Last January the polar vortex was about to plunge into the United States, unleashing an unforgiving and deadly outbreak of Arctic air across much of the nation -- which is why forecasters are keeping a close eye on what it may do as 2020 begins.
During the January 2019 polar vortex invasion, officials warned residents of nearly instantaneous frostbite danger as AccuWeather RealFeel® Temperatures plunged as low as minus 77 degrees Fahrenheit in the Upper Midwest. The punishing cold froze the Great Lakes, creating slushy waves as eerie sea fog blanketed parts of the region. The extreme weather forced the cancellation of thousands of flights at Chicago's O'Hare International Airport and school closures across portions of the Midwest and Northeast.
This year, the polar vortex, which is a large pool of frigid air - typically the coldest in the Northern Hemisphere - that often sits over the polar region during the winter, is forecast to remain strong and hover near the Arctic Circle in the coming weeks.
AccuWeather meteorologists expect blasts of Arctic air to be brief and generally contained to the northern tier states for much of January.
"When you have a strong polar vortex, it tends to keep frigid air pent up so that it is difficult for long-lasting outbreaks of frigid conditions to reach the middle latitudes, including portions of the Midwest and Northeast," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.
"We generally expect the polar vortex to remain strong through at least the middle of the month and possibly longer," Pastelok added. "Even if the polar vortex was to weaken, it could still take another week or two for widespread colder air to move southward toward the contiguous states."
Instead, through at least Jan. 20 and perhaps into the last week of January 2020, the average flow of air across the U.S. is likely to be from the Pacific Ocean. A pattern with a west-to-east flow such as this is likely to create near- to slightly below-average temperatures over the Rockies and mild weather to areas east of the Rockies.
Periodic northward bulges in the jet stream over the eastern half of the nation are likely to bring rounds of well-above-average temperatures to much of the Central and Eastern states.
"We still expect some brief bouts of cold air to knock down temperatures for a couple of days at a time over the northern Plains and Upper Midwest," Pastelok said.
"There will also be some general resistance to warm weather from upstate New York to New England," AccuWeather Senior Long-Range Meteorologist Joe Lundberg said.
Most storms will struggle to tap enough cold air to produce widespread heavy snow due to a fast flow of air from the Pacific across the lower 48 states.
This image shows the approximate coverage and depth of snow over the Lower 48 United States as of Jan. 3, 2019.
Wintry precipitation won't bypass the entire eastern portion of the country as cold air will occasionally infiltrate the upper Great Lakes and central and northern parts of New England -- and forecasters say that some signs point toward a pattern change that may allow for frigid air to grip a larger zone of the U.S. during the latter part of the month.
"There are some signs that the polar vortex may weaken and become somewhat elongated, which may allow Arctic air to move southward late in the month, but we are a little suspicious about that potential this far out," Pastelok said.
However, additional signals may support the pattern flip, according to experts.
"There are also some other signs of potential change late in the month, such as a weakening of the area of high pressure off the Atlantic coast," AccuWeather Senior Meteorologist Jason Nicholls said.
The clockwise circulation around this large high pressure system has been helping to pump warmth over the southern and eastern parts of the nation in recent weeks.
"If this system weakens, it would allow colder air to more easily settle across the Central and Northeastern states," Nicholls stated.
Given the expanse and frequency of warm air through the first three weeks of the month, January 2020 is likely to finish much above average in temperatures over the eastern half to two-thirds of the nation, with the exception of the northern tier, where temperatures may be closer to, but still above, average.
The upcoming pattern may pose some challenges for businesses like ski resorts. However, a number of nights in the weather pattern may still be cold enough for ski resorts to make snow in New England, the central Appalachians and the Upper Midwest. And, since most ski slopes are located on the shady side of the mountains and the sun angle is low in January, the snow that is on the trails may be slow to melt during the day.
The mild conditions may also be a problem for ice fishing interests. The lack of sustained cold may prevent ice from forming on some lakes and ice that does form may be too thin to support the weight of people. Experts say skating and fishing interests should use extreme caution under such conditions.
Prolonged mild weather during this time of year can threaten fruit trees and grape vines where the buds tend to soften. The softening buds can be more prone damage due to harsh cold weather that may follow.
On a positive note, the mild weather will allow tens of millions of home and property owners to save on heating costs during a month that typically brings some of the coldest weather of the entire winter season.
The milder weather pattern will also generally promote ideal traveling conditions across the region. But, brief episodes of cold air can be enough to trigger blinding snow squalls in parts of the Midwest and Northeast, and mild weather and the right ingredients in the winter can cause issues besides snowfall, including fog problems, dangerous black ice and localized flooding.