Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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054 FXUS63 KLOT 042347 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 647 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 236 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: Following Wednesday morning`s cold frontal passage, a weaker, secondary cold front is progged to move across the CWA Wednesday evening and night. A push of low-mid level moisture ahead of the boundary will result in some late day destabilization amounting to as many as a few to several hundred joules of MUCAPE over parts of our area. Models agree on a mid level inversion keeping ELs topped out at around 500mb, which will equate to around -10C. And with the vast majority of forcing for ascent found below the freezing level, such a setup should struggle to produce much lightning, but low- topped convective showers certainly look attainable. Some models were thinking that the moisture advection may be too meager to produce much of anything, but the trend is in favor of a push of showers ahead of the front. The more appreciable moisture, and in turn instability, will be found across the northern and northwestern CWA. It`s here that we`ll find the best odds at seeing precip as well as an embedded thunderstorm, but again the thunder potential overall is seemingly low. All of the precip should be off to our east by Thursday morning. A tight pressure gradient will set up across the region between the departing storm system and an approaching high out west. A 40-50 kt LLJ is also expected to move overhead, both of which will bring windy conditions to the area on Thursday. The LLJ diffuses some for Friday, but the surface pressure gradient will remain in place bringing more breezy conditions on Friday. The high pressure center will propagate across the lower Midwest late Friday into Saturday which should keep precip chances rather minimal heading into the weekend. A large upper low expected to cycle over the eastern Great Lakes region late this week and early next will reel in some cooler air aloft and help keep conditions a tad cooler than normal this weekend with highs progged in the lower and middle 70s. Doom && .AVIATION /00Z TAFS THROUGH 06Z THURSDAY/... Issued at 647 PM CDT Tue Jun 4 2024 Forecast concerns include... Scattered thunderstorms through the early overnight. Mvfr, possible ifr cigs overnight/Wednesday morning. Chance of showers Wednesday morning and Wednesday evening. Gusty westerly winds Wednesday. Scattered thunderstorms are expected to continue through 06z or so and it will be difficult to pin down specific timing for any terminal. Vicinity/tempo mention seems on track and trends will need to be monitored for refinement. Scattered showers may continue overnight but confidence is low for coverage. The best chance for additional showers appears to be mid/late Wednesday morning ahead of a cold front. And then there is another chance for showers mid/late Wednesday evening, though confidence is too low for mention at this time. A lake breeze boundary moved west of ORD/MDW this afternoon, then lifted back north of MDW. This boundary is again being pushed west and will move back through MDW shortly. Wind directions may be problematic near/around any stronger thunderstorms this evening, but overall, prevailing winds are expected to favor a south/southeast direction this evening, then south/southwest overnight. Winds will turn more westerly behind the cold front midday Wednesday with gusts into the lower 20kt range. Mvfr cigs will be possible this evening with any storms. Prevailing low mvfr cigs are expected tonight into Wednesday morning, ahead of the cold front. There is a chance of ifr cigs overnight and scattered mention seems reasonable. Confidence for ifr cigs is low. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago