Weather Blog: Minnesota Tornado Season

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(credit: Ryan McGinnis)

(credit: Ryan McGinnis)

(credit: CBS) Lauren Casey
Lauren Casey joined the WCCO-TV weather team in August 2011, a...
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With no recorded tornadoes in Minnesota history in the months of December, January and February, according to the Minnesota Climatology Working Group, you might say that Friday (Nov. 30) marks an unofficial end of tornado season in Minnesota.

According to the National Weather Service, the average statewide number of tornadoes per year, from 1981 to 2010, is 37. And so far in 2012, we’ve hit average right on the head: 37 tornadoes. Thus, on the surface, 2012 appears to be an average tornado year, but when we look at the months in which these tornadoes occurred, the 2012 season is anything but typical.

The atypical activity began in March with the development of an EF-0 tornado that affected northern Waseca and southern LeSueur counties. According to the Minnesota Climatology Working Group, March tornadoes in Minnesota are very rare with an average number (from 1950-2010) of less than 1, at 0.3.

Then came summer. Climatologically, the most active tornado months in Minnesota are June and July, when the jetstream more often resides above the Upper Midwest. The average number of tornadoes in June is 9.9 and in July, 6.8. This year, according to preliminary data from the National Weather Service, seven tornadoes occurred in June. Only one tornado formed this year in July, traditionally the season’s second busiest month.

Tornado activity in Minnesota in November is extremely rare. But earlier this month, an incredible four tornadoes touched down across the Metro. One briefly touched down in Burnsville, another tornado in Eagan, a third in Mendota Heights and one in Mahtomedi. All the tornadoes were brief, with a lifespan of less than seven minutes, and rated the weakest on the Enhanced Fujita scale at EF-0. Thus it is not the strength, but the mere formation of these tornadoes that is so impressive, as since 1950 there is only one November tornado on record statewide. And statistically, only 0.02 tornadoes form on average in fall’s final full month.

Though this year’s preliminary tornado tally aligns exactly with the average, the timing of the activity occurred outside of the parameters of when tornadoes tend to form according to climate data.

Friday also marks the end of the 2012 Atlantic Hurricane Season, and interestingly a similar trend is observed with the year’s tropical activity. A record two named storms formed even before the official start of the season on June 1, including Tropical Storm Beryl, the strongest pre-season tropical system to make a U.S. landfall. By the end of June, four named storms had formed for the first time on record.

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