


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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506 FXUS63 KLSX 110848 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 348 AM CDT Fri Jul 11 2025 .KEY MESSAGES... - Today will be hot and humid with increasing chances for thunderstorms late this afternoon/evening. A few thunderstorms may be strong to severe, mainly over northeastern Missouri and west- central Illinois. - The active pattern continues with temperatures trending cooler Saturday into Sunday. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 343 AM CDT Fri Jul 11 2025 The northern U.S. will remain active with multiple systems tracking west to east in the vicinity of the U.S./Canadian border. Further south, showers and thunderstorms will navigate around two expansive ridges. One of the ridges is parked over southern California/Baja Peninsula as the second is in control of the western Atlantic. The mid-Mississippi Valley is sandwiched between these features, which will exhibit some degree of influence through the next several days. At the surface, a stationary boundary stretches east to west from the New England Region into eastern Nebraska. Scattered thunderstorms accompany the boundary over northern sections of Illinois with broader coverage being supported by a surface low over western sections of Iowa. Much of this is expected to remain north with the exception of a shower/isolated thunderstorm skimming the far northern fringes of the CWA (NE MO/WC IL). The surface low will lead to modest warm air advection over MO/IL today in conjunction with persistent, but weak south/southwesterly flow at the western periphery of the Atlantic ridge. HREF H8 mean temperatures in the upper teens (C) will warm into the low-20s under mostly sunny skies. This comes with an increase in surface moisture, noted by dewpoints climbing back into the low-70s. The combination will make for a hot and humid day with highs in the low to mid-90s and heat indices in the upper 90s to 105 degrees. Aside from a short-lived, stray shower/thunderstorm, most of the day remains dry with little/no ascent aloft, weak shear (<10 knots), and marginal mid-level lapse rates (6C). The best potential for thunderstorms arrives sometime between late this afternoon into this evening as the surface low and mid-level trough move eastward out of Nebraska. CAMs show another cluster/complex of thunderstorm development over Iowa late this morning into early afternoon with additional development stretching down a trailing cold front that extends southwestward into eastern Kansas and western Oklahoma. Paintball plots (>40 dBZ sim. reflectivity) show CAMs fairly well clustered along and north of the Missouri/Iowa border with the broader complex of thunderstorms (potential MCS). Being that much of the lift associated with the low/trough will move north, thunderstorms along the front will largely rely on frontal convergence and diurnal instability. Development along the front looks to enter the northwestern CWA between 21z-23z. At the peak of diurnal instability, HREF mean SBCAPE values reach 2500-3000 J/kg in conjunction with mid-level lapse rates near 7C and slight backing in surface winds. The limiting factor is the weak flow component with marginal shear values of 20-25 knots. Initial development along the front may support mini-supercells or organized multicells with all hazards possible. The unstable environment may pose the potential to overcome the weaker shear to produce short-lived rotation and damaging hail during the late afternoon/early evening, though hodographs are not elongated to support sustained, strong rotation. While the tornado potential is low, it may extend into northeastern Missouri/west-central Illinois. As thunderstorms evolve downstream, damaging winds become the primary threat as instability wanes and the system becomes outflow dominant through midnight or so. By early Saturday, the cold front drifts along or just south of I- 70, becoming more east-west oriented with scattered showers and thunderstorms along and south of the front. The trough and associated surface low eject northeast into the Great Lakes, removing the better ascent associated with today`s thunderstorms. However, MLCAPE climbs back to 2000-3000 J/kg with plentiful moisture to tap into within a weakly sheared environment. Thunderstorms reemerges Saturday afternoon/evening with much of the convective potential focused along the front. RAP guidance shows weaker lobes of mid-level vorticity rotating around the southern end of the broader trough with isolated/widely scattered convection over top the boundary which looks more diffuse in time. With PWATs of 1.5- 2", any of the thunderstorms will be capable of producing locally heavy rain and localized downburts with damaging wind potential. Thunderstorms weaken once again through the late evening/overnight with decreasing instability. Maples && .LONG TERM... (Saturday Night through Thursday) Issued at 343 AM CDT Fri Jul 11 2025 Sunday into early next week will continue an active stretch of weather as the southeastern surface/mid-level ridge shifts westward into the Gulf States and steers multiple shortwaves northeastward through the central U.S. The first of these arrives Sunday, along with a weak surface low that nudges the surface boundary northward slightly. Though rain probabilities generally increase from north to south (20% near KUIN to 40-45% near KFAM), timing of the shortwave may very well be factor in further adjustments to PoPs in subsequent updates. Considering the diurnal trends with instability, the position of the shortwave is likely to support relatively better coverage with afternoon development. As of now, the best potential looks like it will be just east of central Missouri into east- central/southeastern Missouri, as well as southwestern Illinois. Another round of thunderstorms with weak shear (20 knots) and high PWATs (around 1.8") is possible. I would not be all too surprised to see PoPs increase in later updates, mainly along and south of I-70 along and east of the Mississippi River. The remainder of the period remains rather active as the surface boundary seems to meander through the region. Additional shortwaves track around the southeastern ridge and over top the surface boundary with diurnal peaks in thunderstorms potential. Where exactly the highest potential sets up is less certain with scattered convection in the vicinity of the boundary, aided by ascent from each consecutive shortwave. NBM interquartile ranges remain tightly clustered through Tuesday, but then show greater spread (10 degrees) by mid to late week with guidance likely having trouble resolving the position/timing of surface features and resulting precipitation potential. The one day that may end up mostly dry is Monday, as the boundary is shunted further south in the wake of the first shortwave. Precipitation potential then trends upward through the remainder of the week with a broad spread in PoPs and temperatures with less confidence in the exact position/track of surface features and upper shortwaves. Nonetheless, the pattern favors more active weather through the end of the period with slightly cooler temperatures as a result of precipitation potential. Maples && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1031 PM CDT Thu Jul 10 2025 VFR conditions are expected for a majority of the TAF period, but showers and thunderstorms may impact terminals tomorrow evening. While a few weakening showers will be possible overnight at UIN, VFR conditions are likely to prevail through the night, and most of the afternoon tomorrow in spite of an afternoon deck of cumulus. However, showers and thunderstorms are likely to move into northeast / central Missouri and west-central Illinois by early evening, and will likely impact UIN, COU, and JEF at some point at the end of the period. While we do expect at least some precipitation and lightning in the area, there is some un certainty regarding the timing, and we have opted to represent this with a PROB30 in the TAF for the time being. This may need to be adjusted to prevailing TSRA as confidence in the timing increases. If storms do impact terminals, visibility and ceiling reductions are likely due to heavy rain and low cloud bases, along with strong, erratic surface winds. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX