Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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019 FXUS63 KLSX 250856 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 356 AM CDT Wed Sep 25 2024 .KEY MESSAGES... - Mostly dry conditions are expected today and Thursday, except for isolated showers and thunderstorms across southeastern MO and southwestern IL this afternoon and evening as well as Thursday evening. - Rain associated with tropical moisture from Tropical Cyclone Helene is most likely (>60 percent chance) along and south of I-44 in MO and I-70 in IL early Friday through Saturday, but there is still uncertainty in the exact placement and timing of this rain along with resulting rainfall amounts. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 355 AM CDT Wed Sep 25 2024 An upper-level cutoff low is positioned over the CWA this morning. Associated mid-level clouds have gradually become more scattered with time, allowing fog to develop where clearing has occurred due to radiational cooling of a BL with residual moisture. This fog has been limited in coverage and is mostly present in river valleys, with the expectation for it to dissipate shortly after sunrise. The cutoff low will slowly advance southward today as an upper-level ridge "breaks" across the Upper Midwest/Mississippi River Valley. Rising mid-level heights and temperatures in the wake of the cutoff low will lead to dry conditions aside from isolated showers and thunderstorms during the afternoon and evening in southeastern MO and southwestern IL, more closely beneath the cutoff low during the remainder of the day. There will be less cloud cover today than Tuesday, despite scattered diurnal cumulus and mid-level clouds, with warmer temperatures in the 70s F--close to average for late September. With less clouds tonight into Thursday morning, stronger radiational cooling and some residual moisture still in place will likely allow patchy river valley fog development once again. On Thursday, the upper-level cutoff low will continue to shift southward into the Lower Mississippi River Valley as it begins to interact with Tropical Cyclone (TC) Helene, forecast to become a major hurricane this morning, moving northward and making landfall around the Florida Panhandle. In advance of TC Helene, a plume of increasing tropical moisture wrapping around the cutoff low will begin to surge northwestward into the Ohio Valley and Mid- Mississippi River Valley. This increasing moisture will mostly lead to increasing clouds on Thursday, but isolated to scattered showers, perhaps a thunderstorm, will become possible by the evening in southeastern MO and southwestern IL. Ahead of thickening cloud cover, high temperatures on Thursday should be a degree or two warmer than today. Pfahler && .LONG TERM... (Thursday Night through Tuesday) Issued at 355 AM CDT Wed Sep 25 2024 The forecast remains uncertain late Thursday through the weekend as the upper-level cutoff low meanders across the Lower Mississippi River Valley while interacting with and eventually absorbing TC Helene as it weakens and rotates about the eastern/northeastern flank of the cutoff low. Throughout this evolution, a broad band of showers and steadier, heavier rain at times is expected to arc into the region, associated with the tropical moisture and the potential arrival of TC Helene`s remnants, but its location and movement with time will be dictated by the aforementioned evolution. There is still a considerable amount of variability in model guidance but the trend over the last 24 hours has been for a slight eastward shift in the cutoff low position and, therefore, associated showers/rain. Ensemble model guidance rolling 6-hour probabilities of measurable rainfall are highest (>60 percent) along and south of I-44 (MO) and I-70 (IL) early Friday through Saturday but the 25th and 75th percentiles of total rainfall Thursday through Sunday across those areas are approximately 0.25 to 3", respectively. This spread is quite large and demonstrates uncertainty with some members even having the majority of the rainfall being southeast of the CWA altogether. NBM high temperature distributions Friday and Saturday are largely in the 70s F, but wherever persistent, heavier rainfall occurs temperatures will fall in the lower end of the distributions and in the upper 60s F. Sunday into early next week, the upper-level cutoff low is progged to shift east of the Mid-Mississippi River Valley, with mostly diurnal isolated to scattered showers lingering as any large-scale forcing also exits. Model guidance disparities are present early next week, but there are varying signals for an upper-level trough across the northern CONUS to force a cold front through the CWA around Tuesday, cooling temperatures below average. However, these solutions assume that the evolution of the preceding cutoff low and TC Helene is captured by the current ensemble model envelope. Pfahler && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1052 PM CDT Tue Sep 24 2024 Light winds will persist overnight and as skies continue to clear, fog is forecast at KCOU, KJEF, KSUS, and KCPS. Fog is not forecast at KUIN because of more persistent cloud cover. At KSTL, fog climatologically does not form in these set-ups so confidence in its occurrence is too low to include in TAF. Expect fog to result in MVFR visibilities at KCPS, KJEF, and KCOU and IFR visibilities at times at KSUS. VFR flight conditions will return shortly after sunrise on Wednesday. Winds will be from the north as surface high pressure shifts into the region from the west on Wednesday. MMG/MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX