053 FXUS63 KLSX 042328 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 628 PM CDT Thu Jun 4 2026 .KEY MESSAGES... - Warm and increasingly humid air will become established across the region, persisting well into next week. - Daily chances of showers and thunderstorms are expected into next week. Sunday and Monday are favored for the highest chance and greatest coverage with plenty of dry time otherwise. && .SHORT TERM... (Through Late Friday Night) Issued at 255 PM CDT Thu Jun 4 2026 Low-level south-southwesterly flow continues to transport increasingly moist air into the region, evidenced by dewpoints running about 15 to 20 F higher than Wednesday and widespread diurnal cumulus. A subtle mid-level perturbation is also promoting the development of isolated to scattered showers across southern/southwestern MO that could spread into central and northeastern MO through this evening with a stray thunderstorm possible as well. Further to the west across northern KS/eastern NE, an upper-level trough is driving an MCS that is progged to track slowly northeastward with trailing showers and thunderstorms progressing eastward north of I-70 overnight, supported by a veering, nocturnal LLJ. These showers and thunderstorms are not expected to reach the CWA until Friday morning with the highest HREF probabilities (60 to 80 percent) of measurable rainfall across northeastern MO/west- central IL, but some CAMs have showers and thunderstorm reaching as far south as I-70 and east as I-44 (MO) and I-55 (IL) through late morning and early afternoon. However, coverage should be decreasing through that time as large-scale ascent departs with the through to the east and the LLJ abates. Subsequent development of showers and thunderstorms (locally speaking) is uncertain Friday evening into overnight, seeming to rely on a nocturnal LLJ interfacing a potential remnant outflow boundary somewhere north of I-70 (northeastern MO/west-central IL) and/or a thunderstorm cluster/MCS sagging southward into the LLJ. IF there are thunderstorms in the CWA during the evening, 1500 to 2500 J/kg MLCAPE and 15 to 25 kt of deep-layer wind shear would support transient severe thunderstorms with damaging winds and marginally severe hail. However, confidence is much higher in showers and thunderstorms developing to the north and northwest of the CWA, closer to a quasi-stationary front, which is reflected by HREF probabilities as well, precluding messaging of any severe weather at this time. Even with morning showers and thunderstorms on Friday, breaking clouds and predominantly dry conditions during the afternoon will lead to high temperatures a couple degrees warmer than today. Pfahler && .LONG TERM... (Saturday through Next Thursday) Issued at 255 PM CDT Thu Jun 4 2026 On Saturday, the CWA will spend most, if not all, of the daytime hours between the upper-level trough propagating to the northeast and another southern stream low/trough approaching from the south. With a lack of discernible low-level forcing mechanism either, the location, timing, and coverage of any showers and thunderstorms developing is highly uncertain. However, at least isolated showers and thunderstorms are expected during peak afternoon and evening heating with a weak capping inversion advertised in forecast soundings. Greater coverage would have to depend on either a remnant outflow boundary from morning thunderstorms and/or earlier arrival of the southern stream low/trough. With less overall precipitation/clouds and slightly warmer 850-hPa temperatures, high temperatures on Saturday are forecast to be warmer and in the mid- 80s to around 90 F. The aforementioned upper-level low/trough is anticipated to track northward through the Central Plains and Mid-Mississippi River Valley Sunday and Monday with the greatest corresponding support for showers and thunderstorms in ensemble model guidance those days. Probabilities of measurable rainfall peak each afternoon and evening at 60 to 90 percent across the entire CWA. With greater coverage and prevalence of precipitation and clouds, confidence is high that those days will be relatively cooler, but exact values are very uncertain and sensitive to those factors with wider (5 to 7 F) NBM high temperature interquartile ranges. It is not out of the question that temperatures remain in the 70s F where precipitation lingers most of the day. Deep-layer wind shear Saturday through Monday will be 20 kt or less, indicating that thunderstorms will be unorganized or very loosely organized at best despite potentially moderate amounts (1500 to 3000 J/kg) of instability. These factors limit the threat of severe weather and flash flooding but isolated microbursts with gusty winds and locally heavy rainfall will need to be monitored, considering that warm rain processes will be very efficient with precipitable water projected to flirt with 2" or the 99th climatological percentile in combination with deep warm cloud depths. After the upper-level trough departs late Monday, global model guidance has significant height rises taking place across the Mid- Mississippi River Valley as an upper-level ridge blossoms in part of a developing Omega Block. In effect, this pattern acts to stagnate the resident warm and moist airmass with aided influx of warmth and moisture upon weak low-level southerly flow. This evolution supports high temperatures in the upper 80s to low-90s F and heat index values reaching around 100 F each afternoon with dewpoints in the 70s F. The amount of ensemble model membership with showers and thunderstorms decreases each day after Monday with the increasingly dominant ridge. Pfahler && .AVIATION... (For the 00z TAFs through 00z Friday Evening) Issued at 627 PM CDT Thu Jun 4 2026 VFR conditions continue this evening with south-southwesterly surface winds persisting through Friday. The primary focus this period will be on showers and thunderstorms currently over the eastern Plains. The main support for widespread precipitation remains north of the terminals, though an isolated shower may impact KCOU/KJEF this evening before weakening/fading after sunset. Chances then increase late tonight in to Friday morning as thunderstorms from the Plains move over the Mississippi Valley. KUIN is in better position to be impacted by remnant showers/thunderstorms Friday morning with activity becoming scattered to isolated around KCOU/KJEF, if it survive at all that far south. PROB30s were utilized to convey the low-end potential over central Missouri, while prevailing -SHRA and PROB30 for -TSRA was leveraged at KUIN. Brief dips to MVFR is possible around KUIN. However, VFR is favored much of the time. Gusty conditions will also appear at KUIN as the activity tracks west to east over the terminal. Showers/thunderstorms become less noteworthy through the rest of the period, although an isolated shower/thunderstorm cannot be entirely ruled out through peak diurnal heating. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX