What is the polar vortex? How it can impact snow, freezes in the U.S.
The polar vortex is a large area of circulating cold air above the North Pole. Strong winds keep it contained, but when it weakens, it can wobble and stretch.
- An Arctic blast is expected to bring cold temperatures and potential heavy snow to Michigan and other northern states around Thanksgiving and into early December.
- A rare weather phenomenon, known as a sudden stratospheric warming, could weaken the polar vortex and allow frigid air to move farther south.
- The combination of La Niña and other weather patterns is increasing the chances for cold outbreaks and winter-like conditions across the northern U.S.
- Michigan’s Thanksgiving forecast includes dry but cold conditions, with a growing possibility of lake-effect snow and gusty winds.
Another Arctic blast is on the move toward the U.S., bringing the potential for bitter cold and heavy snow accumulations to Michigan — possibly beginning during the holiday weekend into early December.
It’s unclear whether the forecast event will be as intense as the Nov. 10 system that left up to 18 inches in parts of the Upper Peninsula. However, the cold is unlikely to reach as far south as it did when record-breaking temperatures hit parts of Florida.
“Following a warm start to November in the West and a cold start in the East, the combined effects from (several weather conditions) may usher in a significant pattern change later in the month and into early December,” the Climate Prediction Center wrote. “During Thanksgiving week, a colder pattern is likely for the West and the Northern Plains wild milder air overspreads the East. A transition to a colder pattern is then forecast across much of the central and northern U.S. into early December.
“This pattern change favors a transition to more winter-like conditions across the west-central and central U.S. including much below normal temperatures and the potential for heavy snow,” the CPC added.
According to the Climate Prediction Center, the Madden-Julian Oscillation (MJO) — a moving pattern of thunderstorms in the tropics — can influence how cold or warm air shifts across the U.S., while La Niña — a cooling of the tropical Pacific Ocean — tends to push Arctic air farther south, increasing the chance of cold outbreaks across the northern states.
Combined, the patterns are likely to mean an “increased cold air intrusion across the northern tier of the contiguous United States” over the next couple weeks, the forecast states.
‘An extraordinary and unique event’ could be brewing
Experts say an unusual stratospheric pattern could reshape winter weather across the Northern Hemisphere if it develops.
The phenomenon, known as a sudden stratospheric warming, it’s “the largest type of disruption that occurs to the polar vortex,” climatologist Judah Cohen, a research scientist at MIT, in an email to USA TODAY.
Essentially, the polar vortex acts as a tight rubber band circling the North Pole in the stratosphere, 10 to 30 miles above the surface, keeping frigid Arctic air locked in place. When sudden stratospheric warming weakens it, that band stretches unevenly and becomes wavy, allowing lobes of cold air to slip much farther south. But this doesn’t happen automatically — the jet stream acts as the gatekeeper, or the force reshaping the band, deciding where, or even if, that cold air moves.
So, contrary to what its description may suggest, a weakened polar vortex means circulation around the North Pole isn’t as tight, more likely to spill and bring harsher winter weather to the U.S., while a strong vortex keeps Arctic air contained, leading to milder winter conditions here.
The atmosphere is at a “critical juncture,” and what happens in November could be a “fork in the road” for the rest of the winter, Cohen said.
If this occurs, it would be the first such event in November since accurate satellite records began, he added. There were two known events in the pre-satellite era, November 1958 and 1968.
“I tend not to rely on stratospheric data before satellite data was available,” Cohen said. So if a stratospheric warming event occurs, “that would be an extraordinary, even unique event.”
Michigan weather forecast for Thanksgiving
As of Wednesday, Nov. 19, Michigan’s Thanksgiving forecast calls for dry conditions, though temperatures between 28 and 36 degrees may support the potential for lake-effect snow, which has growing chances of occurring, according to the Climate Prediction Center.
“Anticipated gusty winds combined with snowfall could support blowing snow and resulting decreased visibility, especially across parts of the Northern and Central Plains during the Thanksgiving travel period,” meteorologists wrote in the forecast. “There could be some wrap-around lake-effect snow across the southern Great Lakes but [it is] difficult to determine at this time.”
Across the country, AccuWeather meteorologists are predicting major travel disruptions as a storm system is set to batter much of the western United States and Midwest with heavy snow and cold rain over the holiday, while the Northeast will see mainly rain ahead of Thanksgiving Day.
On the back side of the Thanksgiving storm system, gusty winds are forecast to spread from the Great Lakes into the Northeast, impacting air travel, airports, and high-profile vehicles. Dangerously high wind gusts could affect events like the Macy’s Day Parade in New York City and the Dunkin’ Donuts Thanksgiving Day Parade in Philadelphia, grounding some balloons.
What does the polar vortex look like right now?
Right now, charts show the polar vortex is unusually weak — similar to its strength in early autumn — and is likely to send temps plunging across the U.S. beginning early next week.
This weakness is linked to very low winds high above the Arctic, called zonal winds, which are now declining and approaching 0 m/s, a threshold that can trigger an event called major sudden stratospheric warming (SSW). These events often lead to significant cold outbreaks and even blizzards.
(Note about graphs: The dashed 0 m/s line marks the boundary between westerly winds (above the line), which represent a normal, intact stratospheric polar vortex, and easterly winds (below the line), which indicate a reversal of the vortex and meet the definition of a major sudden stratospheric warming if sustained. In the initial graph, on Nov. 1, the forecast members (blue lines) are at the top of the chart, signaling a strong polar vortex. In the latter graph, many forecast members (blue lines) dip toward or below 0 m/s in late November, showing the potential for a significant weakening or even reversal of the 10 hPa zonal winds, compared with the typical climatological values shown in red.)
The vortex’s instability is also influenced by the Quasi-Biennial Oscillation (QBO), a stratospheric wind pattern that shifts between easterly and westerly roughly every 28–29 months. When the QBO is in its easterly phase, as it is now, the vortex is more prone to wobble south.
Meanwhile, a weak La Niña in the lower atmosphere is shaping winter patterns as well. While it doesn’t directly affect the polar vortex, it can shift the jet stream, influencing where Arctic air travels and how long cold outbreaks linger. La Niña may also leave large bodies of unfrozen, increasing the risk of lake-effect snow in the Great Lakes when Arctic air moves over open water.
What is lake-effect snow?
Lake-effect snow typically happens when cold, dry Arctic air moves over a large, relatively warm lake, creating narrow bands of clouds that can dump snow anywhere from a few minutes to several days.
Wind direction also matters a lot — it can mean heavy snow in one town while a sunny mile or two away sees nothing:
- Westerly winds: These are most common across the central and southern Great Lakes region. When cold air moves from the west over Lakes Michigan or Superior, it picks up moisture and warmth from the water. As the air reaches land, that moisture condenses, producing localized snow showers.
- Northwesterly winds: These can enhance snow totals in northern and eastern Michigan, including the Upper Peninsula and areas along the Lake Superior and Lake Huron shorelines.
Michigan weather watches and warnings
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Brandi D. Addison covers weather across the United States as the Weather Connect Reporter for the USA TODAY Network. She can be reached at baddison@gannett.com. Find her on Facebook.
