July's intense heat may have worked against tornadoes
Curse the heat if you will, but it's brought one blessing: fewer tornadoes.
Across the nation, record heat and drought have reduced tornadoes to what could be a record low, while Minnesota is likely to have a rare July without a single recorded twister.
Minnesota led the nation with a record 113 tornadoes just two years ago but has experienced only about 20 so far this summer.
"It's almost like it's been too hot for tornadoes," said Todd Krause, warning coordination meteorologist for the Twin Cities office of the National Weather Service.
Nationally, there have been only 23 tornadoes reported in July, according to Harold Brooks, research meteorologist at the National Severe Storms Laboratory in Norman, Okla.
Although the combination of heat and moisture, key ingredients in tornadoes, has been plentiful in Minnesota this summer, Krause said a third essential dynamic -- the wind shear which generates rotating storm systems -- has been missing as the jet stream has run steadily to the north and west.
The record heat that has blanketed much of Minnesota and Wisconsin has also led to wide gaps between the air temperature and dew points, reducing low-level cloud formation that might generate tornadoes, Krause added. Spreads of about 10 degrees are typical in tornado weather, but while dew points in the 70s have been common in recent weeks, temperatures have been steadily in the 90s. Also, rising warm air has pushed cloud bases to generally higher-than-usual altitudes, where tornado formation is less likely, Krause said.
But if the winds have been quiet, the warmth has been remarkable. Assistant Minnesota state climatologist Pete Boulay said the Twin Cities are in line for a number of July heat distinctions, such as:
• The only July since 1873 with an average temperature of 80 or greater every day.
• The third July since 1873 with an average daily high of 90 or greater.
Duluth and La Crosse, Wis., are finishing what could be their warmest Julys ever. The Twin Cities, Rochester and Sioux Falls are headed for No. 2 in their local standings.
The heat has dominated the public conversation, but in the Twin Cities, at least, the July average temperature wasn't as far above normal as those in each month December through March. The Twin Cities has seen 14 straight months with above normal average temperatures.
The warmest March on record in many parts of Minnesota seemed to be the perfect setup for a rough tornado season. A tornado on March 19 near Elysian, in southern Minnesota, was the second-earliest ever noted in the state. But there have been few of note since then.
Minnesota averaged 47 tornadoes per year 2000-11. July is on average the peak month, and the frequency drops off steeply in August. The famously hot and dry year of 1988 brought only five, a record low total.
A tornado-free July in Minnesota is rare but not unprecedented. There were none in the Julys of 2007 and 1989.
Bill McAuliffe • 612-673-7646