From the Rockies to the Northeast, a cruel blast of unusually wintry April weather is barreling east. Accumulating snow has already blanketed Denver, while Indianapolis, Cleveland and Buffalo await up to several inches of snow between Tuesday afternoon and Wednesday.

Where it’s not snowing, it’s turning abnormally cold. The National Weather Service has issued frost and freeze watches and warnings for 87 million Americans from Texas to Virginia. By Thursday morning, temperatures will be at least 10 degrees below normal from eastern Montana to north Florida.

But those who don’t care for this February-like weather in April won’t have to wait long for a reprieve. Many of these same areas are on tap to hit 80 degrees and enjoy spring- or summerlike warmth next week.

A major pattern change will bring a virtual seasonal reversal to the eastern half of the country, with the chilly air mass set to be scoured out by insurgences of Gulf of Mexico heat and humidity.

The pattern shift could bring the chance of flooding for some and the risk of severe thunderstorm and tornado activity for others.

Late April snow from Denver to Buffalo

Next week’s warm-up won’t come before wintry weather’s last gasp, however, which was already pummeling parts of the High Plains and Rockies. Jamestown, Colo., situated in the mountains west of Boulder, picked up 15.3 inches. Boulder itself wound up with nine inches, while downtown Denver reported 4.8 inches. Fort Collins measured 6.4 inches.

The snow overachieved in parts of Kansas, like in Hays, along Interstate 70 west of Salina, where seven inches fell. Topeka got more than four inches, and snow was just exiting Kansas City during the morning hours Tuesday, where temperatures were a little too high for accumulation.

It was also snowing in Chicago late Wednesday morning but little accumulation was anticipated before the main axis of precipitation moves east into Indiana and Ohio.

Indianapolis, Cleveland, Detroit and Buffalo are all in line for accumulating snow between Tuesday night and Wednesday.

Indianapolis is expecting one to two inches of snow Tuesday night while two to four inches is possible in Toledo, where a winter weather advisory is in effect.

In Cleveland, which is also under a winter weather advisory for three to five inches Wednesday, so much snow this late in the season is rare. “A 2” snow event in Cleveland on or after April 21st has only happened twice: May 6, 1974 (2.0″) and April 23-25, 2005 (12.4″)," tweeted the National Weather Service.

Just about half an inch of snow is expected Wednesday in Detroit, which may remain just north of the heaviest precipitation.

The snow will continue through western Pennsylvania, Upstate New York and northern Vermont as the cold front slides east on Wednesday. Buffalo is under a winter weather advisory for three to six inches.

At the same time, a line of thunderstorms, some strong, will build south along the front, likely affecting areas along the Interstate 95 corridor from New York City through the Mid-Atlantic. That’s where the National Weather Service Storm Prediction Center has drawn a level 1 out of 5 marginal risk of severe weather. New Jersey and the Delmarva Peninsula could be most impacted, with the storms maturing as they head farther east.

As the front roars across the Interstate 95 corridor from Washington to New York, temperatures may plummet from the 60s to near 50 degrees in just an hour or two.

Widespread freezing temperatures

Behind the front, temperatures some 15 to 25 degrees below average will overspread the central United States from north Texas to the Ohio Valley and Appalachians westward. A number of records could be set on Wednesday morning.

The Weather Service tweeted that 87 million Americans are under frost or freeze watches and warnings from Texas to Virginia, with the heart of the cold focused in the Plains and Midwest on Wednesday and toward the Tennessee and Ohio valleys on Thursday.

On Thursday morning, nearly 100 million Americans may see temperatures at or below freezing from the Rockies through the Midwest to the Northeast.

By Friday, after parts of the interior Northeast and Mid-Atlantic endure freezing temperatures to start the day, the core of the cold will slide off the East Coast.

A pattern switcheroo ushers in springtime

As the high-altitude lobe of frigid air that’s hung over the Midwest and Northeast for much of mid-April scoots away, that will open the doors for ridging, or the building of high pressure and a northward jaunt of the jet stream into next week. This will allow warmth to slowly ooze northward from the Gulf of Mexico and overspread the eastern half of the nation.

The atmospheric shake-up comes this weekend when, behind a pocket of unsettled weather in the Northeast, high pressure will build over the central Plains eastward. That will flip winds southerly, bringing temperatures up to 20 degrees above average to the Plains on Monday, the Great Lakes by Tuesday and likely the East Coast toward the midweek onward.

Oklahoma City, which was only expected to see a high in the 40s on Tuesday, could be basking in mid-80s by early next week. Dallas was planning for lows in the 30s on Tuesday night, but is expecting lows in the 60s and highs in the 80s in just a few days’ time.

Atlanta could see highs approach 80 by Monday, too, with temperatures nearing 70 in the nation’s capital; the warmth will be more moderate on the East Coast.

The outlook from the Weather Service’s Climate Prediction Center is calling for above-average temperatures across the southern tier of the United States and along the Eastern Seaboard to round out April and kick off May.

In the western United States, below-average temperatures may stick around most of next week, part of a temperature seesaw punctuated by a jet stream slicing across the nation’s heartland.

Active weather and severe potential

There are increasing signs the incipient pattern — mild east, cool west — could prove favorable for an uptick in severe thunderstorm and tornado activity. Already, Texas, particularly regions surrounding the Interstate 35 corridor including Dallas, is eyeing the potential for strong to severe thunderstorms on Friday afternoon. A few of them could rotate and become tornadic.

Those storms will likely sweep southeastward toward the Gulf Coast with an attendant severe risk on Saturday before rain and a few storms arrive in the Southeast by Sunday.

The mid-April pattern, which featured a stalled front draped along the Interstate 10 corridor parallel to the Gulf Coast, resulted in excessive rainfall and flooding in parts of Mississippi, Louisiana and Alabama. More than 10 inches have fallen so far this month at New Orleans’ Lakefront Airport. The next week could favor continued rounds of heavy rain exacerbating existing flooding and falling on already-saturated soils.

Attention then turns to the next big jet stream disturbance, which should eject east out of the Rockies early in the workweek. That could brew enhanced odds of a severe weather event, potentially more widespread, somewhere in the central or southeastern United States, with the risk of tornadoes.

Details will become clearer as the evolution of the modeled system evolves in the days ahead.