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The east coast of Greenland, where the melting ice sheets meets the sea, in July. Diminished ice cover can “kink” the jet stream, scientists say, affecting how weather systems move across the hemisphere.

Andrew Testa, New York Times

Autumn forecast: You might not need heavy coats for a while

  • Article by: BILL McAULIFFE
  • Star Tribune
  • September 20, 2012 - 11:12 PM

A warmer-than-normal autumn is likely for Minnesota, and may be followed by another mild winter, a meteorologist from the National Climate Prediction Center said Thursday.

The agency released its latest three-month outlook, and seasonal forecaster Huug van den Dool said the warm regime that's been so pronounced across the hemisphere this year is likely to continue through October, November and December in Minnesota and across the United States.

January through August of this year was the warmest such period on record for both the Twin Cities and Minnesota.

As for winter, "I wouldn't go as far as saying it will be as extreme as last year, but chances are it will be above normal," he said in a telephone conference with reporters. "That's the long-term trend."

The Climate Prediction Center report was noncommittal on precipitation over the next three months, but its drought outlook through the end of the year, also updated Thursday, indicated drought conditions appear likely to increase across north-central and southeastern Minnesota, including the metro area.

With that backdrop, fire danger across the state continues to be high. Some rain across northern Minnesota this week, along with cooling temperatures and higher humidity, has dialed back some of the danger ratings from last week, said Olin Phillips, director of the Minnesota Department of Natural Resources' wildfire program.

Even so, continuing dry weather and the simple progress toward fall, with the drying of vegetation, are keeping the fire risk high.

"Unless we get some rain, it's going to get worse," Phillips said.

Minnesota firefighters have battled 1,059 wildfires on state land so far this year, compared with 706 all of last year, but that's still far fewer than the record of 3,440 in 1976. The 24,227 acres burned so far is about half the annual average.

Relatively warm and dry winters in Minnesota are frequently related to El Niño, the cyclical phenomenon in the Pacific Ocean that affects the weather and which has been intensifying in recent months. It now appears that El Niño might not be as pronounced as previously thought, Van den Dool said.

Sea ice a factor?

Another global factor may be coming into play, however.

Arctic sea ice shrank to a record annual minimum as of Sunday. And while the ice area is expected to start growing with the start of the six-month polar night, the big melt could continue to affect weather in Minnesota and even farther south through autumn and into winter, a researcher said during Thursday's telephone conference.

The diminished ice cover, symptom of a trend that's been in place for the 33 years that the polar ice cap has been monitored by satellites, can "kink" the jet stream in a way that can cause weather systems to stall in their west-to-east travels across the Northern Hemisphere, said Walt Meier, research scientist for the National Snow and Ice Data Center in Boulder, Colo.

With less ice cover to reflect sunlight and more dark sea to absorb it across the Arctic, a big melt-off increases the warmth in the region, which changes the global interplay of warm and cold and major air currents, Meier said.

The Arctic is "like the earth's air conditioner," he added. "It's like it's losing coolant."

A resulting slowdown for weather systems could mean long-lasting snowfalls, or rainfalls, or cold spells, drought or heat waves, Meier added.

Researchers are inclined to associated the prodigious snows that hit the eastern United States, including Minnesota, in 2010-11 with the shrunken ice cap, but need to know more.

"This is an active area of research," he said.

Bill McAuliffe • 612-673-7646

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