Today’s thunderstorm potential all begins with a lifting warm front. The cold front that passed through our area yesterday has stalled over southern Illinois. As we head into the afternoon, it will begin lifting as a warm front along with a low pressure system coming in from the southwest.
Now, how warm we get today and the severe potential all comes down to how far north this warm front lifts. It also depends on how much energy was taken by not only this morning’s shower activity, but also by the line of storms or the mesoscale convective system that is currently moving through Missouri and Eastern Iowa.
Models have shown that the warmest weather will most likely stay off to the south. But if the warm front can lift as far as southern Wisconsin, then that will place us in what is known as the warm sector of the low pressure system. That by definition is the area between the warm front and the cold front. Winds are generally out of the south or southwest in the warm sector which helps transport the moisture and energy needed to produce general or severe thunderstorms.
The SPC did downgrade a portion of the Stateline in the 8 AM Day 1 Convective Outlook that was under a slight risk this morning down to just a marginal risk. For now, a majority of our counties in Northern Illinois are placed under a Marginal Risk (Level 1 of 5) for severe weather. A highest severe threat for today is mainly off to our southwest.
Here is what to expect as we go into this evening. The primary hazards with these thunderstorms will be the heavy rain and flooding issues that will result from these storms. As much as 1 to 2″ of rain could fall between now and Friday morning. The amount of runoff along with a saturated ground could cause a rise in the river levels as we head into the weekend. Runoff from the additional rainfall could cause minor flooding issues along a few spots on the rivers. Severe-wise, the biggest threats if storms do jump into severe criteria will be gusty winds up to 60 mph, and small hail up to quarter size. Tornado threat for the Stateline remains minimal.
As we head into tomorrow, the cold front associated with the same low pressure system will pass through the viewing area and could spark up a few showers and maybe an isolated thunderstorm during the afternoon. Once that passes, we dry up for Mother’s Day weekend. High pressure will move in starting on Friday and will bring not only sunshine to the Stateline, but warmer weather as well!