####018007252#### FXUS63 KLSX 200827 AFDLSX Area Forecast Discussion National Weather Service St Louis MO 327 AM CDT Mon Jul 20 2026 .KEY MESSAGES... - Hot and humid conditions with heat index values of 100 to 108 F will envelop much of the area today and then locations along/south of I-70 on Tuesday. - The highest chance of showers and thunderstorms through Tuesday will be overnight tonight into Tuesday morning north of I-70, including a conditional threat of a marginally severe thunderstorm or two and locally heavy rainfall. - Much cooler conditions will arrive by Wednesday, accompanied by a period of dry weather through Thursday. && .SHORT TERM /THROUGH TUESDAY/... Issued at 326 AM CDT Mon Jul 20 2026 Light winds and mostly clear skies have allowed areas of fog to form across the CWA this morning, especially in the vicinity of a weak stationary front and moistened soils from Sunday's showers/thunderstorms. Near/east of the Mississippi River, this fog has been aided by Canadian wildfire smoke or at least further reducing visibilities. Visibilities down to 1/4 mi are possible, but confidence is low that these will occur on more than a localized basis before dissipation through sunrise. An upper-level trough entering the Upper Mississippi River Valley and a leading shortwave trough will encourage slow northeastward advancement of the front as a warm front today, reaching west- central/south-central IL through the afternoon and evening. Therefore, wildfire smoke is likely to linger in these locations through much of the day, mainly impacting air quality (see Air Quality Alert). With more of the CWA in the warm sector and much less showers/thunderstorms, more widespread high temperatures around 90 to the mid-90s F are forecast along with pooled dewpoints yielding heat index values of 100 to 108 F in the Heat Advisory area. Diurnal showers and thunderstorms are expected again this afternoon, but the HREF membership indicates that coverage will be much lower than Sunday and mainly across southeastern MO where a capping inversion is weakest. Confidence is also low in any showers and thunderstorms associated with the lead shortwave trough this afternoon reaching the northern CWA. These showers and thunderstorms are more likely to remain across eastern IA and northern/central IL. An increasing amount of HREF membership has showers and thunderstorms developing along a cold front across IA and WI this evening, tracking south/southeastward with a veering LLJ overnight into Monday morning including reaching some portion of the CWA north of I-70. However, there is still uncertainty on how far southwestward thunderstorms truly develop along the front/into the LLJ and reach the CWA given northwesterly downshear Corfidi vectors and a dependency on upstream convective evolution. If thunderstorms do reach the CWA, 30 to 40 kt of deep-layer wind shear and 1000 to 2500 J/kg MLCAPE support organized thunderstorms, largely into either an MCS and/or a few supercells with a threat of isolated marginally severe hail and damaging winds. There would also be a threat of locally heavy rainfall along the southwestern flank of thunderstorms with PW around 2" and deep warm-cloud depths. However, both of these threats are conditional with another reasonable scenario still including thunderstorms remaining just north/northeast of the CWA. The cold front will traverse the CWA on Tuesday, reaching locations along/south of I-70 during the afternoon and evening. In these locations, high temperatures will once again warm into the 90s F, accompanied by heat index values of 100 to locally near 110 F depending on how much dewpoint lower with deep BL mixing. To the north behind the front, stronger post-frontal CAA will lag frontal passage with high temperatures still reaching around 90 F, but dewpoints will be lowering more quickly and significantly limit heat index values. With the upper-level trough passing just to the northeast and mid-level heights remaining neutral, it is uncertain how much development of showers and thunderstorms there will be along the portion of the cold front traversing the CWA, with stronger forcing and less prominent capping inversion to the east. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Issued at 326 AM CDT Mon Jul 20 2026 The much cooler and drier airmass will finally reach the Mid- Mississippi River Valley by Wednesday, remaining in place through Thursday. In this airmass, seasonably cool high temperatures in the 80s F are forecast, accompanied by low dewpoints in the 50s and low- 60s F. A surface high pressure center will also dominate the region with dry weather during this time, but a series of upper-level shortwave troughs with northwesterly flow will induce rounds of showers and thunderstorms along/north of a warm front, initially in the Central Plains, Thursday through Saturday. However, global ensemble model guidance indicates that the most favored corridor of showers and thunderstorms will not reach the CWA until Thursday night or Friday as the warm front progresses northeastward. Although moisture will be increasing through this time, the CWA will likely remain largely on the cool side of the front through Friday not to mention potential precipitation and clouds keeping temperatures seasonably cool. Global model guidance is in agreement that an upper-level ridge will be amplifying across the Front Range of the Rocky Mountains through the end of the week, favoring the warm front to continue lifting northeastward along with the more active track of shortwave troughs. Therefore, confidence is high that temperatures will moderate to average, if not above average through the weekend along with increasingly dry conditions. && .AVIATION /00Z TAFS THROUGH 06Z TUESDAY/... Issued at 1042 PM CDT Sun Jul 19 2026 Dry, VFR conditions currently prevail at the terminals, however MVFR visibility restrictions are still forecast to develop at KSUS, KCPS, and KJEF where a combination of today's heavy rain and moisture from nearby rivers will promote fog development. There is some signal for IFR conditions to develop overnight south of the St. Louis metropolitan area, but all indications suggest this threat would avoid the terminals. Following this threat, dry and VFR conditions will resume Monday morning. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 7 PM CDT Tuesday for MOZ041-047>051- 059>065-072>075-084-085-099. Heat Advisory from noon today to 8 PM CDT this evening for MOZ018-019-026-027-034>036-042-052. IL...Air Quality Alert until midnight CDT tonight for ILZ058>060- 065-095>097. Heat Advisory until 7 PM CDT Tuesday for ILZ069-074-079- 100>102. Heat Advisory from noon today to 8 PM CDT this evening for ILZ095-097-098. && $$ SHORT TERM...Pfahler LONG TERM...Pfahler AVIATION...MRB