From Washington, D.C., to Massachusetts, forecasters say the mercury could reach the mid-to-upper 90s. Normally at this time of year, the region enjoys cool springtime weather in the upper 60s to mid 70s, said Marc Chenard, a forecaster with the NWS Weather Prediction Center in College Park, Maryland.
In the western half of the country, a separate weather system is pushing cold air south into Colorado. Between 12 and 18 inches of snow are expected to fall between Friday night to noon Saturday, Chenard said, with temperatures plummeting from 88 degrees F on Thursday to the low 40s on Friday.
"In Denver, it's a pretty big swing from summer back to wintry weather, but it's not unprecedented," he said. "So, no records are being broken. It's just unusual."
Late season snowfalls in the Mile High City are rare but not unheard of. An inch of the white stuff fell May 29, 1975, and a half inch dusted Denver on June 6, 1953
The threat of heavy snow led the Colorado Rockies to postpone its game with the New York Mets scheduled for Friday at Denver's Coors Field.
The irony of the snow-out for the Mets, who escaped the New York heat when the team flew out West, was not lost on those who follow baseball.
"The Mets not playing cause of snow is insane," wrote Twitter user Young YE after the announcement.
New York City, where 73 degrees F is average for mid-May, could hit 93 degrees F on Saturday, while temperatures in Washington could reach 96 degrees or higher, Chenard said. Temperatures should drop to normal on Monday.
Forecasters said the early-season heat wave will result from a high-pressure dome of air in the upper atmosphere that is deflecting the typical flow of cool air from Canada, pushing it westward. That allows hot air from the southern and central states to move into the northeast, he said.
The National Weather Service also issued a tornado watch for Maryland, Delaware, New Jersey and parts of southern Pennsylvania from midday on Friday and to 7 p.m. A watch means that conditions are favorable for the formation of a tornado.
(Reporting by Rich McKay in Atlanta and Keith Coffman in Denver; Editing by Frank McGurty, David Gregorio and Daniel Wallis)
By Keith Coffman and Rich McKay