Many locations Wednesday afternoon reached 90 degrees, with Rockford’s official high topping out at 89 degrees. The afternoon heat did help fuel widely scattered thunderstorms that produced torrential downpours, frequent lightning, and even gusty winds. A severe thunderstorm warning was even issued for a portion of Rock and Walworth counties.
The radar has turned quiet Wednesday evening with skies expected to remain mostly clear to partly cloudy overnight. It’s actually a fairly comfortable evening, especially in the areas that experienced the afternoon rain and where temperatures cooled as a result.
Thursday will be similar to Wednesday, temperatures soaring through the upper 80s and low 90s with a few afternoon thunderstorms. A slightly drier air mass Thursday may limit the overall coverage but look for storms to begin after 1pm/2pm, and last through sunset. Like Wednesday, torrential downpours and gusty winds will be possible. We may actually see the storms push a little further west of I-39 where storm coverage Wednesday was very isolated.
The thunderstorms were not the result of anything from above, but more so from what happened at the surface. The difference in temperature between the waters of Lake Michigan and the adjacent land caused a lake breeze to develop. As the lake breeze moved inland it caused the slightly unstable air mass to rise, resulting in thunderstorms.
Those storms then produced outflow boundaries from rain cooled air. These boundaries produced additional thunderstorms that slowly moved north and northwest. Because the winds aloft were so weak, the storms were not moving fast at all resulting in heavy downpours – for those who were under those storms. For others, it didn’t rain at all or only a few drops of rain fell. Isolated thunderstorms will occur once again Friday, but likely remain confined to northwest Illinois. Winds on Friday will shift to the east, but it won’t do much to cool us off. In fact, highs on Friday are forecast to warm into the low 90s during the afternoon.