Severe weather bypassed the Stateline Saturday evening and overnight, and again Sunday, as the heat and humidity soared Sunday afternoon. Highs both days warmed into the upper 80s and low 90s, but the heat index Sunday reached over 100 degrees for some.
Isolated showers and thunderstorms developed early Sunday afternoon but were quick to lift north into Wisconsin and east towards Chicago. Other than just a quick brief downpour, much of Sunday afternoon was dry. The dry trend will continue for most this evening, however, a cluster of thunderstorms moving north from southern and central Illinois will continue to threaten parts of north and northeast Illinois Sunday evening and night. As the cluster of storms continue to move east they’ll likely remain mostly east of I-39. This places areas such as southern Lee, DeKalb, Boone and McHenry counties at a slightly higher chance for a few thunderstorms Sunday evening after 9pm/10pm.
The risk for a few strong wind gusts, along with heavy downpours, will be possible with any storms that do develop. The remainder of the region will stay mostly dry, and muggy. Additional thunderstorms are possible early Monday morning as an area of low pressure moves in from the northwest. Instability won’t be at its highest point, but there will be plenty of moisture around. Those showers and storms will occur as an upper level trough (dip in the jet stream) moves over southern Wisconsin and northern Illinois. This incoming trough will pull a warm front close to, if not stalling out, very close to northern Illinois. The boundary *could* become the focal point for additional thunderstorms to develop along during the afternoon as instability builds back up following the morning rain activity. Just how much instability, though, is the big question. As the low pulls further away from northern Illinois a ridge of high pressure will move in from the west. As this ridge moves in it’ll work to put a lid, or cap, on our atmosphere during the afternoon.
The departing shower/storm activity from Monday morning may leave behind a remnant low pressure system. The combination of the low and warm front, as mentioned above, could help trigger additional thunderstorms during the afternoon (helping to break the lid). If that occurs the risk for strong to severe thunderstorms will be possible. Right now areas south of the state line are highlighted under a marginal risk for severe weather Monday afternoon and evening. As the picture becomes a little more clear by Monday morning as to what could potentially play out by the afternoon, it’s possible that parts of the Midwest – Illinois and Indiana – could be upgraded to a slightly higher risk.