What’s a polar vortex and why is it menacing Michigan?

What's a polar vortex and why is it menacing Michigan?

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What's a polar vortex and why is it menacing Michigan?

As the Great Lakes region braces for a predicted near-record cold this weekend, Michiganians are again cursing the dreaded "polar vortex."
But what exactly is this meteorological menace? And why is it making Michigan's unpredictable winter weather unbearable?
The answer starts at the North Pole.
Swirling, cold air from the Arctic Circle (also known as the polar vortex) is usually contained to that part of the globe. But every so often, the system that holds it there weakens, pushing it southward.
"The polar jet stream usually contains that Arctic air to the north," said Kevin Kacan, a meteorologist at the National Weather Service's Detroit regional office. "But when there's a disturbance that breaks that northern track of the jet stream, it allows air to sink down into the northern U.S."
In this case, experts said a warmup in Earth's stratosphere dislodged a polar, low-pressure system, bringing frigid air from the North Pole southward.
In the Detroit region, the meteorologists at the National Weather Service in White Lake are forecasting a high near 8 degrees Fahrenheit on Friday, a low of -10 degrees Friday night; a high of 8 again on Saturday and a low around -2; and a high near 15 on Sunday and a low around 3 on Sunday night.
The wind chills will make the conditions feel colder.
The frosty temperatures will rival the coldest daily highs ever recorded in Detroit for Jan. 23 and Jan. 24, which are 6 and 4 degrees, respectively.
In an average year, Jan. 23 and Jan. 24 reach highs of about 32 degrees, according to the weather service's data.
It will be much colder in Michigan on Friday than the forecast for Point Barrow, Alaska, the northernmost point in the United States, where the high is projected to reach a relatively balmy 27 degrees, according to the NWS.
The forecast for the Upper Peninsula is more grim. Parts of the Upper Peninsula likely won't get much warmer than minus-5 degrees during the day on Friday. Overnight wind chills are expected to plummet to 35 below zero.
How cold is minus-35 degrees? That's about how cold it felt Wednesday, Jan. 21, at the Amundsen-Scott station at the South Pole, according to AccuWeather.
The National Weather Service's Kacan said it's difficult to quantify exactly how much of an impact the vortex has on Michigan's frigid temperatures, especially given that late January through early February is, on average, the coldest time of year in Michigan.
The meteorologists said the polar vortex system is set up due north of Michigan over Canada's Hudson Bay. It's at least partly responsible for the frigid temperatures around the region that contributed this week to a 100-vehicle pileup in southwest Michigan and led the city of Detroit to declare a cold weather health emergency.
But the vortex is not the only phenomenon playing havoc with Michigan's weather conditions.
Weather experts said a weak La Niña system, in which relatively cool surface temperatures on the Pacific Ocean can bring slightly colder and more precipitous conditions eastward across the U.S., also has contributed to the recent cold snap.
"In terms of snowfall this year for Metro Detroit, we are above average for the season," Kacan said. As of Wednesday night, the region has received 27.9 inches of snow compared with an average of 19.2 inches.
Polar vortexes, also called Arctic outbreaks or Arctic air intrusions, are fairly common weather events, experts said. According to the National Weather Service, they generally take place "every other year or so."
But they aren't often felt as far south as Michigan. The weather service said similar cold outbreaks felt throughout much of the U.S. took place in 2014, 1989, 1985, 1982 and 1977.
"We definitely see cold, Arctic intrusions push down," said Cory Behnke, another meteorologist at the Detroit regional office. "It varies from winter to winter, year to year, as far as how far south it goes.
"This weekend it's pushing farther southward than anything we saw last year."
mreinhart@detroitnews.com
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