927 FXUS63 KDVN 281945 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 245 PM CDT Sun Jun 28 2026 .KEY MESSAGES... - Lightning-heavy concentrated area of thunderstorms (70-90% coverage) will continue along to push along the U.S. Highway 30 corridor into northwest Illinois through mid-late afternoon. - A prolonged period of hot and very humid conditions will expand area-wide Monday, highlighted by day-to-day peak heat index values of 100 to 110 and nighttime low temperatures no cooler than the mid 70s. Increasing potential to extend the Extreme Heat Warning through midweek. - Thunderstorm chances will be limited tonight through midweek, with increasing chances Wednesday night onward (including the July 4th weekend), with the resultant cloud cover potentially having an impact on how high of temperatures but at least will remain very humid. && .UPDATE... Issued at 240 PM CDT Sun Jun 28 2026 The concentrated but strong storm cluster heading into northwest Illinois has had occasional severe wind and hail due to effective shear of 45-50 kt and MUCAPE of 2000-2500 J/kg. This is feeding on an instability gradient and has had well-defined storm structure at times (including mesocyclone presence and inflow). This has shown its likely closer rooted to the boundary layer than a couple hours ago. However, there is capping impinging on this from the southwest, as sampled by our 18Z sounding and continued warming of the 800 mb layer. The most likely scenario will be 40-50 mph gusts, small hail, and brief but torrential downpours, with a more persistent severe threat being the higher end outcome. Departure time in far eastern CWA in eastern Whiteside County should be near 430 PM, and if it clips northern/northeast Bureau County would be 530 PM. Some temporary wake low effects of non-thunderstorm wind gusts of >35 mph from the east will be seen for one to maybe two hours after the storms along/north of U.S. Highway 30. && .SHORT TERM /THROUGH WEDNESDAY NIGHT/... Issued at 240 PM CDT Sun Jun 28 2026 After thunderstorms depart the area by sunset, we will settle into a true midsummer hot and very humid pattern. This is characterized by an expansive and slightly retrograding 500 mb high pressure ridge just to our southeast (591+ dm center), and channeled low-level thermal ridge directly over the area (850 mb temps of 21-24C). Confidence is high in convective cloud cover being limited over the area, though there is some model signature for some cirrus Monday afternoon. The setup is also not supportive of storms in the CWA through Wednesday morning, and if they were to occur they would likely only be in the northwest portion of the CWA (near/north and west of Cedar Rapids) and during nighttime hours. In addition, confidence is high in very warm nighttime lows, with NBM membership probabilities over 70 percent of 75+ degree lows for most of the CWA through Wednesday night. So their seems little that would work against the going Extreme Heat Warning, issued especially for persistence, and an increasing likelihood that some or all of the CWA will see it extended through Wednesday. The warm nighttime lows start tonight, thanks to 70-75 degree dew points and a more southerly wind direction. That theme looks to be consistent during the upcoming nights. For dew point adjustments, have capped some of the NBM upper 70s to 80 dew points to more mid 70s, which are more consistent with upstream trusted observations. Also in line with that is the fact we are still a little ahead of the peak evapotranspiration season (mid July into early August). On the note of the winds, these will be breezy again on Monday, with gusts of 20-25 mph, maybe slightly higher at times. While that can nudge heat effects a bit down when near thresholds, we are comfortably into the multi-day extreme heat values (heat index of 105+). So that should only offer a slight reprieve. For Wednesday and even Wednesday night, the chances for thunderstorms continue to ease, with the convective breeding ground likely being the western Corn Belt region to Minnesota, and steering vectors still favoring more of a northeast movement. && .LONG TERM /THURSDAY THROUGH SATURDAY/... Issued at 240 PM CDT Sun Jun 28 2026 For this time period, confidence is high in continued very humid air especially through Saturday. However, convective effects may influence temperatures as the upper pattern flattens a bit over the Upper Midwest and Corn Belt. Using ensemble means and deterministic guidance, the current best convective potential looks to be over the weekend as a short wave trough approaches. That said, predictability at this distance is low, and some noteworthy shifting such as MCS effects prior or a change in trough passage timing certainly can occur. Otherwise, synoptically the pattern of 850 mb temperatures of 20-22C support high temperatures at least in the lower 90s, which would mean heat index values into the 100+ range, and potentially 105-110. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1240 PM CDT Sun Jun 28 2026 Primary concerns with the TAFs are MVFR ceilings and nearby thunderstorms through mid afternoon, gusty southeast winds at times this afternoon, and potential for LLWS tonight. Semi-orgnized thunderstorms will continue to move east-southeast over portions of eastern Iowa and eventually into northwest Illinois through mid-afternoon. While DBQ will see some effects, it is less certain at MLI. If they occur at MLI, they would likely be between 1930-21Z, and probably have some gusts 25-35 kt. Apart from storms, areas of MVFR ceilings are present and will be slow to depart through mid-afternoon. DBQ could even hold onto them through the entirety of the daytime hours. For winds, east to southeast winds will have regular gusts this afternoon. There are likely to be what we call "wake low" effects at DBQ and possibly CID, which even gustier east- southeast non-thunderstorm winds that occur in the wake of a persistent area of storms. These could result in temporary (~1 hr) gusts to 30-40 kt, but confidence in that is low. While gusts abate after sunset, sustained winds should hold around 10 kt, maybe higher at times. The setup also favors a 35-40 kt low-level jet AOA 1.5 kft, and that is borderline for true LLWS development given those sustained surface winds. If it occurs atop area airports, 03Z-09Z is the most favored time. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...Extreme Heat Warning from noon Monday to 9 PM CDT Tuesday for IAZ040>042-051>054-066. Extreme Heat Warning until 9 PM CDT Tuesday for IAZ063>065-067- 068-076>078-087>089-098-099. IL...Extreme Heat Warning from noon Monday to 9 PM CDT Tuesday for ILZ001-002-007-009-016>018. Extreme Heat Warning until 9 PM CDT Tuesday for ILZ015-024>026- 034-035. MO...Extreme Heat Warning until 9 PM CDT Tuesday for MOZ009-010. && $$ UPDATE...Friedlein SHORT TERM...Friedlein LONG TERM...Friedlein AVIATION...Friedlein