Confidence is on the rise for severe thunderstorms to occur Saturday afternoon and evening.
A strong storm system is intensifying over southeastern Colorado as of Friday at 7:00 PM. This system will continue to strengthen as it propagates northeastward through the central Plains and eventually into western Iowa come Saturday afternoon. Though scattered showers and thunderstorms are forecast through most of Saturday, the severe threat looks to begin in the late afternoon hours when the storm’s warm front begins to drag northward through central and northern Illinois and last through the mid-evening.
This storm system is situated underneath the lee side of a deep mid-to-upper level trough. A strengthening jet streak on the lee side of the trough will likely contribute to the system’s intensification causing additional upward vertical motion through the night and into Saturday. Lapse rates are expected to steepen as the trough takes on a slightly negative tilt on its way to the Midwest. Low and midlevel lapse rates are forecast to reach between 7 and 8 degrees Celsius per kilometer resulting in a highly buoyant environment. Notable instability with CAPE values between 1500 and 2000 Joules per kilometer will be easily reachable with no significant convective inhibition, steep low-level lapse rates, warm frontal lifting, and the environment’s level of free convection situated below one kilometer above ground level. Strong moisture convergence will allow dewpoints to reach the upper 50’s to lower 60’s. The storm’s shear profile is also suitable for severe convection with surface winds out of the south-southeast at 10-20 kts and 3km winds exceeding 50 kts out of the southwest. All of this comes together to produce an environment suitable for long-lived, rotating updrafts.
The primary concern with Saturday’s storms will be sizeable hail, occasional heavy downpours, and tornadoes. As of Friday at 7:00 PM, the Storm Prediction Center is Norman, Oklahoma has issued a moderate risk, outlook 4 of 5, for much of east central and north central Illinois for Saturday. Most of Whiteside and Lee Counties are included in the moderate risk while much of Carroll, Ogle, and DeKalb Counties are under an enhanced risk. The rest of the Stateline remains under a slight risk. Tornado probabilities are as high as 15% a 10% or higher probability of significant tornadoes (EF-2 or stronger). Hail probabilities are as high as 30% with a 10% or greater probability of significant hail (two inch diameter or larger).
The time frame most favorable for severe weather in the Stateline is 4:00-8:00PM. The greatest chance for severe weather remains, for the time being, confined to areas south of I-88 though severe weather is very possible anywhere across the Stateline.