September's song is same old tune

The calendar may not say so, but many things with "summer" in them are over. Summer vacation. Summer driving season. Summer hours. Summer romances. Meteorological summer.

But just as summer actually has three weeks to go, so may another phenomenon usually associated with warm weather: severe weather.

Last year a tornado on Sept. 16 killed a girl in Rogers. The year before that, tornadoes and high winds across central Minnesota on the final day of summer destroyed three houses in Andover, killed a man in Minneapolis and dropped 3.2 inches of rain on Fridley. Two weeks later, rains of up to 6 inches fell over the southern Twin Cities, a deluge that climatologists described as unprecedented for October.

An almost-as-freakish 4 inches dropped on the Twin Cities at the end of October the year before.

Storms like that are only supposed to happen in May, June and July, and really taper off by mid-August. But in the past decade, the late summer and early fall storms have picked up in severity, said Kenny Blumenfeld, a University of Minnesota doctoral candidate in geography who is studying Twin Cities severe weather trends.

"We're getting near the point where we can look for more late-season severe weather activity than is normal, and less in the peak season," Blumenfeld said.

From 1950 through 2004, Minnesota averaged 1.2 tornadoes each September. But September 2005 saw seven. Three came in September 2003, followed by one on Oct. 29. In 2000, a tornado touched down in Kandiyohi County on Nov. 1.

Tracking the edges of the jet stream, severe weather has customarily moved northward in the spring, then back southward in the late summer, producing a sort of echo storm season in late summer across the Upper Midwest.

Blumenfeld and University of Minnesota Extension climatologist Mark Seeley said that tornadoes, hail and high winds have been appearing earlier as well as later in the season in recent years. Witness the 14 tornadoes that killed two people and did $230 million worth of damage in Comfrey, St. Peter and other southern Minnesota towns on March 29, 1998.

Both men said one explanation might be a warming climate. Many scientists say a warmer atmosphere, by holding more water vapor, will bring more extreme rainfalls.

"It certainly would make sense if we saw an expansion of the potential flooding season," Blumenfeld said.

So a word for September: Get the sweatshirts ready, but hang on to your hats, and boots.

SEPTEMBER BY THE NUMBERS

• The average daily high and low temperature will drop to 65 and 44 by Sept. 30 in the Twin Cities.

• What's possible? The state record high for September is 111, set at Beardsley on Sept. 11, 1931. The record low is 10, set on Sept. 22, 1974, in Thorhult and Sept. 30, 1930, in Big Falls.

• Last September brought 2.44 inches of rain to the Twin Cities, a mere quarter-inch below average.

• Some of the outer suburbs might see a frost before the month is out. The average date for the first 32-degree low of the season is Sept. 27 at Jordan and Sept. 28 at Rosemount. The northern third of Minnesota usually sees a killer frost -- 28 degrees -- by the last week in September.

•The Twin Cities will lose about an hour and 23 minutes of daylight between today and Sept. 30.

SEPTEMBER DATEBOOK

Today: Labor Day

Tuesday: Classes begin at University of Minnesota and most local public schools

Sept. 11: Primary elections

Sept. 13: Ramadan begins

Sept. 16: One-year anniversary of tornado that hit Rogers, killing a 10-year-old girl

Sept. 21-22: Yom Kippur

Sept. 21-23: Hawk weekend (Hawk Ridge, Duluth)

Sept. 23: Autumn begins

SKYWATCH

Because the atmosphere bends the rays of the sun, making sunset partly an illusion, the Twin Cities will see slightly more than 12 hours of daylight on Sept. 23, the day of the autumnal equinox. The day with the closest to 12 hours of daylight will be Sept. 26. That is also the night the full harvest moon rises. The harvest moon rises closer to the same time for several nights than the moon at other times of year.

Bill McAuliffe • 612-673-7646

Bill McAuliffe • mcaul@startribune.com