MANKATO, Minn. (KEYC) - In the office today we talked about the severe weather threat for this evening. We first started talking about the slight risk across parts of southern Minnesota and northern Iowa. The conversation then switched to how there is a 5% chance for tornadoes. Followed by someone saying “but its only a five percent chance, that’s not that high".
So what does it mean when there is a 5% chance for tornadoes and why is the risk higher than you think.
The Storm Prediction Center has a tornado risk scale that ranges from 2% to 60%. Hardly do they ever issue anything above a 10% unless they are certain of a wide spread outbreak. Today there is a 5% risk for tornadoes across parts of southern Minnesota and northern Iowa with a 2% ring around that.
A 5% risk for tornadoes means that there is 5% chance you will see a tornado within 25 miles of any point within the forecast area. So since Mankato is in that 5% risk area, there is a 5% chance of seeing a tornado within 25 miles of Mankato.
This is when you go OK, but 5%, that is not that high.
That is when I will tell you that when you look at these probabilities you also need to factor in climatology averages. For instance lets hypothetically say that on September 9th from 1950 to 2018, there has been a tornado within 25 miles Mankato 1% of the time (Not actual data). That would mean today’s 5% risk would make it 5 times more likely a tornado would occur within 25 miles of Mankato.
Just for reference, from 1991-2010, the entire state of Minnesota has averaged about 2 tornadoes for the month of September.