caused $4.7 billion in damage and 22 deaths, according to NOAA.

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4. Major ice storms are among the most high-impact winter weather events: Thick accumulations of freezing rain can be extremely damaging to trees and power lines because of the weight added to those and other surfaces.

The setup that produces the most damaging amounts of freezing rain usually features fresh arctic air fed by high pressure, which is then overrun by mild, moist air ushered in by southerly winds from the Gulf of Mexico.

Jet stream disturbances riding over the top of this battleground between cold and mild air can create a large area of freezing rain that sometimes extends west to east for hundreds of miles.

Icing impacts from the freezing rain can range from a nuisance, like isolated slick roads, to crippling, with power outages lasting for days and severe tree damage depending on the longevity of the freezing rain, temperatures and whether gusty winds are present.

5. Watching for the lake-effect snow machine to crank up and last for days: Lake-effect snow downwind of the Great Lakes is common from late fall through the heart of winter. Sometimes these events are short-lived, while others can hammer the same areas for days at a time.

Meteorologists take extra notice when a familiar setup for heavy lake-effect snow is projected by computer forecast models.

The pattern features a sharp, persistent southward plunge of the jet stream anchored over the eastern U.S. by a gyre of low pressure over or near Canada's Hudson Bay. In this pattern, repeated rounds of cold air spill over the Great Lakes and manufacture bands of heavy lake-effect snow that can persist for several days in spots, piling up in totals measured by the foot.

December 2022 was a reminder that lake-effect snow can create serious impacts. A blizzard-inducing snow band from Lake Erie waffled over Buffalo, New York, for almost 72 hours around the Christmas holiday, dumping up to 51 inches of snow on the metro area, burying roads, trapping motorists and shutting down the city's airport.

Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.