updated 4/17/2012 12:24:25 PM ET 2012-04-17T16:24:25

Abnormally warm temperatures brought a taste of summer to the eastern part of the United States early this week, testing runners resolve in the Boston Marathon and bringing out sunbathers in New York.

"It's just the extreme warm spring weather pattern continuing," said Mike Pigott, a senior meteorologist at AccuWeather.com, referring to record-breaking weather in the continental U.S. last month.

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In Boston, temperatures were about 20 degrees Fahrenheit (11 degrees Celsius) higher than the normal high when runners set out yesterday morning (April 16), said Pigott. Meanwhile, in New York City, temperatures hit 86 degrees F (30 degrees C) in Central Park. By comparison, the normal high is 62 degrees F (17 degrees C), Pigott said.

While the eastern two-thirds of the lower 48 states have seen unusual warmth so far this year, the west has had a cooler, wetter experience. Yesterday, San Francisco was 2 degrees F (1 degree C) below normal.

"One of the reasons that we have been so warm this spring is most of the cold fronts we see at this time of year have been diving down into the western U.S.," Pigott said.

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The configuration of the jet stream, a band of high-altitude westerly winds, is responsible, and this year, it is directing colder air and storm systems into the West Coast while allowing high pressure and warmer temperatures to prevail in the East, he said. [ A Warm Winter: Is It Climate Change?]

Early last year, the opposite scenario played out with wet, cold weather arriving over the East, and drier weather over the West, culminating in a devastating drought.

On Monday, temperatures in the middle of the country, in places such as Wichita, Dallas and St. Louis, were around normal. In the far southeast, Tampa, Fla., was a few degrees above normal, Pigott said.

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