883 FXUS63 KLSX 060328 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1028 PM CDT Fri Jun 5 2026 .KEY MESSAGES... - Daily, varying chances of showers and thunderstorms will exist through Tuesday. There is a threat of locally heavy rainfall Sunday and Monday. - Temperatures and humidity will increase after Monday with peak afternoon/evening heat index values potentially reaching 100 F. && .SHORT TERM... (Through Late Saturday Night) Issued at 225 PM CDT Fri Jun 5 2026 With this morning's upper-level trough departing and the next trough passing north of the Mid-Mississippi River Valley, any redevelopment of showers and thunderstorms this evening and overnight is uncertain owing to a lack of clear low-level forcing mechanism. A few CAMs have inconsistently indicated potential showers and thunderstorms in northeastern MO/west-central IL this evening as well as clusters/MCS of showers and thunderstorms across IA and northern IL along a quasi- stationary front sagging southward into the LLJ overnight. However, latest HREF probabilities of measurable rainfall are 20 percent or less, further diminishing confidence. IF thunderstorms occur this evening, 2000 to 3000 J/kg MLCAPE and modest 15 to 25 kt of deep- layer wind shear, a briefly severe thunderstorm with damaging winds and marginally severe hail cannot be ruled out. But this conditionality and an already marginal environment keeps confidence too low to advertise the threat. On Saturday, the CWA will be sandwiched between the aforementioned upper-level trough passing to the northeast and another southern stream low/trough approaching from the south. Since the quasi- stationary front is also expected to stay northeast of the CWA, low- level forcing mechanisms are once again not clear. With a weak capping inversion, at least isolated showers and thunderstorms are expected during peak afternoon heating of the warm and moist airmass. There are timing differences in the arrival of large-scale ascent leading the approaching low/tough with the earliest being late Saturday afternoon, which would support increasing coverage of showers and thunderstorms if there is an overlap with peak heating. The exact location, coverage, and timing of thunderstorms is unclear, with CAMs also advertising notably different solutions. An isolated microburst with gusty winds is possible, but sub-15 kt deep- layer wind shear will limit the threat of severe weather. High temperatures will generally be similar to today, if not a couple of degrees warmer where clouds are less prevalent during the afternoon between early day stratocumulus and increasing upper-level clouds. Pfahler && .LONG TERM... (Sunday through Next Friday) Issued at 225 PM CDT Fri Jun 5 2026 The upper-level low/trough will propagate northward over the Central Plains and Mid-Mississippi River Valley Sunday and Monday, accompanied by the most favorable period of more widespread showers and thunderstorms with 60 to 90 percent of ensemble membership containing measurable rainfall. Probabilities are highest during each afternoon and evening but there are indications that nocturnal LLJs could also support some overnight showers and thunderstorms. There will be a locally heavy rainfall threat during this time period with precipitable water nearing 2" (99th climatological percentile) and very deep warm cloud depths combined with kinematic profiles suggesting potential backbuilding or training of some thunderstorms. However, flash flooding would conceptually require locations to receive multiple rounds of heavy rainfall and/or thunderstorms to loosely organize into MCSs or clusters, which cannot be said with much confidence at this range. The majority of the NBM high temperature distribution is cooler (upper 70s to 80s F) both Sunday and Monday, but how cool temperatures actually remain will come down to the amount of precipitation and cloud cover. Following the departure of the upper-level low/trough late Monday, global model guidance continues to advertise a blossoming ridge dominating the Mid-Mississippi River Valley within varying depictions of an Omega Block Tuesday through late next week. In this pattern, the warm and moist airmass will remain in place and even be reinforced by weak, but persistent low-level southerly flow. As 850- hPa temperatures rise to near the 99th climatological percentile and the ridge inhibits daily shower and thunderstorm development, high temperatures will warm to the upper 80s to mid-90s F. With dewpoints in the 70s F, peak afternoon and evening heat index values are projected to reach 100 F, but values will depend on the balance of diurnal boundary layer mixing allowing warmer temperatures with lowering dewpoints. Pfahler && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1027 PM CDT Fri Jun 5 2026 A large majority of the period will be VFR with light, diurnal surface winds around 5-10 knots that become light and variable in the overnight hours. Mid/upper level clouds continue to increase out of the southwest ahead of a disturbance in the southern Plains, which eject northeast later in the day. The system remains departed to southwest until afternoon, when a isolated to widely scattered showers and perhaps an isolated thunderstorm are possible. The limited coverage makes it too difficult to pinpoint any one terminal that could be impacted, but 18z Saturday afternoon to 02z Sunday would be the best timeframe for convection. Once beyond sunset isolated activity fades as we await better chances(30-60%) that arrive early Sunday morning. Maples && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX