


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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294 FXUS63 KLOT 220539 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1239 AM CDT Sun Jun 22 2025 .KEY MESSAGES... - Dangerously hot and humid conditions will continue through at least Monday evening. A Heat Advisory remains in effect through Monday evening for the entire CWA except Cook County IL where an Extreme Heat Warning is in effect. - The magnitude of heat should lessen by Tuesday, though heat indices will still be in the upper 90s to lower 100s through much of the next workweek. (No planned extensions of the ongoing heat products). - Chances for episodic thunderstorm clusters increase incrementally Tuesday onward. Some may be severe with damaging winds and flash flooding. && .DISCUSSION... Issued at 152 PM CDT Sat Jun 21 2025 Through Monday: No meaningful changes needed to the forecast through Monday evening with hot and humid conditions likely to continue. Sunday should be a near carbon copy of today with highs in the low-mid 90s, warmest in the urban corridor of NE IL. Dewpoints Sunday will also likely be very similar to today with low-mid 70s leading to peak afternoon heat indices mostly in the 100-105 degree range. Guidance does suggest somewhat deeper mixing Monday, which could allow dewpoints to mix out some in the afternoon. Confidence is low as to if that deep mixing will occur, and if so, to what extent dewpoints will drop in the afternoon. Deeper mixing and lower dewpoints would likely allow temps to get a couple/few degrees warmer on Monday than today and Sunday. If dewpoints really tank Monday afternoon, it is possible that heat indices could fail to reach 100 degrees, but given low confidence in this scenario, opted to stick with the NBM guidance which has dewpoints mixing into the 60s with another day of 100F+ heat indices. Strong and gusty southwest winds will continue the rest of this afternoon and only gradually ease this evening. As low level jet develops this evening, its possible that we could see a few hours of stronger winds/gusts around the urban heat island of Chicago where higher temps should keep the boundary layer somewhat mixed late into the evening. Wouldn`t even rule out a few gusts to around 40 mph in Chicago until late evening. Sunday should be another windy day, though likely averaging about 5kt less so than today. The relatively high dewpoints and southwesterly winds should result temps being pretty slow to fall this evening and again Sunday evening. In fact, in the urban heat island corridor of the Chicago area wouldn`t be surprising to see low temps only bottom out in the lower 80s tonight and again Sunday night with heat indies unlikely to fall below 90 until midnight or a little after. Skies are expected to remain mostly clear through Monday. By Monday afternoon the mid-upper level ridge is forecast to begin weakening, mainly especially to our northwest. This should allow the "ring of fire" of convection should inch closer to our CWA, though through the afternoon hours Monday it still looks like convection should remain northwest of our CWA. - Izzi Monday night through Friday: Toward the middle of next week, the upper-level ridge responsible for our heat wave is expected to flatten somewhat resulting in quasi-southwest to zonal flow across the Upper Mississippi River Valley. As a result, the low-level baroclinic zone acting as a highway for episodic showers and storms (the "ring of fire") should drift southward toward the Lower Great Lakes by the middle of the week. At the same time, the strength of the low-level thermal ridge is expected to weaken, resulting in gradually less capping. When taken together, confidence is increasing in a pattern change in the Tuesday to Wednesday timeframe toward stormier conditions likely tied to both the diurnal cycle and any incoming subtle shortwaves in the southwest to zonal flow aloft, albeit with continued warm temperatures. At this point, it appears that Tuesday will be the first day to feature low-end chances (30-40%) scattered afternoon thunderstorms, perhaps focused along any residual outflow from prior convection to our north and/or a lake breeze. Coverage of afternoon thunderstorms may then incrementally increase Wednesday through Friday owing to progressively weaker capping and the southward drift of the baroclinic zone. Blended NBM guidance offers nearly continuous mid-range chance PoPs (40 to 60%) Wednesday through Friday, which seems reasonable given the pattern. Of course, many hours will be dry. With the upper-level jet and associated availability of shear currently expected to remain focused north of our area, the airmass within and south of the baroclinic zone will be somewhat stagnant and prone to moisture pooling. As a result, the overall kinematic and thermodynamic regime next week will be characterized by high PWATs (potentially nosing north of 2" at times), weak mid-level lapse rates, daily afternoon MLCAPE >2000 J/kg, and meager deep-layer shear. Such a regime should limit the threat for widespread/high-impact severe weather (e.g., no derechoes), but can nevertheless support episodic clusters of thunderstorms with a threat for torrential downpours, prolific lightning, and strong to damaging winds. Both NCAR and CSU ML probabilities for severe weather hang in the 5 to 15% range Tuesday through Friday, consistent with a Level 1 to 2 out of 5 threat for severe weather each day. Also, could also see a threat for localized flooding where thunderstorms impact the same area. Outside of thunderstorms Tuesday through Friday, it will remain hot and humid. Daily highs in the low to mid 90s with dew points in the low to mid 70s will support daily afternoon peak heat indices in the upper 90s to lower 100s each day (not quite 105 on a widespread basis). With the baroclinic zone so close by or directly overhead, perhaps at times modulated by convective outflow, there will finally be an opportunity for Lake Michigan to provide local cooling to shoreline locations. So, at this point, the end time of the Extreme Heat Warning for Cook County at midnight Tuesday morning appears to be in good shape. Elsewhere, the Heat Advisory is currently slated to also end early Tuesday morning, which appears reasonable. (If heat indices were expected to remain at or above 105 on a widespread basis through Friday, we`d have to consider alterations to the ongoing heat products). All in all, the pattern next week should be quintessential of mid to late June. Borchardt && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 1238 AM CDT Sun Jun 22 2025 VFR and dry conditions are forecast through the period. South southwest winds have eased across the area early this morning around 10kt with only sporadic gustiness lingering. Plan to hold onto low-level wind shear (LLWS) through 12Z this morning with the low-level jet (LLJ) expected to gradually weaken through the remainder of the overnight hours. While not as strong as yesterday, today is expected to be breezy again, with gusts in the upper 20kt range developing by the afternoon. Winds ease again toward sunset with another round of LLWS possible, mainly toward RFD, with the LLJ expected to be a bit weaker further east over the Chicago metro. Petr && .CLIMATE... Issued at 152 PM CDT Sat Jun 21 2025 Temperatures through early next week will threaten record highs and record warm lows for Chicago and Rockford. Here are the current records: Chicago High Warm Low Saturday 6/21 101 (1988) 74 (1923) Sunday 6/22 97 (1988) 76 (1923) Monday 6/23 97 (1930) 79 (1923) Rockford High Warm Low Saturday 6/21 100 (2022) 71 (1995) Sunday 6/22 97 (1923) 73 (1908) Monday 6/23 97 (1923) 74 (1908) NWS Chicago && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Heat Advisory until midnight CDT Monday night for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106- ILZ107-ILZ108. Extreme Heat Warning until midnight CDT Monday night for ILZ103-ILZ104-ILZ105. IN...Heat Advisory until midnight CDT /1 AM EDT/ Monday night for INZ001-INZ002-INZ010-INZ011-INZ019. LM...Small Craft Advisory from noon today to 7 PM CDT this evening for the IL and IN nearshore waters. Small Craft Advisory until 3 AM CDT early this morning for Wilmette Harbor to Calumet Harbor IL. && $$ Visit us at weather.gov/chicago