Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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244 FXUS63 KLOT 170521 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1221 AM CDT Mon Jun 17 2024 .KEY MESSAGES... - Hot and muggy conditions will prevail through the end of the week. Periodic chances for thunderstorms will continue, though many hours will remain dry. && .DISCUSSION... Issued at 227 PM CDT Sun Jun 16 2024 Through Tonight: An impressively organized mesoscale convective vortex anchored by a semi-circular outer-core of convection is currently lifting from northeastern Missouri and into southeastern Iowa. Low- level moisture continues to increase ahead of the MCV, with dew points in the lower 70s now spreading toward the I-39 corridor and Wisconsin state line. Mostly sunny skies have allowed for highs to build into the upper 80s to around 90, which when combined with the steepening mid-level lapse rates and increasing low- level moisture, is contributing to some 2000-2500 J/kg of MLCAPE. As broad southwesterly steering flow continues guiding the MCV northeastward into the destabilizing airmass, coverage of ongoing thunderstorms near the MCV will increase with perhaps a rapid uptick near or west of I-39 sometime in the 2 to 4 pm timeframe. Thereafter, convection will sweep across northern Illinois, primarily along and north of I-80. Locally augmented flow on the eastern side of the circulation (KDVN radar is sampling some 50+kt of flow at 12kft) will provide a mesoscale kinematic environment for the development of supercells and bowing structures as convection matures across northern Illinois this afternoon, with a threat for "upscale growth" into a compact MCS as it crosses northern Illinois this afternoon and evening. For this reason, the threat for a swath of damaging winds (60 to 75 mph) and even brief tornadoes continues to increase, focused near and north of I-80. Accordingly, the Storm Prediction Center has introduced a Level 2/5 threat level for severe weather this afternoon, mainly along and north of I-80. Given the highly organized structure of the MCV, we remain concerned for regenerating convection along the backside of the circulation across northeastern Illinois after sunset. Such a threat will come down to mesoscale details of whether a focused zone of low-level confluence will develop at the intersection of the western side of the cyclonic circulation and any continued southwesterly low-level synoptic flow. Forecast confidence in such a band of regenerating convection remains low, though it`s something we can`t rule out. If radar and satellite trends begin to hint at a mesoscale band of regenerating convection materializing, we will consider issuing a short-fused Flash Flood Watch for part of our area (likely just a few counties). Borchardt Monday through Sunday: The main forecast focus for the week continues to be the hot and humid conditions expected to impact the Great Lakes through much of the week. The main driver for the heat will be a broad upper ridge that will stall across the Mid-Atlantic which will generate stout south-southwesterly flow over the area. This flow will keep dew points in the mid to upper 60s to around 70 which in combination with high temperatures likely to hover in the low to mid 90s, will generate heat indices in the 95 to 105 degree range at times. The warmest days this week continue to look like Monday and possibly Thursday depending on how quickly the ridge breaks down. Though, the latest guidance has started to show that dew points may mix down a couple of degrees Monday afternoon which may help keep heat indices a bit lower as well. Additionally, the high dew points will also aid in keeping overnight lows in the mid to upper 70s through at least the middle of the week which means little relief even after sunset. Therefore, despite the aforementioned heat indices expected to remain below our Heat Advisory criteria; we encourage anyone with outdoor plans to practice heat safety by taking breaks from the heat and staying hydrated. Aside from the heat, there is also the potential for periodic showers and thunderstorms through the week as well. However, the confidence on coverage of showers and storms through Tuesday remain low. The reason is because the ridge will create mid- level height rises which should generate subsidence and work to counteract the robust instability forecast to be in place. Given the uncertainty on which parameter will win out, have decided to maintain the 20% POPs for both Monday and Tuesday afternoons. Heading into Wednesday and the later half of the week the upper- level pattern is forecast to become more zonal as the ridge gets squashed by a trough ejecting across the upper Midwest. As this occurs, a frontal boundary is forecast to develop over the upper Mississippi Valley and gradually drift southeast into Wisconsin towards weeks end. However, exactly when and/or if this boundary will actually get shoved through northern IL remains very uncertain as guidance continues to struggle as to when the ridge will actually break down. Therefore, have made no changes to the advertised NBM forecast which from Wednesday onward which offered 20 to 40 percent POPs, highest across northwest IL near the aforementioned front. Furthermore, high temperatures are currently forecast to remain in the 90s through weeks end but these values could wind up cooler if the front moves through sooner than advertised so stay tuned. Yack && .AVIATION /06Z TAFS THROUGH 12Z TUESDAY/... Issued at 1221 AM CDT Mon Jun 17 2024 Current satellite imagery shows a MCV moving away from the area and into Michigan. Some mid-level clouds associated with this MCV will stick around overnight, but VFR conditions are expected to persist. Tomorrow looks generally quiet with the exception of some gusty (around 25 kts) southwest winds in the afternoon. Mid-level moisture will allow some diurnal low-to-mid level clouds to develop tomorrow afternoon. A few of the CAMs hint at the possibilty of some isolated storms tomorrow, but confidence in this actually occurring is too low to include in the TAFs. Carothers && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Air Quality Alert until midnight CDT tonight for ILZ005-ILZ006- ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108. IN...None. LM...None. && $$ Visit us at weather.gov/chicago