267 FXUS63 KLSX 242330 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 630 PM CDT Thu Jul 24 2025 .KEY MESSAGES... - Locally heavy rainfall is possible this tonight through Friday morning across northeast Missouri and west-central Illinois along with some gusty winds. - While some areas (northeast Missouri and west-central Illinois) will see a brief break in the dangerous heat on Friday, it will continue for most of the rest of the area through at least Tuesday. && .SHORT TERM... (Through Late Friday Afternoon) Issued at 236 PM CDT Thu Jul 24 2025 A weak cold front is currently located near a KSLN>>KDBQ line as of 1900 UTC. Convective initiation has been focused in eastern Kansas near or just behind this boundary where the strongest low- level convergence is located. This activity will spread east/northeast into early this evening, reaching parts of northeast Missouri near or just after 0Z Friday. Cannot rule out a few gusty winds through late this evening in these areas, but the environment is not supportive for organized severe weather. The deep-layer shear is only about 20 knots, which signifies that thunderstorms should tend to be cold-pool dominant. There will also be a bit less instability in these locations due to thicker mid/high level cloud cover which has been in place since this morning. In addition, there is not much in the way of dry air aloft (for entrainment) nor steep low-level lapse rates. Downdraft CAPE from the RAP early this evening is only expected to be in the 400-700 J/kg range, which is quite modest for this time of year. Locally heavy rainfall looks to be the higher potential threat, moreso in northwestern portions of the flash flood watch. While individual storms/clusters should move relatively quickly, there is the potential for training along/near the frontal boundary in an environment characterized by deep warm cloud depths and anomalously high precipitable water values (>2.00"; >99th percentile). The main limiting factor is that the stronger low-level moisture convergence is well to our west tonight (closer to the Missouri-Kansas border). This means that the genesis region for overnight convection will stay well to our west, with storms likely weakening as they head east/northeast into northeast Missouri. Due to the much weaker low- level moisture convergence in our area, the current thinking is that flash flooding, if any occurs, would be quite isolated. Across the watch area, pockets of 1-2" of rain is expected though cannot rule out a spot or two with up to 3-4" if storms train for a long enough duration. In terms of the flash flood watch, only subtle changes were made. The main change was to delay the start to 0Z this evening based on the latest timing of thunderstorms moving upstream, as well as ending it at 18Z Friday. No areal changes were made, though the chances of any flash flooding do look to wane with southeastern extent. Dangerous heat is the other concern for Friday. While there remains some uncertainty on exactly how far southeast thunderstorms reach by late morning, there is not much in the way of any veering of the low- level jet. In addition, mid/upper level heights are neutral and may even increase slightly Friday afternoon. This means that southern sections of the area will feel the influence of the mid/upper level ridge. The current thinking is that widespread morning rain/clouds in northeast Missouri and west-central Illinois will provide a more definitive (albeit short) break to the ongoing heat so will let the heat advisory expire this evening up that way. Further southeast, elected to extend both the advisory and the warning not only through Friday, but all the way through Tuesday. Peak heat index values of 100-105+F are forecast Friday afternoon, with the hottest locations in/near metropolitan St. Louis and southwest Illinois. Confidence in 100+ heat index values increases the further southeast you travel. There probably will be some redevelopment of showers and thunderstorms along the remnant outflow boundary Friday afternoon, but exactly where is uncertain. The best guess is somewhere near the I-44/I-55 corridors in Missouri/Illinois respectively. Gosselin && .LONG TERM... (Friday Night through Next Thursday) Issued at 236 PM CDT Thu Jul 24 2025 (Friday Night - Tuesday Night) Heat, heat, heat is the main story this weekend through at least Tuesday of next week. As alluded to in the paragraph above, heat headlines were extended through Tuesday over most of the region. For portions of northeast Missouri and west-central Illinois who will get a definitive break on Friday, the heat may return as early as Saturday. Once it returns, it will also last through at least Tuesday. There likely will be another round of showers and thunderstorms Friday night into Saturday morning, but this round is expected to be further north and east compared to tonight`s activity. The mid/upper level anticyclone really builds in nearly directly overhead Monday night into Tuesday. The strength of this anticyclone is very impressive, with the center of the high approaching 600 dm at 500 hPa! To put that in perspective, that would be record-breaking for the lower/mid Mississippi Valley. This type of anomalous midlevel high is a bit less rare in summer in the central Plains, but is something not really ever seen further east. Given that the 850-hPa thermal ridge is expected to be oriented west- east across the area on Tuesday, the expectation is for Tuesday to be the hottest day of this entire stretch with widespread upper 90s forecast. Some readings topping out at or slightly above the century mark are likely given the pattern, particularly in/around the urban heat island of metropolitan St. Louis. In terms of dewpoints, they should gradually decrease day by day as you lose any semblance of near-surface moisture from recent rainfall and mixing increases into early next week. However, slightly less humid air will be offset by increasing temperatures. The net result is for peak heat index values of 105-110F for much of the area through Tuesday. (Wednesday - Next Thursday) There is increasing confidence in a major pattern change across the CONUS midweek next week as a very anomalous mid/upper level trough digs into southeast Canada. The air mass associated with this trough is also very noteworthy, with both the GEFS/EPS showing 850-hPa temperature anomalies of -2 to -3 sigma. While the brunt of this air mass will plunge into the Great Lakes/northeast US, guidance has shifted steadily cooler behind the surface cold front in our neck of the woods. The pattern is also very amplified for mid summer standards, with strong mid/upper confluence between the western ridge and aforementioned southeastern Canada trough. This strong confluence portends to a seasonably strong surface high (~1024 hPa - >90th percentile of climatology on the NAEFS) sliding southward into the Mississippi Valley. This should allow for much cooler and less humid air to be ushered in behind the cold front; it is just a matter of exactly when and how cool. Speaking of which, there remains some spread on the timing of the frontal passage, with Wednesday being the day of most uncertainty for temperatures. Spread between the 25th/75th percentile from the NBM for Wednesday`s highs are in the 5-8+F range. A faster front would put an end to the heat Tuesday evening, while a slower passage would likely mean that Wednesday would the last of this stretch of dangerous heat. In terms of precipitation chances, showers and thunderstorms are possible along and possibly behind the frontal passage. Chances are not particularly high currently, peaking out in the 20-30%. However, as the timing of the frontal boundary decreases, look for PoPs to increase sometime in that window. Gosselin && .AVIATION... (For the 00z TAFs through 00z Friday Evening) Issued at 630 PM CDT Thu Jul 24 2025 A complex of thunderstorms has developed across western and northern MO, which is expected to track eastward this evening, mainly north of the I-70 corridor. Therefore, confidence is highest in impacts at KUIN, which include IFR flight conditions and gusty winds, with lower confidence at KCOU and KJEF where thunderstorms will likely come close but may pass just to the north. The probability of thunderstorms is even lower at St. Louis metro terminals this evening. Overnight, showers and thunderstorms will continue at times across northeastern MO and west-central IL including KUIN. It is uncertain how long showers and thunderstorms will linger into Friday morning as well as the timing, location, and location of redeveloping thunderstorms later Friday. Therefore, confidence on if and when impacts at any given terminal occur, no explicit mention of showers and thunderstorms has been included with this TAF package. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 7 PM CDT Tuesday for Boone MO-Callaway MO- Cole MO-Crawford MO-Gasconade MO-Iron MO-Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Warren MO-Washington MO. Heat Advisory until 7 PM CDT this evening for Audrain MO-Marion MO-Monroe MO-Pike MO-Ralls MO. Flood Watch through Friday morning for Audrain MO-Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO. Extreme Heat Warning until 7 PM CDT Tuesday for Franklin MO- Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO. IL...Heat Advisory until 7 PM CDT Tuesday for Bond IL-Calhoun IL- Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL- Madison IL-Marion IL-Montgomery IL-Randolph IL-Washington IL. Heat Advisory until 7 PM CDT this evening for Adams IL-Brown IL- Pike IL. Flood Watch through Friday morning for Adams IL-Brown IL-Pike IL. Extreme Heat Warning until 7 PM CDT Tuesday for Monroe IL-Saint Clair IL. && $$ WFO LSX