Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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437 FXUS63 KLOT 050526 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1226 AM CDT Sat Oct 5 2024 .KEY MESSAGES... - Gusty winds developing Saturday. A few gusts to 35 mph late in the afternoon. - Low chance (around 20%) for showers and possible thunderstorms late Saturday night into early Sunday morning (mainly N. of I-80). - Combination of gusty northwest winds and dry conditions looks to create elevated fire threat on Sunday - Dry and above normal conditions expected through at least the end of next week && .DISCUSSION... Issued at 307 PM CDT Fri Oct 4 2024 Through Saturday: Moisture channel loops reveal the shortwave--in part responsible for this morning`s showers--shearing and gradually diminishing across northern Missouri. Lingering modest warm advection associated with this feature will drift eastward into our forecast area this evening and overnight. As this occurs, a reservoir of mid-level moisture currently situated across central Illinois will slosh back across the region resulting in some increase in cloud cover, particularly south of I-80. At this point, it`s a bit unclear just how widespread/expansive cloud cover will be, but have increased sky cover grids a touch. Suppose it`s not totally out of the question a few sprinkles materialize late tonight into Saturday morning if cloud cover remains deep enough, but chances for this look much too low to include any mention in the gridded forecast. Tomorrow will feature increasingly gusty south to southeasterly winds through the day along with notably warmer temperatures as a robust surface low takes shape across Manitoba. Expecting gusts to peak late in the afternoon (3 to 5 pm or so) near 30-35 mph as mixing deepens PBL heights to around 4-5 kft. Any morning cloud cover should scatter through the early afternoon, and with 850 mb temperatures warming through about +15 C, high temperatures will push into the low to mid 80s, with cooler conditions along the Illinois lakeshore. On Saturday night, the aforementioned surface low will push rapidly into Ontario as it occludes. With the core of the associated upper low displaced well to our north, we`ll be on the southern flanks of the more notable large scale ascent. This forcing won`t be totally negligible, however, with 60 to 80 m/12 hour 500 mb height falls forecast, along with at least a glancing blow of DCVA scraping the Illinois/Wisconsin state line. While essentially every piece of global guidance (and respective ensembles) are precipitation-free in our area, a plume of very steep mid-level lapse rates will advance ahead of the incoming cold front. Just a slight misdiagnosis of moisture in the 800-650 mb layer will be the difference between a totally dry outcome and one featuring unforecast storms. For this forecast iteration, it does look like guidance has generally dried this layer out a bit, as well as warmed the base of the lapse rate plume, resulting in more formidable capping to elevated parcels. While the latest 12z ECMWF isn`t producing precipitation over our area, forecast soundings near the state line really don`t look all that far away from supporting convection, however. As a result, don`t want to entirely rule out the potential for a few storms, however, so have maintained some low-end PoPs across the northeastern half of the CWA on Saturday night. If storms do materialize, effective inflow bases look like they`d be elevated (above 925-850 mb or so), resulting in more muted effective deep layer shear compared to surface-based parcels. This should limit the severe threat, but a few stronger storms can`t be ruled out. Carlaw Saturday Night through Friday: Following the passage of a cold front late Saturday night, the forecast in the long term looks awfully quiet for us here. We pick on Sunday morning where we should find dewpoints circling the drain behind the departing front with midday Tds forecast in the middle and upper 30s. Highs in the lower and middle 70s yield midday RH values in the 25 to 30% range. Breezy northwest winds will spread over the area during the day with gusts getting up to around 25 to 30 mph. The breezy winds and low RH in conjunction with dry fuels brings about fire weather concerns for Sunday. The going forecast keeps us below red flag criteria for now, but it doesn`t miss by much. The windward side of the departing upper trough will drop a broad area of surface high pressure across the Plains on Sunday. The trough will be very slow to move out of the NE US with a prominent upper ridge building in behind it, which will then spill over the Midwest later next week. Consequently, this should lead to several days of dry, uneventful weather while surface high pressure meanders about the region for the better part of next week. Sunny and seasonable to slightly warmer-than-normal conditions are expected through late week. Doom && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1209 AM CDT Sat Oct 5 2024 Aviation Forecast Concerns: - Strong/gusty SSE winds today. - Strong/gusty SSW winds tonight shift WNW toward morning. - Period of LLWS conditions mid-evening into the pre-dawn hours. Modest east winds will gradually shift southeast early this morning as surface high pressure passes off to the northeast of the terminals. The surface pressure gradient then tightens during the day in response to an approaching upper disturbance and cold front, with winds becoming gusty around 20 kt by late morning and into the mid/upper 20 kt range this afternoon. Winds should gradually continue to veer more southerly through the day, but will likely not shift west of south until this evening. Winds this evening will likely remain at least sporadically gusty after sunset in strong gradient flow, as a low level jet strengthens winds at around 2000 ft AGL to 50-55 kts by mid/late evening. A cold front is expected to push across KRFD roughly 06-07Z and across the Chicago terminals in the 08-10Z time frame, shifting winds WNW. Cloud-wise, VFR conditions are expected through the period. Initial warm advection aloft is currently producing a 6000-7000 foot deck which will spread into the terminals early this morning, before eventually scattering out early this afternoon. Model guidance has trended drier with the cold front later tonight, focusing best precipitation chances north of the IL/WI border and limited to a 1-2 hour period around the time of the frontal passage itself. Can`t totally rule out scattered SHRA or even an isolated TSRA, though probabilities are too low for TAF mention at this time. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 4 AM CDT Sunday for the IL nearshore waters. Small Craft Advisory until 4 AM CDT early this morning for the IN nearshore waters. && $$ Visit us at weather.gov/chicago