Several rounds of rainfall appear likely to occur beginning Friday, scheduled to continue off and on through much of Saturday and Saturday Night, perhaps continuing into the opening hours of Sunday. In addition to locally heavy rainfall being a rather widespread threat in the Stateline, thunderstorms are also likely to be a player during this time, including one specific window during which there could be a rather significant risk for severe weather in much of Illinois, possibly including the Stateline.
Much of Friday's daytime hours are to be dry, though at any point during the afternoon there could be a few stray showers that develop. Come Friday Evening, and more so Friday Night, rain is likely to become more widespread as a slow moving warm front to our south begins a northward climb. The severe threat Friday Night is extremely low, though there'll likely be several embedded thunderstorms capable of producing heavy downpours, small hail, and frequent lightning. This activity is likely to continue until the early part of Saturday.
Rain should take a break by mid to late morning, and will likely continue through the early to mid-afternoon hours. What happens during that break will go a long way in determining the extent of a severe weather threat in the Stateline. We'll closely pay attention to two things in the early afternoon. First, we'll need to keep a close eye on the position of a warm front that's to continue lifting northward. Should the warm front move through the area, and temperatures start to surge, the amount of energy in our atmosphere would skyrocket. Should it stay just to the south of our area, we'd see much less energy, and thus a much lower severe potential. Additionally, we'll need to pay very close attention to how little or how much sun we see during that time. As a rule, more sunshine equals more energy, which equals more of a severe weather threat. Less sun means less energy and an inherently lower severe threat. This will be one of the few cases where we'd greatly prefer cloudiness over sun, and cooler temperatures over warmth.
Latest model projections do suggest the warm front eventually does reach into the Stateline very late Saturday Afternoon or very early Saturday Evening. This would send temperatures soaring into the low to mid-60s, if only for a brief moment. It would open up the opportunity for a brief severe weather threat between roughly 3:00 and 7:00pm.
At this stage in the game, all modes of severe weather would be in play. The atmospheric setup in the late afternoon and early evening would be one conducive to large hail, very gusty winds, as well as a few tornadoes. Right now, it appears as though the greatest risk for severe weather and tornadic storms will set up along and south of Interstate 80, where atmospheric energy levels will be maximized.
Breaking down our local risk, our southernmost counties will see the greatest severe risk, with the threat going down rather quickly the farther north you go. Here's our assessment as of 11:00 Thursday Evening, on a scale of 1 to 10, with 1 being the lowest risk for severe weather and 10 being the highest.
Whiteside, Lee, and Southern DeKalb Counties: 5 out of 10.
Carroll, Ogle, and Northern DeKalb Counties: 4 out of 10.
Jo Daviess, Stephenson, Winnebago, Boone Counties: 3 out of 10.
Green, Rock, Walworth Counties: 2 out of 10.
Stay tuned as we continue to monitor this developing situation.