137 FXUS63 KLSX 162229 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 529 PM CDT Wed Apr 16 2025 .KEY MESSAGES... - An active weather pattern will commence tonight and continue through Sunday featuring multiple rounds of showers and thunderstorms. - A very low threat of severe storms will exist across far northeast Missouri and west-central Illinois on Thursday night, with a greater albeit still low threat of severe storms across the entire area late Friday afternoon and evening. - There is an increasing threat of heavy rain from Friday night into early Sunday with total amounts of 3-5 inches coming into focus centered near the I-44 corridor. This rainfall would bring at least a low flash flood threat but a greater threat of river flooding. - A conditional threat of severe storms exists on Sunday, highly dependent on the track of a low pressure storm system moving across the area. && .SHORT TERM... (Through Late Thursday Night) Issued at 227 PM CDT Wed Apr 16 2025 An uptick in the coverage of showers and perhaps a thunderstorm will occur this evening within a broad north-south zone centered through eastern MO. This will be in response to a weak mid-level perturbation tracking across the area, and more importantly, intensifying low- level WAA. This region of precipitation will persist into the overnight hours and shift E/NE into IL with time. Overnight and primarily after 09-10z a separate area of more organized showers and thunderstorms will spread/develop into central MO attendant with a more well-defined mid-level impulse along with a veering and increasing southwesterly LLJ and associated deeper moisture transport/lift and instability. Showers and thunderstorms are expected to be ongoing on Thursday morning, mainly centered west of the MS River. This activity should shift east of the region by early Thursday afternoon in association with the eastward migration of the mid-level disturbance, leaving much of the afternoon void of precipitation. Temperatures should warm nicely in the wake of the morning precipitation as skies clear and WAA increases. Present indications are there will be another threat of showers and thunderstorms on Thursday night. Warming aloft, mid-level height rises, and advection of the low-level moisture pool to the northeast would suggest this threat is limited in both coverage and area and largely confined to NE MO/WC IL. Better forcing and instability is forecast further north into IA, however weak but sufficient MUCAPE and steep mid-level lapse rates would support a small hail threat with any elevated storms which may form, a low chance of possibly marginally severe hail with the strongest of storms. Glass && .LONG TERM... (Friday through Next Wednesday) Issued at 227 PM CDT Wed Apr 16 2025 The weather/pattern will become increasingly active Friday through the weekend. Confidence is low on the severe weather threat with numerous caveats, however the threat of heavy rainfall continues to increase. The jury is still out on the convective evolution and trends on Friday and Friday Night, but there is an increasing amount of data suggesting that Friday itself will be largely void of surface- based convection due to the presence of a stout EML/strong capping inversion through the daylight hours throughout the warm sector ahead of the advancing cold front. Like discussed yesterday the more plausible solution appears to be that height falls/cooling aloft on Friday evening (and perhaps some cooling from mid-level showers) will help create a more favorable environment for thunderstorms along and in the wake of the advancing cold front as forcing with the upper system and LLJ increases. In this scenario, thunderstorms are largely rooted above the boundary layer and a primary severe threat would be large hail and to a lesser extent strong winds. Even with any storms that might manage to from along the front itself, the flow is quite veered leading to unfavorable hodographs for a tornado threat. An increasing threat of heavy rain will unfold later Friday night into Sunday as the cold front moves southward and eventually stalls across SE MO/SRN IL with a slow-moving deep upper trof upstream. The evolving pattern closely resembles the hybrid synoptic-frontal pattern for heavy rain recently highlighted in Gerhardt (2024) supporting multiple waves of showers and thunderstorms and training. The location and axis of the heaviest rainfall and overall extent of the rainfall footprint has been nudging northward and there is normal uncertainty still in the exact location. However the AXIS of greatest rainfall appears to be setting up near or just north of the I-44 corridor. The latest probabilities of rainfall in excess of 3 inches have increased to 50-70 percent. The 100-member LREF shows the total QPF distribution from the 90th percentile in the 4-5+ inch range, which largely supports our new messaging of 3-5 inches of rainfall. If these rainfall amounts materialize, then the most likely impacts would be AT LEAST minor river flooding on the Meramec, Big, and Bourbeuse, and Black Rivers/Basins and other small river systems in central and east-central MO. The flash flood threat will be dependent on high rainfall rates, with at least a higher threat of nuisance flooding. We will also need to keep a close eye on the potential for severe storms. Perhaps immediately along the stalled front across SE MO/SRN IL Saturday afternoon/eve. Any threat on Sunday will be dependent on the eventual track of the surface low and position of the warm front associated with the ejecting upper trof. The models continue to diverge on specifics with the timing/location of the upper trof and surface systems. The more northern track shown in the operational ECMWF showing a negatively tilted trof, bigger warm sector and more favorable instability would support greater severe thunderstorm potential, both in coverage and the magnitude of threats. Glass && .AVIATION... (For the 00z TAFs through 00z Thursday Evening) Issued at 524 PM CDT Wed Apr 16 2025 VFR conditions are expected for the remainder of the evening. Overnight through tomorrow morning, waves of showers are expected to affect all terminals, with the most prolonged period of showers likely at St. Louis area terminals (roughly 05 to 16Z). A few thunderstorms will also be possible at all terminals, but confidence is low in terminal impacts due to sparse coverage. Ceilings are also likely to lower overnight through tomorrow morning, perhaps to MVFR. Category reductions due to ceilings are most likely at UIN. Meanwhile, breezy south winds are expected from late this evening through the end of the period. Low clouds are expected to gradually diminish through the day tomorrow, and most sites should end the period with VFR conditions. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX