Last weekend, we saw very warm temperatures throughout the Eastern US before a powerful cold front swept those warm temperatures out the door and with it brought the coldest air mass of the season.
Before the big cool down, a few thunderstorms, some severe, brought hail and even a few tornadoes in the Minneapolis metro area. Now, you may be asking, how can there be tornadoes at this time of year?
Well, the setup for this event was quite interesting. I’m going to share with you a few graphics of the meteorological setup for this event.
In the second surface map, around sunset in Minneapolis, a stationary warm front is situated over central Minnesota and a cold front extends from the area of low pressure just to the west of the Twin Cities, somewhere around the St. Cloud area. The Twin Cities happen to be in a favorable region for tornadoes to develop, as winds at the surface are southerly and winds in the mid-levels (not shown) are from the west. This creates what is known in the world of meteorology as shear. Shear is key to the development of tornadoes, along with moisture, instability, and a source of lift. On this day, the main ingredient that was lacking, was instability. If this system had had a more warm, moist unstable airmass to tap into, their may very well have been a much greater tornado threat.
In the first surface map, around 9PM, the system begins to occlude, meaning the low pressure at the surface begins to become displaced with the center of the storm in the middle and upper levels of the atmosphere. The area where the warm front and cold front intersect with the surface low, is called the triple point. This is often a place where the greatest shear and instability are located, and it is no coincidence that it is over the area where these tornadoes occurred.
Take a look at the links below to see the surface maps from this event.
Ultimately, these tornadoes occurred because of they formed in an environment that was favorable for development. However, if there was more lift and instability in the atmosphere on this day, things could have been much worse.
Take a look at a few images of the damage from the NWS in Chanhassen. They surveyed the damage and determined the EF ratings for the tornadoes.
For more on this event, check out the National Weather Service’s damage assessment on their webpage at the link below.
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Until next time.