A cold, stormy winter is here. Five maps show how the jet stream affects forecasts
Although the U.S. will see a milder pattern this week, there's probably cold, stormy weather still in store for us this winter.
Large-scale weather and climate patterns, guided in part by the jet stream, play a major role in the storms and cold outbreaks that define winter weather across the nation.
From the North Atlantic Oscillation to the polar vortex and from "atmospheric rivers" to lake-effect snow, here's a look at some of those patterns and what they mean for our weather.
The recipe for an ice storm
According to Weather.com meteorologist Chris Dolce, "the setup that produces the most damaging amounts of freezing rain usually features fresh arctic air that is fed by northerly winds near the surface, which is then overrun by mild, moist air ushered in on southerly winds from the Gulf of Mexico."
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Disruptions to the jet stream riding over the top of this combination of mild and cold air can produce a zone of freezing rain that can stretch hundreds of miles from west to east.
The North Atlantic Oscillation can bring cold, snow to the East
The North Atlantic Oscillation is an index forecasters track that has both positive and negative phases that vary in degrees over periods of days or weeks, Weather.com said.
"When the NAO goes 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," Dolce said.
The negative phase of the NAO can result in extreme cold and memorable snowstorms for the central and eastern U.S.
Polar vortex can bring a deep freeze to central US
The polar vortex is actually a gigantic, circular area of cold air high up in the atmosphere that typically spins over the North Pole (as its name suggests). It's a normal pattern that's stronger in the winter and usually tends to keep the coldest weather bottled up near the North Pole.
When the polar vortex is "strong," cold air is less likely to plunge deep into North America, Weather.com meteorologist Jonathan Erdman said. The stronger the polar vortex, the milder the USA's winter is.
But occasionally, some of the vortex can break off or move south, funneling freezing temperatures to the southern U.S. and into Europe and Asia.
When the polar vortex weakens or even splits, it allows frigid air to escape and push southward toward the United States, funneled by the jet stream.
'Atmospheric rivers' bring havoc to West Coast
Sometimes called "rivers in the sky," atmospheric rivers are a major factor in extreme rain and snowfall in the West.
Long, narrow corridors of air heavy with moisture form when warmer air from the tropics (lower latitudes) moves toward the poles ahead of powerful storm fronts.
In fact, these "rivers in the sky" are responsible for up to 65% of the western USA's extreme rain and snow events, a 2017 study said. And on average, up to 50% of the annual precipitation on the West Coast occurs in just a few atmospheric river events.
Lake-effect snow can bring blizzard-like conditions
Lake-effect snow, which can last for only a few minutes to several days, develops from narrow bands of clouds that form when cold, dry arctic air passes over a large, relatively mild lake.
As the cold air passes over the unfrozen "warm" waters of the Great Lakes, warmth and moisture are transferred into the lowest portion of the atmosphere, the National Weather Service says. The air rises, and clouds form and grow into narrow bands that produce 2 to 3 inches of snow an hour or more.
These snows typically occur only in the fall or early winter, before the lakes freeze over. (But if the lakes don't freeze, lake-effect snow can occur throughout the winter and into the spring.)