862 FXUS63 KLOT 102336 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 636 PM CDT Tue Jun 10 2025 .KEY MESSAGES... - Wednesday (tomorrow) will likely feature the warmest temperatures for the area since May 15th. - Shower and thunderstorm activity Wednesday night onward may initially focus primarily near or north of the IL/WI state line and then sag southward Friday-Saturday. - There is a signal for hot and humid conditions to develop across the general region next week with periodic shower/storm chances. && .DISCUSSION... Issued at 257 PM CDT Tue Jun 10 2025 Through Wednesday Night: Following a downright cool start to the day today (Tuesday), the warm-up to near/slightly below normal highs this afternoon is merely the appetizer for the substantial warm-up in store for Wednesday. Breezy west-southwesterly winds will subside by or before sunset this evening, with light to moderate southwest winds the rest of the night. Overnight low temperatures will be propped up by warm advection and the modest southwest winds to upper 50s/around 60F, except low-mid 60s in and near Chicago. On Wednesday, it appears that the general haziness from lingering wildfire smoke should be less pronounced than today per HRRR vertically integrated smoke forecasts. 925 mb temps in the upper teens Celsius this afternoon will jump about 5C by this time tomorrow. With plenty of sun and better moisture return flow still blocked off by high pressure to our southeast translating to dew points only in the upper 50s-low 60s in the afternoon, the stage will be set for mid-summer like warmth. Breezy west-southwest winds gusting to 25-30 mph and deepening mixing will further contribute to highs in the upper 80s for most of the area (except far southeast) and a smattering of highs up in the 90-91F range. This will particularly be the case for the heart of the Chicago urban heat island. If Chicago (ORD) and Rockford reach 90+F, it will be the first 90F temps there since May 15th (94F at both sites that day). Turning attention to convective potential Wednesday night into early Thursday, there was a distinct "less convectively active" flavor within the CWA in the 12z 6/10 cycle versus previous cycles. This was even as a previously quasi-stationary boundary is still progged to make slow southward headway down toward the I-80 corridor by pre-dawn Thursday (a bit faster southward progression of the boundary near the lake, as is common). A positively tilted short-wave impulse or two (one in the early evening and another overnight possibly) should move east-southeast from the upper MS Valley to Lake Michigan and potentially interact with the slowly sagging frontal boundary. The change in the guidance today to lesser shower/t-storm chances Wednesday night looks to be tied to lower RH in roughly the 850-700 mb layer serving to keep 1500-2500 J/kg of potential MUCAPE largely capped. At the risk of subsequent cycles trending back towards more of a convective footprint across parts of northern IL, opted to take an incremental step of nudging PoPs downward with this issuance. Chance (30-40%) PoPs are now confined to the IL/WI state line counties, and tapering quickly southward from there (<15% in the central and southern Chicago metro). Fairly steep mid-level lapse rates and up to 30-35 kt of effective deep layer bulk shear do introduce the possibility for isolated organized convection, if any storms indeed form. Conditional upon MUCIN being eroded sufficiently for a few thunderstorms to develop into far northern Illinois, there is a level 1 of 5 (marginal risk) threat for a severe storm capable of producing localized damaging hail and/or damaging downburst winds. Aside from the lower-end/uncertain/lower confidence shower and thunderstorm chances for far northern Illinois, it will be a seasonably warm overnight with lows in the 60s, except upper 50s near the IL North Shore. Castro Thursday through Tuesday: The extended forecast is looking a bit more soggy with temperatures in the 80s, and 70s along the shoreline (on day when the winds are out of the east-northeast). It will not be a total wash out with some dry periods, but there is lower confidence on the exact timing of showers and thunderstorms given so much model disagreement. For Thursday, models have trended farther north with that stalled front and slower with any southward progression. The GFS tries to bring it in during the day, whereas the Euro holds off until later. As a result, PoPs were pulled back slightly in the northwest portion of the CWA for the lack of certainty in coverage. That being said, there should be less cloud cover away from the boundary (around and south of I-88), with south- southwesterly flow providing warm air advection and isentropic upglide, decent CAPE, and around 20 knots of shear leading to the chance for some showers and storms, especially in the afternoon and early evening. There are discrepancies on specific timing, but most ensemble members are coming into agreement on finally moving the frontal boundary and its surface low over the area Thursday night through Saturday. It would not be surprising if Thursday night ended up being slightly drier given the poor time of day and lack of confidence initial start of frontal movement. But from Friday through Saturday, there should be better forcing with the front as it passes through to allow for better coverage of showers and thunderstorms. Felt no need to change what the NBM provided with highest PoPs around 60 to 70 percent around the Rockford Metro Friday afternoon, and southeast of Interstate 55 on Saturday afternoon. There is about moderate confidence in these trends for now, but whether or not the guidance is on the right track with this slower progression of the front remains to be seen and timing will no doubt be adjusted in future forecast updates. A weak mid-level wave is forecast to move over the area on Sunday. While models are in poor agreement with both timing and strength of the wave moves over, there is some agreement in a slightly drier air mass in the mid levels. Sunday may be drier, but with residual moisture and diurnal heating, there is the slight chance for additional showers and an isolated storm in the area. Heading into next week, the Euro depicts a broad upper ridge building across the central CONUS on Monday which persists through a good portion of the week. If this pattern plays out, it would favor a period of above average and humid conditions with highs in the 80s and possibly 90s at times. Being on the eastern periphery of the ridge, it would lead toward additional showers and storms. However, the GFS is now resolving an upper level trough that drops down from Saskatchewan moving toward Wisconsin with a reflected surface low and better forcing for more vigorous convection with it. The Canadian is sort of an average of the two, with a weak trough that passes to the north. Given all the uncertainty, the NBM chance PoPs were left in for now leaving quite a murky yet wet at times extended forecast. DK && .AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 636 PM CDT Tue Jun 10 2025 Key aviation messages for the 00Z TAFs: - Brief period of marginal LLWS possible prior to sunrise. - Breezy west-southwest winds again Wednesday. - Scattered TSRA threat late Wednesday expected to remain well northwest of terminals through end of this taf period. Quiet aviation weather conditions are expected tonight, as breezy west-southwest winds diminish with sunset this evening. Weak lake breeze noted over southeast WI is expected to push inland into far northeast IL, but remain northeast of ORD/MDW with no wind shift expected this evening. A modest west- southwest 35 kt low level jet is forecast to develop toward sunrise and may produce a very brief marginal LLWS potential before the jet weakens and shifts off to the east. Surface winds will become breezy as low-level mixing commences after sunrise, and can`t rule out a brief 25+ kt gust as we mix into the departing shallow jet. Overall however, wind gusts in the 20-25 kt range are expected during the day Wednesday before easing again with sunset tomorrow evening. A weak cold front is forecast to sag into far northern IA and southern WI Wednesday night, and become a focus for scattered thunderstorm development. At this time, it appears that the threat of storms will remain well northwest of the terminals through the end of the current TAF period. Some model guidance dose indicate that the front may sag into northeast IL after 06Z Thursday which could produce a northeast wind shift for ORD/MDW, but again this is beyond the end of the current TAF. Otherwise, skies remain hazy aloft due to lingering wildfire smoke. Little in the way of surface visibility impacts have been noted across the region, and GOES-19 satellite experimental cross sections and HRRR model forecasts indicate much of the smoke is in the 050-110 layer. While making for a hazy appearance aloft, the smoke layer is transparent enough that no mention is necessary in TAFs. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago