Image: HBO

The old saying that the weather can change on a moment’s notice is about to get very real for the northeastern United States.

The last few days have felt freakishly spring-like up and down the mid-Atlantic, with temperatures in our nation’s capital smashing a 117 year record by 6 degrees Fahrenheit today. Philadelphia saw highs of 65, while New Yorkers barely needed a light jacket for their balmy, 60 degree commute this morning. But that all changes in a big way tomorrow, when an advancing cold front and storm system cause the mercury to plummet and the snow to fall fast, and hard.

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Between Wednesday evening and Thursday morning, temperatures up and down the northeast are expected to fall a staggering 30 to 40 degrees Fahrenheit. Winter storm and blizzard warnings are in effect from Philadelphia to Boston, with up to a foot of snow accumulation possible in certain areas by Thursday night. According to Weather.com, snowfall rates of an inch or more per hour are likely in the most intense storm bands.

This sort of dramatic meteorological about-face is unusual but not unprecedented in the Northeast. As Weather Underground meteorologist Jeff Masters explained to Gizmodo, such events typically occur when the pressure and temperature gradients across an atmospheric front are more exaggerated than normal.

“What’s happening is that the Jet Stream, which marks the boundary between cold air to the north and warm air to the south, has an unusually sharp temperature contrast on either side,” he said. “If the boundary moves over, you’re gonna notice it, with a huge swing in temperatures of up to 40 degrees.”

According to Masters, these sorts of wild temperature swings occur more often in La Niña years—which promote a wavier jet stream that dips slightly more southward, on average, over central and eastern North America. We’re currently experiencing a weak La Niña, following one of the strongest El Niño events on record, from 2015 to 2016. “You expect to see a little more variability in your winter extremes when you’ve got a La Niña in place,” Masters said, adding that other climactic patterns like the North Atlantic Oscillation can play a role, too.

In any case, a cold front passing through the northeast and mid Atlantic late Wednesday and Thursday is expected to spin up quite the mighty winter storm, complete with rip-roaring winds and blinding snow. In fact, as Mashable’s Andrew Freedman notes, central air pressures in the storm could drop rapidly enough—at least 24 millibars in 24 hours—for it to qualify as a “bombogenesis” event, which is quite possibly the most badass meteorological term we’ve heard all week.

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So there you have it: Spring today, wintry chaos tomorrow. 2017 clearly has a flair for drama.