


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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619 FXUS63 KLSX 080234 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 934 PM CDT Mon Jul 7 2025 .KEY MESSAGES... - Scattered thunderstorms are possible this afternoon and evening from the eastern Ozarks through southwestern Illinois. Isolated to perhaps scattered thunderstorms are possible over central Missouri. - Another round of thunderstorms is possible Tuesday. A few thunderstorms may become strong with isolated severe potential. The primary threat will be damaging wind gusts to 60 mph and a localized threat for heavy rain. && .SHORT TERM... (Through Late Tuesday Night) Issued at 231 PM CDT Mon Jul 7 2025 Regional analysis shows a diffuse or remnant cold front slowly sinking southward across sections of southern Missouri and Illinois. Scattered showers and thunderstorms have largely been focused along this boundary early this afternoon. Curving back to the northwest through central Missouri is isolated activity that is largely supported by mid/upper level vorticity riding over top the washed out surface boundary and theta-e gradient that extends through mid- Missouri from NW to SE. The boundary, which is essentially washed out/quasi stationary, will remain the primary driver for additional showers and thunderstorms through this evening. Trends over the last few hours have showed a slight reinforcement toward the south as a result of cooler air produced by ongoing thunderstorms in this region. If the cooler air produced by thunderstorms continues to push the boundary south, which come CAMs suggest, much of it will shift that direction over the next few hours. That leaves us with isolated to perhaps scattered convection over central Missouri, where mid/upper vorticity is currently tracking east-southeast along a theta-e gradient that separates the moist atmosphere from the copious moisture further southwest. Fortunately, thunderstorms are developing in a weak shear environment and other than the semi-linear orientation along the diffuse boundary, there isn`t much organization expected. Therefore, should a thunderstorm become strong, the primary threat would be gusty winds. Even that potential is low considering the inverted-V signature isn`t as pronounced as I`d like to otherwise see (marked by 10 dewpoint depressions). Thunderstorm chances never really zero out, but will wane late this evening as diurnal instability is lost after sunset. Tuesday sets up another complicated scenario which poses slightly better chances for stronger thunderstorms. CAMs show some semblance of an MCS taking shape over sections of eastern Nebraska, then sliding southeast near the IA/MO border overnight. By Tuesday morning, the complex weakens, if not entirely dissipates as it outruns its H8 theta-e source and become outflow dominant. Slight chance/chance PoPs cover this potential over central MO into west-central IL. The biggest question was how far east to spread slight chance PoPs considering the uncertainty with how long the complex survives. This will certainly be an area of opportunity for fine tuning in subsequent updates. The best potential for thunderstorms comes Tuesday afternoon into Tuesday evening with the caveat that prior convection will be long gone, allowing instability to build through the afternoon. The complex addressed in the prior paragraph will send and outflow boundary southward, encountering dewpoints in the low to mid-70s and SBCAPE values of 2500-3500 J/kg per HRRR/RAP/NAM guidance, as well as interaction with the remnant surface boundary to its south. How it plays out is dependent on timing and placement, which has not been so consistent across individual guidance. Should a slow solution play out, thunderstorms would initialize further northwest and take advantage of an unstable atmosphere that is largely undisturbed or merely riddles with isolated to scattered activity. Should the quicker solution play out, the interaction with the surface boundary could result in development sooner, which would have broader geographical coverage that Monday. This would then lead to competing updrafts in a low shear (20 knots or less) environment with moisture loaded thunderstorms falling into a surface layer that is marginally supportive of wet downburst. Therefore, the SPC`s marginal outlook seems reasonable with the potential for isolated damaging wind gust to around 60 mph. Additionally, PWAT values around 2 inches suggest heavy rainfall will be possible within thunderstorms. However, these look somewhat progressive and while localized ponding or nuisance flooding is possible, it does not look like a widespread issue at this time. Maples && .LONG TERM... (Wednesday through Next Monday) Issued at 231 PM CDT Mon Jul 7 2025 The main features that draw attention in the long term period are the strengthening upper level ridge over the Desert Southwest and the stacked ridge centered over the western Atlantic that extends westward through the Gulf States. By Wednesday, the surface front that aids in thunderstorm development Tuesday into Tuesday evening will be reinforces southward as the mid/upper trough passes over the mid-Mississippi Valley. As is typical for mid-summer patterns, guidance varies on the exact position and resulting precipitation potential. Nonetheless, the general consensus is that thunderstorms will be concentrated along and south of I-70 as high pressure builds in front the northwest and northwesterly flow draws in slightly cooler and drier air. The remainder of the forecast is somewhat of a squeeze play, as the strong upper level ridge over the southwestern CONUS begins to flatten shift slightly west. What initially steers the active track over the northern Rockies into the Plains, will eventually give way to zonal flow aloft. Meanwhile, the ridge over the southeastern U.S. could be a player in convective trends further out. In summer patterns like this, convection will often get hinged up at the northwest side of the surface/mid-level ridge, but can also be invigorated by shortwave/troughs that are introduced from the west. Considering that upper level flow turns zonal in time, additional chances for thunderstorms will press through the region, along with the enhancement of southerly flow at the back side of the southeastern ridge and ahead of approaching systems. While no single day looks completely dry, the most of the dry time might end up being Wednesday afternoon into Thursday as surface high pressure crosses the region. By late week into next weekend, a better organized trough tracks west to east across the central Rockies, resulting in surface cyclogenesis over the Plains. A cold front trails the system and could be best chance at showers and thunderstorms beyond Wednesday. Maples && .AVIATION... (For the 06z TAFs through 06z Tuesday Night) Issued at 934 PM CDT Mon Jul 7 2025 Dry conditions are expected for the most part overnight tonight, though we are watching organized convection moving southward across eastern Nebraska. The most likely scenario is for this activity to largely or completely miss us to the west. In addition, it should weaken with time. Additional thunderstorms are forecast to develop by early-mid afternoon, mainly from central Missouri east/northeast. The best chances for thunderstorms may be at the metro terminals, largely between 2000 UTC Tuesday and 0000 UTC Wednesday. Wasn`t quite confident to add a TEMPO, as there remains enough uncertainty on timing and where convective development will be preferred. However, any terminal that does see a direct impact from a thunderstorm Tuesday will be capable of producing gusty winds and brief downpours reducing visibilities. The other concern is for some fog tonight at the river valley sites. The best chances look to be across southeast Missouri and southwest Illinois, but cannot rule out a brief period of say MVFR visibilities at KSUS, KCPS, and/or KJEF. Left this mention out of the TAFs for now, but this will be something to monitor overnight. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX