043 FXUS63 KLOT 101124 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 624 AM CDT Tue Jun 10 2025 .KEY MESSAGES... - Dry conditions are expected through Wednesday, along with warming temperatures. - Shower and thunderstorm chances return Wednesday night and last through at least the weekend. - There is a signal for hot and humid conditions to develop across the general region next week. && .DISCUSSION... Issued at 315 AM CDT Tue Jun 10 2025 Through Wednesday: The Great Lakes upper trough will exit to the northeast today while a surface ridge of high pressure over the Ozarks this morning gradually shifts eastward across our region. This will foster a period of dry and quintessential weather across our area today through the daylight hours of Wednesday. The only weather item of note for today is the Canadian wildfire smoke, which should be streaming overhead area again today. With the increased particulates overhead through the day, the sky is likely to have a hazy look in spite of an otherwise mostly sunny to partly cloudy day. It also remains plausible that some of smoke particulates could mix down to the surface this afternoon as boundary layer depths increase to around 5,000 feet. As such, the surface air quality forecast today (courtesy of AirNow.gov) will be in the moderate category. Otherwise, expect somewhat breezy westerly winds this afternoon, with gusts of 20 to 25 mph. Temperatures will top out well into the 70s to around 80 in some locations. A quasi-zonal upper-level pattern characterized by broad but low amplitude ridging over the northern CONUS will expand eastward toward the Great Lakes on Wednesday as the surface high settles over the southeastern CONUS. This shift in the pattern will promote a much warmer day across the area Wednesday as a lower-level thermal ridge (featuring 850 mb temps of +18 to +19C) advects eastward from the Plains into the lower Great Lakes. Currently, we are anticipating lower smoke concentrations for Wednesday, which should support mostly sunny skies for the area. Altogether, this is expected to result in temperatures topping out in the upper 80s in most locations, including along the Lake Michigan lakeshore areas due to the breezy offshore winds. KJB Wednesday Night through Monday: An active stretch of weather is forecast to be getting underway Wednesday night as a stalled baroclinic/frontal zone establishes across portions of the Mississippi Valley and western Great Lakes. While there remains a near 50/50 split as to whether or not this frontal zone will be in northern IL or just to the north in southern WI, there has been a notable trend in the deterministic guidance to favor the zone setting up more into IL. Regardless, a cluster of thunderstorms is expected to develop Wednesday afternoon in northeast NE and western IA beneath a modest shortwave impulse and then track along the boundary into southern WI and/or northern IL Wednesday night. Given that the thunderstorm cluster will likely arrive well after dark, guidance continues to show the cluster weakening with eastward progress due to lack of surface instability and drier mid-level air. However, forecast soundings do show a modest elevated mixed layer in place across northern IL and southern WI characterized by 6-7 C/km mid-level lapse rates and around 1000 J/kg of MUCAPE. Meaning that if the cluster is indeed organized as guidance suggests then it should be able to overcome the mid-level dry air and utilize the instability to maintain itself. Couple this with the fact that modest 500 mb flow will be overhead generating 30- 35 kts of bulk shear, conditions do look to be in place for a conditional threat for severe weather mainly in the form of damaging winds and hail Wednesday night. Again whether or not this threat is our problem or that of our neighbors to the north is still uncertain, but the marginal (level 1 of 5) risk from the Storm Prediction Center for the northern half of our CWA during this period seems appropriate. The thunderstorm cluster should move east of the area early Thursday morning and give us a break in the weather as weak mid- level ridging builds in. However, the storms will likely push the aforementioned frontal boundary further into northern IL (if it is not already) which may allow for the development of diurnally driven scattered showers and thunderstorms Thursday afternoon/evening especially across the northern half to two- thirds of the area. While the kinematic parameters on Thursday do not look as impressive due to the weaker flow aloft, there is still forecast to be around 20-30 kts of shear which could support an isolated strong to possibly severe storm. Otherwise, temperatures on Thursday likely to be near to slightly above average of mid-June with highs in the lower to mid-80s. Though areas to the north of the boundary and near the lake will likely be notably cooler (highs in the mid to upper 70s and mid-60s respectively) due to onshore winds. For the Thursday night through Sunday time period, the forecast becomes much more uncertain as not only will we be dealing with the vacationing frontal boundary but also potentially a developing upper low forecast to be ejecting out of the southern Plains towards the Ohio River Valley. That said, if the arrival of the upper low is slower as some guidance (roughly 60% of the members) suggest then Thursday night may end of drier than currently forecast with Friday looking nearly identical to Thursday with diurnally driven showers and storms. Though with storm coverage on Friday being a bit further south due to convective modification of the frontal zone. However, if the faster solution pans out (as roughly 40% of ensemble members suggest) then a much broader coverage of showers and storms would materialize on Friday. With these uncertainties, have maintained the 40-60% POPs offered by the NBM Thursday night through Friday night which seem reasonable at this point but suspect further refinement will be made over the coming days as trends converge on a solution. Regardless of which solution comes to fruition, Saturday and Sunday basically look the same in either scenario. Thus, expect showers and thunderstorms to linger through the day on Saturday as the upper low moves through the region and the front begins to get pushed south by another shortwave diving in from the Upper Midwest. Rain will gradually taper through the day on Sunday as the front pushes the better moisture and instability out of our area. Outside of the rain, temperatures for Friday, Saturday, and Sunday will be nearly identical with highs in the upper 70s to lower 80s inland and cooler near the lake due to onshore winds. Overnight lows to remain in the lower to mid-60s during this period. Heading into next week, guidance remains in good agreement that a broad upper ridge will begin to build across the central CONUS on Monday and persist through a good portion of the week. This pattern is forecast to favor a period of above average and humid conditions with highs in the 80s and possibly 90s at times. Given that northern IL and northwest IN looks to be on the eastern periphery of the ridge, there will also be periods of showers and thunderstorms next week too especially towards midweek. Yack && .AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 624 AM CDT Tue Jun 10 2025 Forecast concerns for the terminals... - Breezy west winds this afternoon. - Hazy skies due to wildfire smoke aloft. - Period of gusty winds Wednesday morning as we mix into the low-level jet. A surface high will continue to build into the region today resulting in VFR conditions through the period. However, some wildfire smoke aloft will keep a haze to the skies through this evening. Given that smoke should be somewhat thin have opted not to formally include in the TAFs just yet. Winds will increase as diurnal mixing commences with gusts around 20 kts this afternoon from the west. Gusts will subside this evening with directions becoming more southwesterly, but speeds will increase once again Wednesday morning for a brief period as the atmosphere mixes into the 35-40 kt low-level jet centered over WI. With the jet core being centered to our north low-level wind shear is not expected. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago