At the same time, there are increasing odds of significant winter storminess in the eastern half of the nation between Wednesday and Christmas Eve. While far from a lock, Mother Nature may deliver a white Christmas for a swath of the Midwest and eastern United States.
The frigid weather and possibility of snow will coincide with a peak time for holiday travelers.
At roughly a week out, it’s not possible to forecast exactly where a storm might form and what areas will see snow or rain or remain dry. But between Dec. 22 and 24, the chance of a significant storm between the Midwest and East Coast is above normal.
The primary setup for our potential outbreak of wintry weather involves strong high pressure building over the eastern Pacific Ocean toward the Alaska Aleutian Islands.
That high acts as a force field, deflecting the jet stream around it. The jet — which separates frigid and more mild air — will bulge toward the Arctic Circle in central North America before crashing southward over the central and eastern United States.
We can glance at a model of the trajectories of air parcels in the atmosphere for clues about the origins of next week’s air mass. If we run the model for Christmas Eve in the Midwest, it traces the air back to Nunavut, Canada, between the Northwest Passages and Baffin Bay, adjacent to Greenland.
The initial blast of cold will drive southward into the northern Rockies and northern Plains on Monday and Tuesday, reaching the Great Lakes Tuesday into Wednesday. Some of the coldest areas near the border with Canada could see temperatures 30 to 40 degrees below normal, meaning highs around minus-10 and lows from minus-20 to minus-30. Subzero temperatures could reach as far south as the central Plains.
The core of the cold will probably remain over the north central United States through midweek, although temperatures 10 to 20 degrees below normal could reach parts of the South and the eastern United States late in the workweek.
A second, reinforcing blast of cold may dive into the northern Plains and Upper Midwest Wednesday and Thursday, pushing frigid air even farther south and east. Minneapolis should expect lows well below zero while Chicago may see highs only in the teens for much of the second half of next week.
By Christmas Eve and Christmas Day, much of the eastern half of the country may see temperatures 10 to 30 degrees below normal.
There are signs a third shot of frigid air could enter the northern Plains around Christmas Day before barreling south and eastward about 10 days from now.
The National Weather Service’s Climate Prediction Center places high odds of below average temperatures in the central and eastern states both 6 to 10 and 8 to 14 days into the future.
The potential for snowfall will stretch from the Plains to the East Coast during the second half of next week. Here’s what we know:
- Central states: As the jet stream dives south over the Plains, a disturbance embedded within that flow could cause some snow to break out over the central and northern Plains before sweeping across the Midwest and Tennessee and Ohio valleys. The exact of track of any disturbance is still unclear, as is the location of the rain/snow transition line; the timing would probably be around Wednesday into Thursday.
- Mid-Atlantic and Northeast: The disturbance that could bring snow to the central states could evolve into a major East Coast storm. It would have plenty of jet stream energy to feed off and could draw abundant moisture from the Atlantic Ocean. However, it’s still highly uncertain if and where a coastal storm might form and what areas will be hardest hit. The timing of this storm would probably be between Thursday and Christmas Eve.
It’s important to remember the approximate timeline with which meteorologists can cast predictions — understanding that the first blast of cold is still three days away and the possibility of a storm five to seven days away:
At seven to 10 days in advance, forecasters can identify large-scale patterns favorable for cold (or warm) air outbreaks and big storms. That means estimating the approximate shape of the jet stream, which would offer insight into temperature trends — and where storms might form.
At five to seven days in advance, forecasters can start estimating temperature differences from normal. That’s why they can say, for instance, that the impending air mass over the Plains has the potential to be 30 degrees or more colder than average. They can also begin spotting the ingredients needed to make a storm — but can’t tell yet whether they’ll overlap just right.
At three to five days in advance, forecasters can provide a prediction of high and low temperatures with adequate accuracy for planning purposes. They can also tell if a storm will form, and gauge roughly how strong it will be. They might also be able to determine where, within 100 miles or so in either direction, a storm will track. They can’t yet reliably give forecasts of specific rain or snow totals because small shifts in the track could markedly change those amounts.
At one to two days in advance, forecasters know where the storm will go, about how much rain or snow will fall, how bad the winds will be, and how long it will last. They can provide specific snowfall forecasts and guidance for planning purposes.
On the day of, forecasters can identify “mesoscale” influences, or gauge how smaller-scale (about the size of a few counties) features will locally affect conditions. That might mean pinpointing where a 10-mile-wide stubborn snow band will set up, or where the greatest tornado risk of an afternoon might be.