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Vernal equinox: Spring arrives, but for much of US it already feels like summer

The Spring equinox arrived at 10:14 p.m. Alaska Standard Time Monday. But for much of the country, 2012 appears to have brought not just a leap day in February, but a leap season in March, with temperatures this month in many parts of the country hitting summertime highs.

With a major storm system moving through the central US, the conditions for violent weather, including tornadoes, are in place.

The potential for the most violent weather, including summer-like “super cell” thunderstorms and tornadoes, is centered in Texas, according to the National Weather Service's Storm Prediction Center in Norman, Okla. But a tongue-like swath stretching from the Texas Gulf through northwestern North Dakota also faces an elevated risk of tornadoes over the next 24 hours. The possibility of severe thunderstorms spans much of the eastern US.

On Sunday, the storm system triggered severe thunderstorms over the Great Plains, with reports of tornadoes touching down in five communities sprinkled among southern South Dakota, western Nebraska, and western Oklahoma.

The system, centered over the Montana-North Dakota border, is drawing energy from unusually warm, unusually moist air in the central US, explains Jeff Masters, director of meteorology for the Weather Underground, a web-based weather forecast and analysis service.

On Sunday, Winner, S.D., hit 94 degrees, he notes, the earliest date the Northern Plains has posted a 90-degree day. International Falls, Minn., enjoyed a balmy 79 degrees – a record for the day. St. Patrick's Day weekend came to Houghton, Mich., with back-to-back 76-degree days – 44 degrees above normal.

“I've always said I could do without March in Michigan. I'm finally getting my wish. Now I'm not so sure it was a good request,” says Dr. Masters, who is based in Ann Arbor.

On Sunday, Boston posted a high of 74, 28 degrees above normal for the day. And Chicago has been experiencing a mini-heat wave, with five consecutive days above 80 degrees.

Hearkening back to the late 1800s, when the nation first started recording weather information, “I don't think we've seen such a moist and warm air mass in March over the center of the US,” Dr. Masters says. He points out that sea-surface temperatures in the Gulf of Mexico are among the five warmest on record for the month. Those record moisture levels extend as far north as Minnesota, he notes.

“February showers bring March flowers,” National Weather Service forecasters wrote in an early morning discussion of upcoming weather patterns, posted on the Chicago Forecast office's website. Forecasters say they expect rain and cooler temperatures Friday, bringing an end to the “dog days of March.” Until then, however, “our historic and unprecedented stretch of record-breaking temperatures generally looks to continue.”

As it collides with this unusually warm, soggy air, the storm working its way east is expected to dump between 7 and 11 inches of rain over parts of Texas, Louisiana, Arkansas, Oklahoma, and Missouri.

Broader atmospheric conditions contributing to these records begin with La Nina, part of a warm-cool cycle that sloshes back and forth along the tropical Pacific. During a La Nina event, the ocean surface in the eastern Pacific is colder than normal, while water in the western tropical Pacific is warmer than normal.

The effects of a shift between La Nina and its mirror opposite, El Nino, are felt most strongly in the tropics. But the siblings can affect atmospheric circulation patterns much farther north and south. Alaska, for instance, has been experiencing record snowfalls – increased winter precipitation in keeping with previous La Ninas in kind, if not magnitude. The mountain West also has seen heavy snow this winter.

La Nina on average tends to drive storm tracks over North America farther north than usual, sending powerful storms off the North Pacific across the northern tier of the US.

For most of this winter, storms have scooted across the northern US and into Canada, allowing warmer-than-usual temperatures to hit the usually frigid northern US.

This pattern received a boost from another in the North Atlantic – the North Atlantic Oscillation. The NAO shows up as a cycle in which the difference in air pressure between a zone of low pressure around Iceland and high pressure near the Azores swings between strong and weak. The NAO set up this winter in ways that helped keep the storm track from dropping down into the eastern US.

Finally, another feature of the tropics, dubbed the Madden-Julian oscillation, has had La Nina's back. The feature consists of a region of thunderstorms that travels along the equator, making one circuit of the planet every 30 to 60 days. It's behaved in ways this winter that have contributed to warmer temperatures in the eastern US.

Despite the arrival of this week's storm system and the rainfall it's expected to bring, much of the southern tier continues to register moderate to exceptional drought, with the most-severe conditions centered in Texas and Georgia.