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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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433 FXUS63 KLSX 050334 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1034 PM CDT Thu Jul 4 2024 .KEY MESSAGES... - Another round of thunderstorms is expected this evening into overnight, primarily along and south of I-44 in MO and I-70 in IL. Isolated damaging winds are the primary threat, followed by large hail and heavy rainfall that could lead to localized flash flooding. - After slightly below average temperatures on Friday, near average temperatures will persist through next week with the next opportunity for showers and thunderstorms late Sunday and Monday. && .SHORT TERM... (Through Late Friday Night) Issued at 248 PM CDT Thu Jul 4 2024 In the wake of the morning MCS and as clouds have decreased, the atmosphere has recovered with MLCAPE increasing to between 1500 and 3500 J/kg along and south of I-70. Despite this instability, the combination of weak convective inhibition and weak forcing has prevented any additional development of showers and thunderstorms so far this afternoon in the CWA, but isolated to scattered development could still occur this afternoon ahead of a weak surface front in southeastern MO and southwestern IL as heating persists and inhibition decreases. Greater confidence in development of showers and thunderstorms exists this evening as broad ascent leading an upper-level trough arrives and another cold front reaches the reservoir of instability. CAMs focus showers and thunderstorms initially across southern and south-central MO before expanding/progressing northeastward along and south of I-44 (MO) and I-70 (IL) late this evening into overnight along a strengthening of the front at 850 hPa. Instability will only slightly decrease and, with 25 to 40 kt of deep-layer wind shear, strong to severe thunderstorms are possible with a hazard of damaging winds with an opportunity for a few microbursts and upscale growth into one or more clusters. However, large hail is a secondary hazard. Although the pattern is not quite as favorable for sustained heavy rainfall as last night with more progressive thunderstorm motions forecast, ingredients are still present to support a threat of heavy rainfall with these thunderstorms including anomalous PWs and deep warm cloud depths. The latest HREF 24-hour QPF LPMM highlights localized areas of 1 to 4" somewhere along I-44 in MO and I-70 in IL where brief training of thunderstorms along the 850-hPa front is possible. Given that some of that area has lowered flash flood guidance from saturated soils, the Flood Watch remains in effect and has not been changed. Showers and thunderstorms will continue to the east of the CWA early Friday morning, with dry conditions thereafter on Friday. With scattered to broken diurnal cumulus/stratocumulus and weak low-level CAA amidst deep broad cyclonic flow, temperatures on Friday will also be slightly below average accompanied by lower humidity. Pfahler && .LONG TERM... (Saturday through Next Thursday) Issued at 248 PM CDT Thu Jul 4 2024 Global model guidance are in agreement that the upper/mid-level cyclonic flow will give way to southwesterly flow within deepening longwave troughing over the Northern and Central Plains. Multiple shortwave troughs rounding the longwave trough will provide one or more opportunities for periods of showers and thunderstorms Sunday through next week. The largest membership of ensemble models contain rainfall late Sunday and Monday. NBM interquartile temperature ranges also straddle daily average temperatures, increasing confidence in temperatures remaining near average through next week. Pfahler && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1030 PM CDT Thu Jul 4 2024 Some lingering smoke from 4th of July fireworks may periodically reduce surface visibilities to MVFR levels this evening, but this is likely to occur prior to 06Z. The primary concern continues to be the potential for a round of showers and thunderstorms overnight, particularly at St. Louis metro area terminals. St. Louis will likely be located along the northern periphery of these storms, so there remains some uncertainty regarding whether local terminals, including STL, will be directly impacted. The TAF will be amended as confidence increases enough to include a prevailing mention in the TAF. Otherwise, ceilings may briefly drop to MVFR levels during and shortly after showers/storms move through the area. VFR conditions are expected to persist the remainder of the period, with breezy northwest winds developing tomorrow afternoon and ending quickly after sunset. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch until 5 AM CDT Friday for Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO- Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. IL...Flood Watch until 5 AM CDT Friday for Monroe IL-Randolph IL- Saint Clair IL-Washington IL. && $$ WFO LSX