680 FXUS63 KLOT 122337 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 637 PM CDT Thu Jun 12 2025 .KEY MESSAGES... - Scattered showers and possibly a couple of thunderstorms Friday into Saturday, especially south of I-80. - Very warm and humid with occasional chances for showers and thunderstorms early to mid next week. && .DISCUSSION... Issued at 255 PM CDT Thu Jun 12 2025 Through Friday night: Weakening surface boundary is draped across northern IL, but outside of areas influenced by the marine layer, there is very little theta-e difference across the boundary. We are seeing a modest, albeit rather mushy, cumulus field developing near and south of this weak boundary beneath the cirrus canopy. Aided by little more than some weak low level convergence, the chances for any showers developing late this afternoon appears low. Opted to hang onto slight chance pops late this afternoon until around sunset on the off chance a couple of showers manage to develop. Upper low over the southern Plains is still progged to devolve into an open wave as it moves slowly northeast toward the mid Mississippi Valley. Scattered showers could encroach on our far southern late tonight with the shower chances spreading slowly northward Friday morning. Still appears the best chances of rain will be south of I-80, though some showers can`t be ruled out farther north. Any filtered sunshine through the cirrus shield Friday afternoon could theoretically allow for some weak boundary layer destabilization. Nearly moist adiabatic mid level lapse rates will further limit destabilization, but cannot rule out some isolated thunderstorms Friday afternoon, with chances increasing the farther south. Some scattered showers could continue into Friday night as the slow moving system continues to meander ever so slowly east-northeastward across the region. - Izzi Saturday through Thursday: On Saturday, the open wave (remnant closed low) to our south and its associated weak surface low pressure reflection will track toward the central Ohio Valley. With this farther south low path and stabilizing influence of high pressure over the Great Lakes region, shower chances have trended markedly southward over the past few model cycles. There should still be some shower activity south of I-80, but probably more isolated to widely scattered in nature, with highest coverage (55-60% PoPs) generally south of US-24. Better mid-level moisture being focused down near the surface low (and 700-500 mb RH will be trending drier PM hours) and already sub-marginal (5.5C/km or less) mid-level lapse rates, there`s only a slight chance for a few isolated embedded thunderstorms where measurable precip may occur. High temperatures inland of Lake Michigan will range from the upper 70s to mid 80s inland, warmest west/northwest due to increasing sunshine and "coolest" southeast due to thicker clouds. Dew points generally in the 60s will translate to moderate humidity levels, common for mid June. Following a quiet and seasonable Saturday night into Sunday morning, a convectively modified impulse (possible MCV) may move toward the mid MS Valley by early evening. This could conceivably bring some threat for showers and isolated thunderstorms to far interior sections in the afternoon, particularly near/west of I-39 in north central/NW Illinois. With a sharp west-northwest to east-southeast instability gradient resulting in limited instability locally, suspect that the 12z ECMWF/EPS (ECMWF Ensemble) cycle was too bullish with the convectively driven QPF footprint. The 20-30% PoPs well away from the lake, highest northwest, in our official forecast, appear appropriate for now in light of the rest of the guidance being less bullish/farther west with their QPF output than the EC. Expecting high temperatures a couple degrees warmer on Sunday vs. Saturday, in the mid to upper 80s away from the lake, along with muggy conditions, especially south of I-80 (near 70F dew points). Temps may be able to climb a couple degrees lakeside as well, but still notably cooler. Looking ahead to next work week, we`ll enter into a classic summer-time pattern with very warm (aside from any lake cooling) and humid conditions and occasional shower and thunder chances. Modest forcing and weak flow aloft Monday-Tuesday should translate toward largely diurnally driven convection (peak coverage in the afternoon-evening) and the main thunderstorm threats being lightning, downpours, and localized gusty winds. Despite the day 6 lead time, there`s a decent signal in the ensemble guidance for a stronger synoptic system and associated cold frontal approach into the region in the Wednesday-Wednesday night timeframe. This may be a period to watch for organized convection and a threat for strong to severe thunderstorms in the region. If the Wednesday- Wednesday night timing comes to fruition, then Thursday would likely end up mainly dry and slightly cooler/less humid in the wake of the cold front passage. Castro && .AVIATION /00Z TAFS THROUGH 06Z FRIDAY/... Issued at 636 PM CDT Thu Jun 12 2025 Key Messages: - Spotty shower or storm possible near RFD early this evening (15% chance) - Showers lift into portions of the area on Friday, with the best potential N of I-80 mainly in the afternoon (30% chance) Dry conditions are forecast through tonight at area terminals. Will have to keep an eye on RFD early this evening, however, as spotty showers/storms are beginning to develop across far northwest Illinois. If this activity expands further east along the stalled surface boundary, a VCSH/VCTS mention would need to be added to the TAF. Confidence in shower trends during the day on Friday north of I-80 remain rather low and opted to not make any changes to the inherited PROB30s at ORD/MDW during the afternoon and added them to the other sites for consistency. Coverage would be isolated to widely scattered and could persist into the evening as a slow moving disturbance lifts across the region. An eventual VCSH or TEMPO for -SHRA mention may be needed with later updates. Winds will remain east to northeasterly through this evening, becoming light and variable overnight. Winds then settle back into a mainly southeast direction during the day on Friday. There is some hi-res guidance that attempts to turn winds briefly SSW late morning into early afternoon at ORD/MDW. VFR conditions are forecast through the period, though some MVFR stratus may lift toward the area late Friday evening. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago