Threat for Severe Weather on Monday
One of the better severe weather chances is moving towards southeast Wisconsin on Monday in two waves. The first arrives very early Monday and the second (dependent on the first) arrives late afternoon / early evening.
As of Sunday evening, four tornadoes have already been reported as a part of this storm system in northern Minnesota. Those thunderstorms are expected to combine and form a \"bowing line\" before midnight and quickly move across Wisconsin, through southeast Wisconsin, and into Illinois. \"Bowing lines\" are typically responsible for very strong wind gusts and the heat and humidity of the day will bring plenty of rain. This line is forecast to begin moving through southeast Wisconsin between 3-6 AM and last no longer than three hours.
The greatest threat with Round #1 of storms will be damaging wind and some heavy rain. The current SPC (Storm Prediction Center) outlook places us in the \"slight\" risk for severe weather. A much better chance for severe storms exists across Minnesota into western Wisconsin, NE Iowa and NW Illinois.
If this timing stays true, Round #1 will be done by late morning, giving the atmosphere plenty of time to destabilize and bring us plenty of heat and humidity. Temperatures on Monday are expected to rise into the middle 80s with dew points in the middle 60s.
Two of the must have \"ingredients\" for severe weather are instability (the more unstable the atmosphere the better) and shear (or the spin/rotation of a thunderstorm). When analyzing instability meteorologists look at a parameter called CAPE (convective available potential energy), anything over 2,000 is good and on Monday CAPE values are forecast to exceed 4,000 in southern Wisconsin. For shear, we typically like values over 30 and forecast values for Monday are 30-40. If these forecast values can materialize, severe storms will be likely Monday late afternoon / early evening. Right now 4-7 PM looks like the best window for severe weather.
If these storms develop they will start out scattered and then quickly move southeast into Illinois where they are expected to form a bowing line and travel all the way into Kentucky.
This Round #2 of storms looks more impressive than Round #1. Damaging wind will still be the main threat, but large will also be possible and the threat for a tornado does exist.
Round #2 of storms will likely exit southeastern Wisconsin shortly after sunset.
As is often the case with these two round type of thunderstorm events, Round #2 is heavily dependent on Round #1. The faster Round #1 leaves Wisconsin and the sun starts to shine, the better our chances are for Round #2 and vice versa. If for some reason, Round #1 misses the Milwaukee area completely, uff da, Round #2 could be a doozy.
Look for a forecast update Monday morning after Round #1 passes through.