Cold air in the upper levels of the atmosphere is conducive to the formation of tornadoes — cold air at the surface, not so much.
In 2013, cold air has been plentiful in Minnesota. Its prevalence has contributed to reduced numbers of tornadoes during the months which are climatologically most active in the state — May, June and July.
You may remember, or perhaps blocked out of your memory, our colder-than-normal May in which we kicked the month off with snow.
In order for tornadoes to form, warm and moist air needs to be present in the low levels of the atmosphere, hence a sky that makes snow is not primed to make tornadoes. This May, the state experienced only two tornado touchdowns. Last year, 12 May tornadoes formed in Minnesota.
June, on average, is the most active tornado month of the year. According to 1950-2010 averages, 37 percent of yearly tornadoes in Minnesota occur in June. This June did yield the highest tornado count, but that count is only three. In June 2012, the state experienced 7 tornadoes. The second most tornado-prone month is July. This month, only one tornado was reported in Minnesota.
Primary in producing the tame June and July was likely the high frequency of stagnated weather patterns that affected Minnesota. The two main systems that repeatedly set up shop included a closed upper-level low and strong high pressure; systems contrary to one another, but both effective in eliminating a tornado threat.
In the case of a closed upper low, the cool pool center yields an environment too cool and too stable for tornadoes to form. Strong high pressure also creates very stable conditions by promoting sinking air, thereby denying the atmosphere of two main elements needed for thunderstorm formation; lift and instability.
Intensely hot days, as we sweated through the first two weeks of July, do not serve up tornadoes well as warm air in the mid-levels of the atmosphere can create a ‘cap’, preventing air from rising efficiently and thus storms to form.
You may have already done the math, but our 2013 tornado tally so far is six. With our most active severe weather months behind us, our final count may pale in comparison to the annual average number of Minnesota tornadoes from 2001-2010 of 51, and far fewer, than the totals for 2012 and 2011, at 37 and 31 tornadoes respectively.
The 2010 totals made for a record year in Minnesota tornado history, as 113 tornadoes formed across the state including 4 EF-4s and 4 EF-3s.
In Minnesota, tornadoes have occurred in every month from March through November. And as we experienced last year, with four November tornadoes in Dakota and Washington counties, rare occurrences do occur.
Only time will reveal what the rest of this year in weather has in store, but as we know; the one guarantee with Mother Nature is there are no guarantees.