Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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562 FXUS63 KLSX 231138 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 638 AM CDT Mon Sep 23 2024 .KEY MESSAGES... - A severe thunderstorm or two is possible across southeastern MO and southwestern IL this afternoon and evening with damaging winds and marginally severe hail. - Multiple rounds of showers and thunderstorms are expected to persist into Tuesday with additional beneficial rainfall across the region. - Additional showers are possible Thursday through the upcoming weekend, but the forecast is highly uncertain and dependent on the evolution of a tropical disturbance currently in the Caribbean Gulf. && .SHORT TERM... (Through Late Tuesday Afternoon) Issued at 418 AM CDT Mon Sep 23 2024 Multiple rounds of numerous showers and thunderstorms are expected through Tuesday morning as at least two mid-level shortwave troughs pass over the Mid-Mississippi River Valley, increasing large-scale ascent and enhancing prevailing, broad low to mid-level WAA/isentropic ascent across the region. Showers and thunderstorms associated with the first of these rounds are already developing across southern and southwestern MO and are expected to increase in coverage and spread northeastward into a broad portion of the CWA, continuing through the morning. By afternoon, CAMs start to disagree on the coverage of showers and thunderstorms and whether or not there will be any breaks in cloud cover. At least modest elevated instability (MUCAPE up to 750 J/kg) sufficient for thunderstorms is indicated by short-term model guidance through tonight up to the I- 70 corridor, but the development of surface-based instability this afternoon across southeastern MO and southwestern IL is much more uncertain and contingent on breaks in cloud cover. The latest HREF interquartile range of SBCAPE across southeastern MO and far southwestern IL spans 100 to 1500 J/kg. With 25 to 35 kt of deep- layer shear forecast, the higher end of this SBCAPE distribution would be sufficient for a severe thunderstorm or two with damaging winds this afternoon and evening, but even an elevated thunderstorm could be capable of marginally severe hail. Similarly, high temperatures today will be limited by the cloud cover and precip to the mid-60s to mid-70s F. Short-term model guidance and CAMs agree that there will be at least one additional round of numerous showers and thunderstorms this evening and/or early Tuesday morning as the main mid-level shortwave trough begins to approach and then cross the Mid-Mississippi River Valley. This trough is also expected to facilitate gradual low-level cyclogenesis across the region, providing additional low-level moisture convergence that could further augment shower and thunderstorms coverage. Most showers and thunderstorms will exit the CWA during the morning on Tuesday as the shortwave trough departs, but the combination of another upper-level trough quickly arriving in its wake and steepening low-level lapse rates from weak low-level CAA in tandem with modest insolation, will be catalysts for scattered showers and developing during the afternoon. With precip less prevalent on Tuesday, temperatures will be slightly warmer in most locations than today and around 70 to the mid-70s F. The showers and thunderstorms through Tuesday are expected to provide additional widespread beneficial rainfall, with the latest HREF QPF LPMM indicating some swaths of 1 to 3" somewhere in the general vicinity of I-44 in MO and I-70 in IL. Since this rainfall is likely to fall in multiple waves, prolific training of thunderstorms is not expected, and soils are moistening but still relatively dry, the threat of any flooding is low. Pfahler && .LONG TERM... (Tuesday Night through Sunday) Issued at 418 AM CDT Mon Sep 23 2024 On Wednesday, drier conditions are expected across the Mid- Mississippi River Valley as mid-level heights rise in association with an upper-level ridge "breaking" and leading to cutoff cyclogenesis from the trough across the Mid to Lower Mississippi River Valley. Less cloud cover and subsiding low-level CAA support temperatures similar or slightly warmer than Tuesday. Thursday through the upcoming weekend, sensible weather across the Mid-Mississippi River Valley will be greatly influenced by the position of the upper-level cutoff low and its interaction with a tropical disturbance, currently in the northwestern Caribbean Gulf (AL97), that will likely form into a tropical cyclone in the next 48 hours and track northward across the Gulf of Mexico and then Gulf Coast. Global model guidance are coming into better agreement that a plume of moisture ahead of the to-be tropical cyclone will begin to wrap northward around the cutoff low into the Mid-Mississippi River Valley by Thursday, resulting in periods of showers. Friday through the weekend, ensemble model guidance has the tropical cyclone itself tracking into the Ohio River Valley and/or the Mid-Mississippi River Valley as a result of a complex interaction with the cutoff low. The complexity of this evolution and the fact that the tropical cyclone has yet to develop leads to a highly variable and low-confidence forecast in terms of precip through the weekend. As a result, ensemble model membership QPF ranges from little to no rainfall across much of the CWA to over 1" of rainfall. There will at least be increasing cloud cover limiting high temperatures to the 70s F, but NBM interquartile temperature ranges span 5 to 8 F from the differences in precip amounts. Pfahler && .AVIATION... (For the 12z TAFs through 12z Tuesday Morning) Issued at 638 AM CDT Mon Sep 23 2024 MVFR to IFR flight conditions will persist through much of the TAF period with low ceilings and periods of showers and thunderstorms. The most most widespread showers and thunderstorms are expected to be this morning into early afternoon and again this evening and possibly overnight. However, there is less confidence in exact timing of showers and thunderstorms and coverage of thunderstorms beyond this morning. Although ceilings may fluctuate some through tonight, significant improvement will likely not take place until later Tuesday and beyond the TAF period. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX