The high humidity from the last few days took a little break Sunday afternoon. While still hot; temperatures warmed into the upper 80s and low 90s, the dew point temperature dropped into the low to mid 50s across much of northern Illinois, low 60s a little further south, and even into the 40s for a time in Rockford! This actually brought the heat index temperature down below the air temperature at times Sunday afternoon.
Under a mostly clear sky Sunday night temperatures will dip into the low 60s for the start of Monday. Winds will turn calm but fog isn’t expected to form. As high pressure shifts a little further to the southeast of Rockford Monday, southerly winds will bring in a little more moisture for the afternoon. Temperatures Monday will warm quickly once the sun comes up, reaching the upper 80s and low 90s once again. With a slight increase in the dew point temperature, the heat index for the afternoon may make it feel a degree or two warmer than the actual temperature.
A return of moisture will be in full force Tuesday and Wednesday as southerly winds bring dew point temperatures into the upper 60s and low 70s. This will push the heat index into the upper 90s Tuesday and then near 100 degrees Wednesday. The one limiting factor would be if we see storms during the afternoon, but right now that chance appears to remain low.
Thunderstorms will be likely to the north over the Plains and Midwest Monday evening, Tuesday and Wednesday. As a strong ridge of high pressure builds over the central Plains it’ll work to push the eastern edge of the jet stream down through Wisconsin and Illinois. A cap, or lid, on the atmosphere will be in place during the day Tuesday, and for most of Wednesday. This will likely prevent thunderstorms to the north from becoming widespread across the area those days, but isolated thunderstorms are still possible; especially if any outflow boundaries make it this far south. The best period for rain/thunder will be late Wednesday into Thursday as a cold front moves in. Right now any risk for severe weather looks to low. That front will then settle down to the south leaving us with a little less humid air mass for the end of the week.