FXUS63 KLSX 030349 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1049 PM CDT Wed Jul 2 2025 .KEY MESSAGES... - Seasonable temperatures will give way to warm and gradually more humid conditions on Friday/4th of July, into the holiday weekend. - Largely dry conditions are expected into Saturday, including the 4th of July, but opportunities for showers and thunderstorms will exist late Saturday into next week. && .SHORT TERM... (Through Late Thursday Night) Issued at 230 PM CDT Wed Jul 2 2025 Upper/mid-level northwesterly flow above a very slowly departing surface anticyclone will remain the dominant features of the Mid- Mississippi River Valley tonight through Thursday, followed by an upper-level ridge arriving Friday. The remnants of a small thunderstorm cluster continues to dissipate along the IA-MO border before reaching the CWA this afternoon. A similar case will exist Thursday and Friday as well, with greatest forcing from passing mid- level perturbations and nocturnal LLJs focused incrementally northeastward into the Upper Midwest as the ridge arrives. This notion is supported by 90 percent of ensemble model guidance keeping the CWA dry through Friday/4th of July with a capping inversion also lowering the threat of airmass diurnal thunderstorms. As low-level flow/WAA gradually increases coupled with strong insolation through mainly just diurnal cumulus, a warming trend will also exist with high temperatures around 90 to the mid-90s F expected across most of the CWA by Friday. Modest moisture transport will also increase dewpoints, but it will be a slow increase that competes with deep BL mixing each afternoon, generally limiting peak heat index values to the mid to upper 90s F. Pfahler && .LONG TERM... (Friday through Next Wednesday) Issued at 230 PM CDT Wed Jul 2 2025 Over the holiday weekend, the upper-level ridge will progress eastward into the Great Lakes as a weakening trough arrives sometime late Saturday into Sunday accompanied by a cold front on Sunday. Much of Saturday is expected to be dry but 30 to 40 percent of ensemble model membership have showers and thunderstorms reaching northeastern and central MO late in the day. A greater chance of showers and thunderstorms will exist on Sunday, supported by 40 to 60 percent of membership, with relatively greater low-level forcing associated with the cold front and less convective inhibition evident in forecast soundings. Large-scale ascent/forcing is not expected to be particularly strong, which leaves coverage of showers and thunderstorms unclear. Confidence is high in high temperatures again in the 90s F on Saturday, but the spread in the NBM increases to between remaining steady and cooling closer to average on Sunday with showers/thunderstorms and the cold front. Peak heat index values are forecast to be in the mid-90s to near 100 F with some additional increase in dewpoints. Global model guidance is in loose agreement that time-mean upper- level flow will become quasi-zonal early next week and navigated by multiple shortwave troughs above Sunday's cold front wavering across the Mid-Mississippi River. In this pattern, details are difficult to discern at this point including the front's position at any given time and timing of individual shortwave troughs. As a reflection, ensemble model guidance has broad-brushed 30 to 50 percent probabilities of measurable rainfall through early next week. These probabilities are lower at night, supporting a nocturnal minimum in coverage with little indication of nocturnal LLJs being present to maintain/develop showers and thunderstorms. Unsurprisingly, the NBM interquartile high temperature range is also 4 to 7 F next week, ranging between near average and further into the 90s F. Pfahler && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1047 PM CDT Wed Jul 2 2025 High pressure is keeping winds light or calm overnight, with a return to light southerly winds tomorrow. VFR conditions are expected to continue overnight and on Thursday. There's some potential for valley fog by morning, but after a warmer start to the evening and lower dewpoints, combined with some passing mid level cloud cover, we are unlikely to see strong enough cooling overnight to produce significant valley fog. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX