With flash flooding occurring just west of our area already Thursday, this could be a sign of things to come. The recent rain has soaked much of the area. But soil can only absorb so much water at one time. When it's already wet, it will have a hard time absorbing large amounts of water in a short amount of time because there is just nowhere for the excess water to go. This is what can create a flash flood situation. There are many factors used in calculating the flash flood risk at any one location like soil type, moisture content, average temperature, elevation, etc… With these factors, the National Weather Service issues a product that shows the general amount of rain it would take over a specific amount of time to produce flash flooding. The levels are issued by county. The image below is the flash flood guidance for the state of Minnesota and surrounding areas at 11am on Thursday. This will show how much rain each county needs in a 3 hour period to create flash flooding.
Hennepin County for example will need 2.25" of rain over a 3 hour period to create flash flooding. Remember though that the flash flood potential is not a static number, it changes constantly depending on its variables. So this number will go up or down depending on the situation. However, thunderstorms the next few days could produce 2 inches of rain in an hour or less, so 2.25" may sound like a lot, but in reality, it's not that much.