FXUS63 KLSX 181137 AFDLSX Area Forecast Discussion National Weather Service St Louis MO 637 AM CDT Sat Jul 18 2026 .KEY MESSAGES... - Seasonably hot and humid conditions will be most persistent in the Heat Advisory area through Tuesday including peak heat index values up to 105 F. - Daily chances of at least isolated showers and thunderstorms will also exist through Tuesday. A strong thunderstorm or two could contain gusty winds and locally heavy rainfall today and Sunday. && .SHORT TERM /THROUGH SUNDAY/... Issued at 322 AM CDT Sat Jul 18 2026 An upper-level trough will traverse the Great Lakes today, but the Mid-Mississippi River Valley will be displaced from associated large- scale forcing and stronger mid-level flow. Despite this fact, strong heating of the moist airmass in place suggests that isolated to perhaps scattered showers and thunderstorms will develop late this morning through afternoon along/south of I-70 as a weak capping inversion is breached in some locations. Additionally, a cold front trailing the aforementioned trough will progress southwestward into northeastern MO and west-central/south-central IL this evening providing greater coverage of showers and thunderstorms, although this timing is out of phase with peak heating, suggesting that thunderstorms will generally be weakening by that time. Gusty winds will be possible with a stronger thunderstorm or two today given 1500 to 2500 J/kg MLCAPE and "inverted-V" sounding signatures, but weak deep-layer wind shear of 10 to 15 kt and later timing of the cold front keeps confidence in severe weather (damaging winds) still too low to message. Considering PW near 2" (>90th climatological percentile) and slow storm motions, thunderstorms may also contain locally heavy rainfall but the threat of flash flooding is low. With high temperatures around 90 to the mid-90s F and the moist airmass, many areas will see afternoon heat index values rise to near or just above 100 F. As it becomes well-detached from large-scale forcing, the cold front will slow and then stall near or just west of the Mississippi River late tonight through Sunday. Following a lingering shower or thunderstorm near the front overnight, diurnal redevelopment of showers and thunderstorms is expected again during the late morning through afternoon in the vicinity of the front. Exact coverage showers and thunderstorms is uncertain on Sunday, but CAMs indicate some thunderstorm aggregation that may allow for periods of greater coverage. Ingredients for gusty winds with the strongest thunderstorms will once again be in place along with locally heavy rainfall, but the threat of severe weather and flash flooding is still diminished by weak deep-layer wind shear and a lack of thunderstorm organization. Along/west of the front, the net effect of slightly cooler high temperatures but higher dewpoints pooled near the front will result in afternoon heat index values again reaching 100 to 105 F, increasing confidence in four or more days with 100+ F generally in the Heat Advisory area. However, there is a low concern for early development and greater coverage of showers and thunderstorms limiting heat index values. East of the front (west-central/south-central IL), a cooler airmass will arrive with high temperatures in the 80s F and high confidence in heat index values staying below 100 F on Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Issued at 322 AM CDT Sat Jul 18 2026 Another upper-level trough is progged to traverse the Northern Plains toward the Great Lakes Monday through Tuesday, helping to lift the front back northward out of the CWA on Monday and then forcing passage of a cold front on Tuesday. This evolution leads to higher confidence in Monday being the hottest day for most of the CWA as 850-hPa temperatures rise to around 21 to 22 C (95th climatological percentile) with high temperatures firmly in the low to mid-90s F, accompanied by afternoon heat index values of 100 to 107 F. Cold frontal passage on Tuesday introduces more uncertainty in the temperature and heat index forecast, but there is a growing consensus in ensemble membership for one more day of 100+ F heat index values along/south of I-70. Aside from isolated diurnal showers and thunderstorms on Monday, global ensemble membership indicates greater support (20 to 50 percent) for showers and thunderstorms on Tuesday with remnant activity along the LLJ skirting northern parts of the CWA during the morning and then development during the afternoon/evening near the front. With stronger deep-layer wind shear (20 to 35 kt), the low threat of a few severe thunderstorms will need to be monitored for Tuesday afternoon/evening; however, uncertainties in the strength of a capping inversion decreases confidence in how much development, especially surface based, occurs decreases confidence. Behind Tuesday's front, a much cooler and drier airmass will be filtered into the Mid-Mississippi River Valley with below average high temperatures in the 80s F. There is strong support for dry conditions on Wednesday as well with a surface high pressure center building into the area, but a series of upper-level shortwave troughs in the northwesterly flow will provide periods of showers and thunderstorms regionally through the end of the week. That being said, the favored corridor of showers and thunderstorms will be closer to the front west/southwest of the CWA through Thursday, but the front will attempt to lift northeastward with time, eventually shifting this corridor closer to the CWA. Until then, the main impact of the shortwave troughs will be increased cloud cover, which may further reinforce the seasonably cool temperatures ahead of the warm front. && .AVIATION /12Z TAFS THROUGH 18Z SUNDAY/... Issued at 637 AM CDT Sat Jul 18 2026 Largely dry and VFR flight conditions are expected through tonight across the region but there are low chances of showers and thunderstorms, late this morning through afternoon along/south of I- 70 and this evening north of I-70. Confidence is relatively higher (although not high) for thunderstorms to impact KUIN this evening, warranting a PROB30 group. Thunderstorms will be capable of brief MVFR to IFR flight conditions and gusty winds. Very patchy lower ceilings (sub-VFR) cannot be ruled out late tonight into Sunday morning, but the probability of impacts is too low to include in the TAF at this time. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory from noon today to 7 PM CDT Tuesday for MOZ041- 047>051-059>065-072>075-084-085-099. IL...Heat Advisory from noon today to 7 PM CDT Tuesday for ILZ069- 074-079-100>102. && $$ SHORT TERM...Pfahler LONG TERM...Pfahler AVIATION...Pfahler