Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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806 FXUS63 KLOT 041848 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 148 PM CDT Tue Jun 4 2024 .KEY MESSAGES... - Scattered showers and thunderstorms develop mid to late afternoon, and increase in coverage this evening. A few stronger storms may produce with gusty winds and small hail, in addition to heavy rainfall and a localized flooding threat. - Following isolated to scattered showers Wednesday evening, mainly dry and comfortable (seasonably cool) through Sunday. - Breezy to windy on Thursday with gusts up to 30-40 mph. && .DISCUSSION... Issued at 145 PM CDT Tue Jun 4 2024 Through Wednesday: Before the thunder activity concerns, there is a lake breeze from Chicago northward, keeping a lid on temperatures near the lake (70s) whereas most inland areas remain on the warm and muggy side as temperatures, even with the clouds, march on into the mid 80s. A mesoscale convective vortex (MCV) continues to shift northeast into eastern Iowa. Ahead of the MCV There is a fair amount of cloud cover across northern Illinois and northwest Indiana, which is limiting instability. There are some weak waves in the south-southwest flow and we are starting to see some of the radar echoes on KLOT reaching in the ground in the form of mostly light shower activity. Expect this to drift northward, with some erosion on its northern edge. Thunder coverage will low with this leading activity until the MCV drifts a bit farther NE, given that MLCAPE will remain a bit more limited closer to 500 J/Kg. Behind the opaque mid and upper level cloud shield continued moist advection with dewpoints in the mid to upper 60s will lead to increased instability approaching 1000 J/Kg or so of MLCAPE closer to the MCV across central and western Illinois where capping is weaker. Therefore expect an uptick in shower and thunderstorm coverage west of I-39 and to the southwest, but also there is a narrow corridor across northwest Indiana. Some of this activity will migrate closer to the Chicago area toward early evening. MLCAPE will not be quick to erode this evening as the MCV advances into northern Illinois. This would likely be a more favored period for a bit more storm coverage. Gusty winds would be more favored severe hazard given some elevated low level lapse rates through early evening. PWATs will also increase ahead of the MCV as low level flow gets enhanced, which suggests pockets of heavy rain this evening. While the flow aloft is initially somewhat weak, enhanced of the low-mid level southwesterly flow would suggest the individual storm elements will be moving fast enough to preclude any significant flash flooding in spite of the high PWAT airmass. However, increased low level jet/storm regeneration is of some concern for localized flooding/ponding and is probably the hazard of most concern this evening into the early overnight hours. The main area of concern will be west and south of the core Chicago metro area closer the low/slightly better instability. A fairly sharp upper trough, currently across the northern plains will steer a cold front through the area in the area in the morning. Ahead of the trough showers should expand in a scattered to numerous nature again overnight, with storm coverage gradually decreasing overnight into early Wednesday. With a deep moist profile, pockets of heavy rainfall may lead to ponding on area roadways into the morning commute. The cold front will usher in cooler and drier air in the afternoon in the Chicago area and into northwest Indiana, and the front will also bring the end to any lingering showers, at least for a brief time in the afternoon. KMD Wednesday Night through Tuesday (previous discussion) A secondary cold front associated with the unusually deep (for June) low pressure system and upper trough over north-central North America will move across the area Wednesday night. A short window of column moistening ahead of the front, but more importantly, strong upper jet support, will result in a period of isolated to scattered showers north of US-24. The highest shower coverage should be near and north of I-88. Forecast soundings depict fairly low freezing levels for this time of year, though equilibrium levels will also be low (at or under h5) with temps generally warmer than -10C, only yielding 100-200 J/kg of MUCAPE. Thus, while thunder chances with the evening and early overnight shower activity aren`t zero, they`re fairly slim and only warrant slight chance mention. Confidence is higher in shower occurrence, hence coverage wording in the official forecast. Skies will clear out in the pre-dawn hours of Thursday, allowing temperatures to drop into the upper 50s to lower 60s. Deep mixing in a dry (dew points 45-50F) post-secondary frontal air mass on Thursday along with a tight pressure gradient will result in gusty west-northwest winds peaking during the afternoon. Official forecast gusts are up to 35 mph and upside potential from model soundings suggests at least sporadic 40 mph gust potential. The strongest winds will be over our Illinois counties. High temperatures will reach the mid to upper 70s thanks to the deep mixing and dry air mass in place. A blocking pattern will effectively trap anomalously deep upper level low pressure (from the Wednesday and Wednesday night cold front passages) over the central and eastern Great Lakes through the weekend. Our area will be in dry northwest flow on the southwest flank of the upper low, resulting in very little if any rainfall during our what is typically our wettest time of year. Atypically comfortable conditions can be expected Friday through Sunday, with below normal high temperatures in the 70s away from lake cooling, 40s-50s dew points, and overnight lows in the 50s. Low rain shower chances may return early next work week if the upper low stays in close enough proximity to our east as it finally shifts south from its weekend home. Below normal temperatures will likely continue until the mid to latter part of next week. Castro && .AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1149 AM CDT Tue Jun 4 2024 Key Aviation Impacts for the current TAF period: - Scattered thunderstorms near the terminals in the late afternoon. - Gusty southeast winds expected before the second round of storms - A better chance (50 percent) for showers and storms at terminals this evening as showers continue overnight. - Cigs are expected to lower overnight into Wednesday morning, low confidence in clearing time VFR conditions prevail through this afternoon. Winds will be out of the southeast and gusts are expected to strengthen through the afternoon ahead of the showers. An isolated gust up to 30 knots cannot be ruled out completely, but better chances for gusts to be 20 to 25 knots. As a weak boundary moves northward in the late afternoon, chances increase for showers and thunder to pass near the terminals. Confidence in coverage for the first round remains low. The chance for rain and thunder occurring at the terminals is at about 25 percent or less before 00Z, but some thunder will be around thus we will carry a VCTS mention. A southeast lake breeze is expected to develop to help reinforce confidence in southeasterly winds as the prevailing direction. However, as the next round of storms move northward, it would not be surprising if some occasional flipping to 190 happened between 00Z-02Z. The next, more robust wave is expected to move over the terminals after 00Z to bring the next round of showers and thunderstorms over the region. Confidence increased enough to convert the chances for thunder to a TEMPO group for TSRA at area terminals, though timing may be adjusted in the next TAF installment. Showers will continue through the overnight as a cold front moves over the area. Cigs are expected to lower tonight down to at least low MVFR levels. There is a growing signal for IFR conditions early Wednesday morning, but confidence was just low enough to keep the SCT group in the TAFs for now. VFR conditions are expected on Wednesday as conditions dry out and winds become westerly with gusts around 20 knots. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago