Along with record breaking heat the past week or so, many of us have seen the year’s first thunderstorms. Storms have come a lot sooner than normal than previous years, especially, severe thunderstorms. We saw our first severe thunderstorm in the Chippewa Valley last Friday with another round of storms just this past Monday.
That round of storms on Monday was a line of storms that worked up from Iowa feeding on the warm and moist air across the Upper Midwest. There were a few severe thunderstorm warnings in the Coulee region. The main threat from the storms was some severe winds. The line of storms knocked down a power pole in Onalaska. Here’s a look at the storm reports from March 19th. You notice several wind damage reports in the Midwest, but look in Minnesota…a tornado report.
The line of storms on the 19th above was for the most part a typical spring-type line of thunderstorms, but one of the storms had enough rotation to drop a very weak tornado. That storm is circled in white. You can kind of see there may be some circulation based on the radar reflectivity. Notice how in that circle the reds and oranges kind of make an “L” shape, and it kind of notches. That usually can be a sign of some rotation in a line of storms. That rotation is confirmed by the velocity reflectivity below. This is a measure of the wind speeds within the storm and their direction. This is from the Twin Cities radar site which, which is north-northeast of where this tornado touched down in Le Sueur and Waseca counties. The greens are winds going towards the radar while the reds are the winds blowing away from the radar. Check out the yellow circled area. Notice how the two colors look as if they are trying to wrap around each other, and the colors are quite bright. This is the rotation within the storm that produced a tornado. On radar we call these couplets. This is a pretty text book case of couplet, and it produced a tornado, albeit, a weak one.
The tornado was a weak EF-0 twister with winds of around 80 mph. The tornado was on the ground for 7 miles. You can see the path below. It came very close to hitting the heart of the small town of Elysian, MN. The tornado touched down around 6:25pm and was only on the ground for 10 minutes.
Below are some damage photos taken by the National Weather Service. The damage is consistent with EF-0 damage. You can see the weaker farm structure on the south side of Lake Elysian was blown down pretty good, but notice the sturdier home only had some minor roof damage on the north side of Elysian, MN.
There wasn't just a tornado that caused damage along the path of this twister. On the east bank of Lake Francis as the map shows below, the damage was across a much wider area. Up to a mile wide. This was due to what's called a downburst. Downbursts are just a strong downdraft in the thunderstorm. They are formed from precipitation falling through an area of drier air up in the cloud. Some of the precipitation evaporates which cools the air being dragged down by the precipitation. Colder air is denser and falls faster. The downburst hits the ground and the air rolls. As the cold air rolls out it compresses causing the winds to increase dramatically. That's why the can be so dangerous. Normal downdrafts in a thunderstorm are just less severe and not as localized.
What's truly significant about this event is the fact how early in the year this EF-0 tornado touched down. This is the second earliest in recorded history a tornado has touched down in Minnesota...by one day. The earliest recorded tornado touchdown in Minnesota is on March 18th, 1986 in the town of Truman. 2012 continues it's strange ways.