WINTER111607

Winter nights get warmer, make that, less cold

Experts say average winter temperatures have been rising for 30 years, mostly driven by much warmer nighttime temperatures.

By Bill McAuliffe, Star Tribune

Last update: November 15, 2007 - 10:35 PM

Throw a smaller log on the fire.

This winter will likely be warmer than normal across most of Minnesota, according to a forecast released Thursday. And if trends continue, it will be winter nights where warming could be most pronounced.

Twin Cities lows in December and January, averaged over the most recent 30 years, have increased 1.6 and 2.0 degrees over the official normals. It's an increase that University of Minnesota Extension climatologist and meteorologist Mark Seeley called "emphatic."

He noted that the dynamic is right in line with what most global-warming science has outlined. Warming should be most obvious in northern latitudes on winter nights because heat is more effectively trapped by greenhouse gases during long sunless hours than it is during the more volatile days of summer.

Also, Seeley said, recent studies have indicated that a warmer atmosphere, which has more heat-trapping water vapor, may be generating more heat-trapping clouds.

While more moisture may also result in more snow, National Weather Service meteorologist Karen Trammell added that warmer conditions can also mean less continual snow cover, which allows the ground to absorb more radiation.

The Twin Cities, of course, ought to be getting warmer overall as heat-absorbing pavement spreads outward. But the numbers also show that winter temperatures across the entire state have been on a nearly steady and steep upward trend for about 20 years, with seasonal averages since 1998 running above anything in the 106-year record.

Assistant State Climatologist Pete Boulay cautioned that any winter can reverse the trends, reintroducing Minnesotans to heavy parkas and enduring snow and making something as simple as taking out the trash a life-threatening exercise. But climate outlooks tend to reflect trends, he said. And trends suggest winters more like recent ones than like those of the epic past.

Thursday's three-month outlooks from NOAA calls for the strongest chances of a warmer-than-normal winter across southern Minnesota. Chances of above-normal averages are cast across all but the northwest corner of the state, where NOAA's winter temperature outlook is a tossup. Similarly, the precipitation forecast for the entire state is that anything goes.

The outlook for the months of December, January and February relies most prominently on the effects of a "La Niña" -- a cooling of the Pacific Ocean temperatures off the coast of South America. La Niña and its warm counterpart, El Niño, are believed to have dramatic effects on short-term weather around the globe.

The National Weather Service in Duluth has noted that 10 La Niña episodes since 1950 have resulted in average winter snowfall totals 6.9 inches above normal.

Bill McAuliffe • 612-673-7646

Bill McAuliffe • mcaul@startribune.com