U.S. is shivering through polar vortex: The sequel

A photographer braves the cold to shoot photos along Lake Michigan, Monday, Jan. 5, 2015, in Chicago.  

And you thought the polar vortex was so last year, eh?
In an event that has many similarities to 2014's record cold outbreak, including a southerly wobble in the polar vortex itself, dangerously cold wind chills have forced authorities to cancel school on Wednesday in typically cold-tolerant cities such as Minneapolis and Chicago.
The Twin Cities may stay below zero all day Wednesday, with wind chills well below zero, which echoes last January, when there were three such days in Minneapolis.
The temperature in Chicago at 9 a.m. ET was hovering at the big goose egg (0 degrees Fahrenheit), with a forecast high of just 2 degrees. If the high temperature is 3 degrees Fahrenheit or below, the city will tie or break its daily temperature record for Jan. 7. A wind chill warning was issued for the "windy city" due to wind chills that were dipping as low as minus-30 Fahrenheit, which is cold enough to cause frostbite on exposed skin in just a few minutes, according to the National Weather Service.
According to a computer model analysis, as of 9 a.m. ET, 70.3% of the contiguous U.S. had an air temperature below the freezing mark (32 degrees Fahrenheit), with an average temperature for the same region of just 20.5 degrees Fahrenheit.
The cold is coming courtesy of a wobble in the polar vortex, which is enabling pure, Arctic air from Siberia to migrate across the North Pole, head south across Canada, and cross the border at high speed — like a tourist without a passport. The Arctic invasion is occurring in the wake of a phenomenon that is well-known to temporarily destabilize the polar vortex, which is a sudden stratospheric warming event.
The unusually cold air mass is rotating around Hudson Bay, Canada, with spokes of frigid air descending into the U.S. like Antarctic "brinicles" descending from sea ice cover above.

The frigid weather is going to be especially apparent in the big cities of the Mid-Atlantic and Northeast by Wednesday evening, when temperatures are forecast to tumble into the single digits in New York City and Boston, Massachusetts. These temperatures will be made more uncomfortable since they will be accompanied by snow showers and winds gusting to 50 miles per hour.
This will yield wind chills well below zero, presenting health risks for those outside longer than just 10 to 20 minutes.
Then again, perhaps Bostonians shouldn't complain. After all, in International Falls, Minnesota, which is typically the first location in the lower 48 states to experience brutally cold air masses, the wind chill dropped to 41 degrees below zero Fahrenheit on Wednesday morning. 

Ouch.


The cold will surge south and east through the end of the week, causing temperatures to stay below freezing through Friday in Memphis, Tennessee, Atlanta, Georgia, and other typically milder locations. The peak of the cold in the Mid-Atlantic and Northeast will come Wednesday night and Thursday, but temperatures are now forecast to remain below average for this time of year into early next week.
The forecast low temperature in Nashville, Tennessee on Thursday morning is projected to be in the single digits, which is much colder than the upper teens seen there on Wednesday morning.

According to weather.com, Pensacola, Florida could set a record low temperature on Thursday, as could Wilmington, North Carolina. Both of those locations have present records in the low 20s Fahrenheit.

Sprawling, record-breaking high pressure

A massive high pressure area is associated with the cold, dense air mass, causing strong winds to blow across the Great Lakes into the Northeast. The high pressure system is so intense that it is already setting records that rarely fall. Although records for the highest pressure on record at a particular location are among the nerdiest records in the weather world, they are important because they help indicate the unusualness of the cold weather.
As meteorologist Bob Henson reported for the Weather Underground, the strongest high pressure areas are often associated with some of the coldest air outbreaks in recorded history.
"Many all-time pressure records across the plains were established during the memorable cold wave of late December 1983, which sent temperatures down to 9°F as far south as San Antonio and kept Omaha lodged below 0°F for more than a solid week," Henson wrote.
In December, 1983, the lower 48 states' highest pressure reading on record was set at Miles City, Montana, with a reading of 1064 millibars (mb), or 31.42 inches of mercury. The current high pressure area is weaker than this (about 1055 mb), but it is intense enough to set some records at locations other than Miles City, including in South Dakota.