Warmth Peaks Today, Significant Cool-Down Ahead

Warmth Peaks Today:

Northern Illinois and Southern Wisconsin experienced a beautiful September day to kick off the work week. Skies were sun-filled from start to finish, humidity remained low, and highs peaked in the upper 70s and low 80s. Aside from an isolated early morning thunderstorm or two, Tuesday also features a good amount of sunshine. 

However, Monday's northeasterly wind shifts to the south-southwest today, becoming a bit breezy by the afternoon. This will allow highs to inch closer to the 90° mark, a feat Rockford has hit 19 times this year. However, it's not uncommon for us see temperatures this warm so late in the season. In fact, Rockford witnessed its latest 90° day back on October 9th, 2010. So a little over a decade ago.

Capped Atmosphere:

Now, model guidance does show sufficient parameters in place for the potential for a few late-day showers and thunderstorms. However, with a decently-sized layer of warm air sitting just above the surface, a "cap" will be present. Typically, you need the temperature profile in the atmosphere to cool with height for thunderstorm growth and development.

However, when you have this "cap" or "atmospheric lid" in place, air rising into this layer becomes cooler than the surrounding air, limiting the ability for thunderstorms to mature. If anything does manage to break the cap later, the odds of it becoming severe are very low. Tonight remains quiet for the most part, with temperatures falling on either side of the 70° mark.

Wednesday's Cold Front:

Right around sunrise is when we can expect our next cold front to arrive. Cloud cover is then expected to increase shortly after the frontal passage takes place, with winds quickly turning to the north and northwest. 

This cooler breeze will allow a much cooler and fall-like air-mass to filter in from the Upper Midwest, resulting in a steady temperature drop. By Wednesday afternoon, temperatures will be sitting in the mid to upper 70s, with upper 40s expected overnight. Despite Thursday featuring a bit more sunshine, that gusty northwesterly wind will limit highs to the lower 60s. To put this upcoming chill into perspective, we haven't seen highs in the low 60s since late May.