Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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218 FXUS63 KLOT 150153 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 853 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. && .UPDATE... Issued at 853 PM CDT Wed Aug 14 2024 Fairly quiet conditions tonight over the city with easterly winds helping to slowly bring temperatures down into the 70s. Meanwhile, mid level moisture is moving in from the west- northwest. Showers will continue to move in from the west and have arrived as far as the Fox Valley at the time of this update was published. However, observations of light rain making it to the surface have been isolated at best. Instability is fairly meager and mesoanalysis shows fairly widespread capping over northern Illinois. There have been a few flashes in north central Iowa on GOES GLM data, but confidence is lower for lightning activity here for the next few hours. So thunder chances were capped at just a slight chance (up to 24 percent) through 7 AM, with the chance (up to 40 percent) for thunder west of I-39 just before day break. Otherwise, no major changes to the rest of Thursday`s forecast listed below, as thunder chances increase as the front arrives. DK && .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 /00Z TAFS THROUGH 06Z THURSDAY/... Issued at 619 PM CDT Wed Aug 14 2024 Aviation Forecast Concerns: Surface high pressure continues to drift off to the east of the area across the central Great Lakes this evening. Surface winds have turned east-southeasterly on the back side of the high, and a lake breeze which has pushed across KGYY/KMDW/KORD. Winds will remain southeasterly and increase (with gusts around 20 kt) into Thursday, as a warm front approaches the area from the southwest. Additionally, moisture will spread into the area aloft later tonight, with scattered high-based showers developing eastward into early Thursday morning. Greatest thunder potential looks to remain mainly west of the terminals (except perhaps KRFD) overnight, with a low-probability (~20-30%) of some embedded thunderstorms Thursday morning. Scattered showers may continue to fester midday/afternoon Thursday in the vicinity of the warm front boundary, but should become less numerous later in the afternoon as the front eventually shifts just northeast of the terminals. With a very moist low level air mass in the warm sector, guidance depicts solid IFR/MVFR ceilings across the terminals. Would expect some diurnal rise in cloud bases toward the higher end of guidance through the afternoon, though have maintained MVFR ceilings for the duration. Surface winds are expected to turn southerly behind the warm front, with some lingering gusts. Low-confidence in direct thunderstorm impacts to the terminals later Thursday night, though current high-res guidance trends suggest the best chances would be toward or just after the end of the current 30-hour KORD/KMDW TAFs. Ratzer && .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