FXUS63 KLSX 210528 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1228 AM CDT Sun Jun 21 2026 .KEY MESSAGES... - Thunderstorms are expected across the area late tonight through Sunday night. Some of these storms may be strong to severe Sunday afternoon/evening with damaging winds and tornadoes. - Locally heavy rainfall is also possible through Sunday night, mainly across northeast Missouri and west-central Illinois. - Seasonably cool temperatures are forecast next work week. && .SHORT TERM... (Through Late Sunday Night) Issued at 252 PM CDT Sat Jun 20 2026 Dry weather is expected for much of the night, but all eyes were to our west and an MCS that is expected to develop overnight. Forward- propagating Corfidi vectors are due west to east, though the vectors do veer more toward the northwest as you head into eastern Kansas late tonight into early Sunday morning. Given that convection is expected that far west, at least some southward movement is expected. This is also buttressed by a veering low- level jet with time as well. Storms should continue to develop on the western flank of the MCS, and travel southeast with time. Therefore, while there is still some uncertainty with respect to the track of the heart of the MCS, it does appear likely that at least some thunderstorms will move through central and east central Missouri by mid to late morning. Most CAMs generally weaken this activity with time/eastward extent, but this is a common bias. Given the low-level jet and associated moisture convergence does not abate at all Sunday morning, this weakening trend is likely overdone in model guidance. There is still concern for heavy rainfall very late tonight (after 0900 UTC) through Sunday morning across northeast Missouri and west- central Illinois. This is where the heart of the MCS is most likely to travel through. Some training appears likely as well in an environment that should be conducive to efficient warm rain processes (deep warm cloud depths, anomalously high precipitable water values). The duration of thunderstorms in this area is also notable (~6 hours). Amounts of 1-3" are likely, with some spots of up to 4" possible (similar to HREF's LPMM). In terms of a severe threat, some small hail and gusty winds are possible, but the overall severe threat is low with the MCS itself. There is a low possibility that storms may strengthen with time toward midday into the early afternoon as instability climbs. If this occurs, parts of western Illinois may see an increasing risk for damaging winds. The morning MCS should put out a composite outflow boundary to its south/southwest. Exactly where this boundary will be is uncertain and very crucial for the afternoon/evening severe threat. Past experience suggests it will be farther south/southwest than most model guidance. Given the positive factors detailed above re: storms continuing through the morning hours at least with a southerly component, the most likely location still appears to be from central to southeast Missouri around midday. This boundary may try and move northward into the early afternoon if convection completely dies off. Additional convective initiation is expected by mid afternoon, with the aforementioned remnant outflow boundary likely the focus. The convective mode is likely to be mixed, with multicell clusters, small bowing segments, and transient supercells all possible given effective deep-layer shear of ~35 knots. Large hail is possible in supercells along with tornadoes. The tornadic threat may be even more heightened along/near a weak surface low which could transverse the boundary. Similar to yesterday, whether this actually happens or not is unclear. Regardless, there does appear to be enhanced low- level shear/helicity for tornadogenesis near the composite outflow. The threat for damaging winds also will exist, mostly within any bowing segments. As alluded to above, the severe risk remains highly uncertain for Sunday afternoon/evening and largely will be dependent on exactly how the morning MCS evolves. Currently, the best chances for severe thunderstorms appears to be from central- southeast Missouri and points southwest, but this area may expand northeastward if the morning MCS ends up further north and/or weaker than currently anticipated. Gosselin && .LONG TERM... (Monday through Next Saturday) Issued at 252 PM CDT Sat Jun 20 2026 (Monday - Monday Night) There are signs of another midlevel shortwave trough moving across the mid south early Monday. This feature may help continue the threat for showers and a few thunderstorms across southeast Missouri and southwest Illinois. Farther north, drier and cooler weather will continue to advect southward behind Sunday night's cold front. High temperatures in the 70s are forecast on Monday, with lows Monday night in the upper 50s to low 60s. (Tuesday - Next Saturday) Broad northwest flow aloft is expected through the remainder of the week per ensemble guidance. There are signs of a cold frontal passage sometime between Wednesday and Thursday associated with a midlevel shortwave crossing the Great Lakes. There is a fair amount of timing uncertainty with this feature, but this likely will be the only rain chance for the work week. There remains high confidence in at least slightly below normal temperatures through the end of the work week, owing in large part to 850-hPa temperatures running 2-4C below normal. Even the 75th percentile of the NBM for highs is at least a couple of degrees below seasonal normals, with the 25th percentile closer to 6-8 degrees below average. Gosselin && .AVIATION... (For the 06z TAFs through 06z Sunday Night) Issued at 1218 AM CDT Sun Jun 21 2026 showers and thunderstorms are expected to affect the terminals during the period. At UIN, showers may begin as early as 08Z and are likely between 14-20Z and may linger through 00Z with thunderstorms most likely between 16-20Z. At COU/JEF, one round of showers and thunderstorms is mostly likely between 13-19Z with another chance between 22-24Z. At the St. Louis area terminals, showers and thunderstorms are most likely between 15-19Z with another chance between 22-02Z. Any of the heavier showers and thunderstorms could produce MVFR possible IFR ceilings and visibilities with the stronger thunderstorms this afternoon capable of producing hail and wind gusts over 35 knots. Winds will turn out of the south this afternoon around 20-25 knots and then veer out of the north tonight with MVFR (possible IFR) ceilings behind the front. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch from 7 AM CDT this morning through late tonight for Franklin MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO. Flood Watch from 7 AM CDT this morning through Monday morning for Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO. Flood Watch through this evening for Audrain MO-Boone MO- Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO- Pike MO-Ralls MO-Shelby MO-Warren MO. IL...Flood Watch from 7 AM CDT this morning through late tonight for Bond IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Montgomery IL. Flood Watch from 7 AM CDT this morning through Monday morning for Clinton IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL-Washington IL. Flood Watch through this evening for Adams IL-Brown IL-Calhoun IL-Pike IL. && $$ WFO LSX