The polar vortex has been absent from the United States for most of winter thus far, which led to a very warm January.
The Eastern Seaboard through the central and southern Plains recorded temperatures that averaged well above normal by the end of the month. The average temperature was also above normal for the interior portions of the West.
February is forecast to start off with more abnormal warmth across the Plains and East, but some changes are expected as we move through the second month of the year.
The colder air expected to slice into the Central states during the first part of next week may be a sign of even more brutal Arctic air that could take root later in the month.
"We are anticipating that a strong push of Arctic air will take place into the U.S. during the second and third week of February in response to a displacement or weakening of the polar vortex during the first week of February," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.
When the polar vortex-a pool of bitter air that often sits over the North Pole during winter, weakens or becomes elongated, the frigid air that is normally locked up above the Arctic Circle can eventually break loose and move southward.
There is at least one factor working against a large sweep of frigid air into the Eastern states, especially across the Southeast and mid-Atlantic.
An area of high pressure at most levels of the atmosphere has persisted in the southwestern Atlantic this winter -- and it has been helping to pump mild air up the Eastern Seaboard. Indications are that this pattern may continue during much of February.
"For this reason, we expect the upcoming big discharge of Arctic air to target the interior West and northern Plains initially, where it might be more persistent as February progresses," Pastelok said.
"While much colder air could work into the Eastern states during the middle to latter parts of February, there is some uncertainty as to how long-lasting and/or severe this may be, depending in part on the persistent area of high pressure over the southwestern Atlantic," Pastelok said.
The last significant polar vortex shift was during mid- to late-November, which triggered the cold outbreak from late November to early December.
Even though a few major Arctic outbreaks in years past have reached much of the United States, it is rare for the entire Lower 48 states to be cold all at the same time. Typically, cold air will punch into one-third to two-thirds of the contiguous states as other regions remain warmer than average. So, if cold air were to sprawl over the Western and Central states, there would be a tendency for the East or the Southeast to remain warmer for a longer period of time.
"No matter what, we do not see a six- to eight-week outbreak of Arctic air, which is usually what happens with a polar vortex shift, but rather seven- to 14-day cold episodes or shorter," Pastelok said.
"There can still be a couple of days and nights where cold air sneaks its way into the Southeast with a frost or hard freeze and that could be a problem with the leaf-out and blossoming running two to three weeks ahead of schedule," he added.
The warmth coming early next week is likely to increase that leaf-out anomaly.
The spread of colder air into the Central states and perhaps the Northeast could set up an active storm track as February progresses. Depending on which side of the temperature contrast zone places end up, would determine where areas of heavy snow, ice and rain occur.
Snowfall has generally been close to average over the heartland with a few pockets of below-average snowfall in cities such as Chicago.
Much of the northern tier of the Central states has received above-average snowfall thus far with near- to above-average snow for the northern tier of the Northeast. However, a definitive snow drought has persisted over much of the central Appalachians to the mid-Atlantic and southern New England coasts.
A mere 0.3 of an inch of snow has fallen on Philadelphia, compared to a seasonal average close to 9 inches by the end of January. Perennial snow spots such as South Bend, Ind.; Erie Pennsylvania; Cleveland and Buffalo, N.Y.; have had 25 percent to 50 percent of average snowfall thus far.
Very few cities have received above-average snowfall, and the ones that have are mostly in northern New England.
At least for skiers, snowmobilers and snow lovers in general, there is still some hope for more of the white stuff in some of the snow-starved zone if the cold press and storm track pans out. But even if winter storms take an ideal path, there is no guarantee that storms will bring all snow.
In the meantime, home and business owners are saving on their heating bills from the Central and Eastern states with temperature departures that recorded well above normal in January.