Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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995 FXUS63 KLOT 142046 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 346 PM CDT Wed Aug 14 2024 .KEY MESSAGES... - Period of showers with some embedded storms likely late tonight into Thursday morning. - After a likely break in the rain most of Thursday afternoon, another round of showers and storms is possible Thursday night, with some threat for severe weather. - Isolated to widely scattered showers and thunderstorms possible on Friday. A few strong storms are possible over parts of east central Illinois into northwest Indiana. - After a few showers on Saturday, primarily dry and seasonable through mid next week. && .DISCUSSION... Issued at 346 PM CDT Wed Aug 14 2024 Through Thursday Night: This afternoon, we find ourselves on the southwestern periphery of a surface high pressure bubble positioned over the Great Lakes. To our west, a double-barreled surface low brews over the central Plains, flanked to its east by an MCV over northern Iowa and to its southeast by a warm frontal zone. The leading wing of warm air advection aloft is only now encroaching upon the Mississippi River, and it will eventually spread over our forecast area this evening as it slowly presses eastward. Gradual saturation of the tropospheric column will ensue as this occurs, eventually yielding a northwest-southeast oriented arc of showers that will provide us our initial raindrops tonight. Later on in the night, a cluster of storms originating from the central Plains should approach from the west. It will likely reach our western counties around daybreak in a decaying phase, having outrun the instability plume still centered west of the Mississippi River. As a result, the expectation is that a band of widespread rain showers will pass through our forecast area over the course of the morning, likely clearing most or all of the area by the afternoon. This band will still likely contain some lingering embedded convective elements that will pose a threat for lightning, but it is unlikely that the threat for lightning tomorrow morning will be widespread with the lingering convection expected to weaken with time and eastward extent as it continues to separate itself from its parent instability plume. Going into the afternoon, there should be a general lull in shower activity after the morning rain band departs due to a lack of instability and forcing mechanisms and the presence of large-scale subsidence on the back side of the departing convectively-enhanced shortwave. Cloud cover lingering around into the afternoon will hinder destabilization efforts a good bit, but temperatures and humidity will still build throughout the afternoon as a warm front lifts northward through the area. High temperatures are currently forecasted to reach the 80s across only the southwestern half of our CWA (with mid-upper 70s across the northeastern half), but could potentially end up being a little warmer if the rain departs earlier or if the cloud cover scatters out faster and/or to a greater extent than presently expected. Regardless of how much clouds end up clearing out, between ongoing low-level warm/moist air advection and mid-level lapse rates steepening markedly as an EML plume drifts eastward into the area, it appears that the air mass in northern Illinois should eventually become unstable to surface- based air parcels, possibly by as early as the mid-late afternoon. While it is possible that storms try to get going in or near our forecast area by the late afternoon, large-scale forcing will still largely be lacking on the whole at that time, and unless some sort of low-level convergence axis materializes for ascent to focus along, most or all of our forecast area should remain free of thunderstorms until after sunset. By mid-evening, large-scale forcing for ascent will become much more abundant as DPVA increases along the southern periphery of an upper-level low swirling over the Upper Midwest, and convective coverage will likely begin to increase across the area as a result, with most of the latest CAMs focusing it along a surface cold front in spite of relatively weak convergence along the front. Shear profiles appear to be favorable enough to potentially support an initial supercellular storm mode with a threat for large hail and damaging winds. Shear vectors oblique to the front should then tend to promote some degree of upscale growth with time, potentially yielding at least a loosely-organized MCS that would track across or just south of our area during the late evening or overnight hours. While a slowly-stabilizing boundary layer should begin to mute the severe weather threat to some extent later on into the night, it likely won`t curtail the threat for strong to damaging winds entirely with this storm cluster given the overall degree of low-level moisture, instability, forcing, and shear that will be in play. One potential failure mode to keep an eye out for would be if extensive convection were to develop well to our south (possibly along an outflow boundary laid out by the morning convection) during the afternoon or early evening. If that were to occur, this convection could inhibit some of the northward moisture return into the area, which, in turn, would likely limit to some extent both the coverage and potential intensity of any convection that we`ll see Thursday night. This scenario likely still wouldn`t prevent thunderstorms from occurring here altogether, though, with the stout large-scale forcing support and mid-level buoyancy at our disposal likely to still be sufficient enough to support at least isolated thunderstorms in the area in spite of the better moisture remaining fenced off to our south in this scenario. Ogorek Friday through Wednesday: The mid-level low responsible for the active period of weather on Thursday-Thursday night will slow over far northern Wisconsin by Friday morning. An associated surface cold front is expected to cross the forecast area Friday morning into the afternoon. Meanwhile, guidance is in decent agreement that a embedded trough axis will swing across southern Wisconsin and into northern Illinois/Indiana Friday afternoon and evening. This trough axis will intercept an air mass with temps in the 80s and dew points in the mid to upper 60s. While the kinematic profile on Friday looks potentially favorable for strong storms, dry-air advection and modest mid- level lapse rates near and behind the front should greatly diminish thunderstorm coverage and intensity. However, any slowing of the front will result in increasing strong to severe storm chances across the southeast CWA Friday afternoon. With veered surface winds limiting convergence and dry air at the mid-levels until evening, low afternoon PoPs in the 20-40% range (highest far north and in parts of NW IN) appear reasonable, for isolated to widely scattered showers and storms. The upper-level low will drift eastward toward Lake Huron through Saturday night. While some wrap-around precipitation is possible on Saturday, substantial capping is progged to develop above low-level cold air advection (CAA). Diurnally- driven isolated to widely scattered shallow showers appear plausible, supporting 20-30% PoPs over mainly the eastern 1/2 or so of the CWA. Thunderstorm chances continue to look very low due to the aforementioned capping. Beyond Saturday, latest trends continue to suggest that Tropical Cyclone Ernesto will remain far enough east that any blocking or Fujiwhara effects with the upper-level low should also remain well east of the forecast area. This favors a generally quiet and seasonable period Sunday through Monday as a weak ridge shifts across the western Great Lakes. Overall ensemble guidance does suggest that the impacts of the tropics will result in a highly amplified pattern and loose omega block across North America toward the middle of next week. While this could place us in a typically favored area for active weather on the eastern edge of the ridge, moisture return looks to be questionable while the better kinematic environment remains displaced well northwest into south central Canada due to expansive surface high pressure across the Great Lakes and upper Midwest. Kluber/Castro && .AVIATION /18Z TAFS THROUGH 00Z FRIDAY/... Issued at 100 PM CDT Wed Aug 14 2024 The main aviation weather concerns for the 18Z TAF period include: * A period of showers and embedded thunderstorms that will occupy the majority of Thursday morning * Gusty near-southerly winds on Thursday * MVFR to IFR cigs expected for the better part of Thursday A system of rain showers will approach northern IL from the west late tonight. It appears that a few spotty lighter showers could attempt to go up ahead of the main body of precip during the overnight tonight. More widespread coverage is expected to make it to RFD by around 10Z and the Chicago sites closer to 12/13Z. Rain will remain in the area through the morning before fizzling away for the afternoon. At least a few embedded thunderstorms are expected although thunder coverage should be fairly isolated. Impacts to vsbys will be possible underneath these showers during the morning. There is high confidence for at least MVFR cigs to develop mid-late morning with the rain. Lower cigs will then carry on through the rest of the period. The signal is currently strongest for predominantly MVFR during the day but IFR is certainly an option, especially up at RFD. Meanwhile, light E or SE winds will pick up just a tick this afternoon as near-10 kt winds are expected to push off of the lake and over the Chicago sites. SE winds will be mostly under 10 kt tonight until the more widespread rain moves in. During the morning, SSE winds should be regularly gusting to around 20 kt with occasional gusts as high as 25 to 30 kt possible. Winds will remain gusty and may want to teeter to west of south (190-200) for a time during the afternoon. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 9 AM to 7 PM CDT Thursday for the IL nearshore waters. && $$ Visit us at weather.gov/chicago