The 2013 Tornado season (March through August) in Minnesota has been quiet in respect to past years, bringing in only 5 confirmed tornadoes as of July 11th, 2013. Although tornadoes haven't been rampant, severe weather has occurred several times throughout 2013, causing flooding, hail damage and fallen trees.
If you're wondering if these conditions are a little odd in respect to other years, you are correct. Looking at data found from the Storm Prediction Center, Minnesota hasn't had this quiet of a season since the pre-doppler era of 1988. The Storm Prediction Center has created products that show our probability of severe weather and for tornadoes, each showing that this 2013 season is not following past trends. The SPC also provides another view the lack the tornadoes for this season through showing the amount of watches the nation has seen up to July 11th, 2013.
This season's pattern can be partly attributed to our cool and gloomy spring/early summer in 2013. Throughout many locations of Minnesota, average monthly temperatures were reported below normal for March, April and May. Tornadoes thrive off of warm and moist air at the surface with cooler air aloft, so while the surface-air was saturated during March-early June, it wasn't anywhere warm enough to consistently produce tornadoes. Another reason can be found in the jet stream's positioning over Minnesota and surrounding regions. The jet stream, a large current of air aloft, has had zonal flow (in the west to east direction) for June 2013 and early July 2013, meaning that wind direction was generally homogenous around our region. This inhibited the ability for veering winds to form, which are integral to the genesis of tornadoes. Aside from winds, the jet stream also pocketed areas of high pressure consistently over the upper midwest, creating fair and mostly tranquil conditions. These conditions, however, were still able to produce significant storms and wind events so 2013 has not been inactive by any means.
We went through the SPC's raw tornado information and singled out all of the Minnesota events. The SPC's homepage also has plenty more information for national severe weather, and of course your closest NWS office will keep you informed on any severe weather outbreaks.
Prepared by John McCarty, Climatology Volunteer
Pete Boulay, DNR Climatologist