After shivering through bitterly cold temperatures thanks to Winter Storm Grayson, large portions of the country will experience a serious January thaw late this week. In Boston, New York City, Philadelphia and Baltimore, for example, temperatures on Friday are projected to top 50 degrees — and in Washington, D.C., and Richmond, Va., they’re expected to surpass a balmy 60 degrees.
“It’s a big change,” admits Paul Walker, a senior meteorologist for AccuWeather. “We’ve had some wild swings with these temperatures.”
Be that as it may, Walker says the dueling forecasts aren’t cause for concern, and says it’s fairly common to see swings of this nature. “There’s often a point during the winter where the cold relaxes, and that’s what often people think of as the January thaw,” Walker says. “It doesn’t seem like it’s possible most of the time for cold air to persist in one place for a long period of time.”
Instead, Walker says, the cold air is eventually ushered out by changes in the jet stream: the typically horizontal, high-altitude winds that dictate temperature and other weather patterns based on their strength, location and orientation. Changes in the jet stream are responsible for the wild weather this week, Walker says.
During the cold snap, a range of high pressure on the West Coast pushed the jet stream far to the north, causing air to come down from central Canada, across the Great Lakes and east, Walker explains. As that pressure dissipated, a storm moved into California, causing cooler temperatures, heavy rains and mudslides. And without the ridge of high pressure over the West Coast blocking its path, the jet stream began moving freely from the west to the east, carrying “milder Pacific air across the continent” and resulting in unseasonably warm temperatures, Walker says.
But while highs are set to breach 50 degrees on Friday, Walker cautions that “it is going to have rain with it, so it’s not going to feel quite as mild as that.”
The early spring conditions aren’t set to last, either. By the second half of the weekend, Walker says the Eastern U.S. will again see wintery temperatures “pretty typical for this time of year” thanks to another pressure ridge building on the West Coast. On Sunday, Walker says, temperatures in the Northeast will be right around freezing — so watch out for slick conditions as Friday and Saturday’s rain turns into Sunday’s ice.