393 FXUS63 KLSX 262331 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 631 PM CDT Sat Jul 26 2025 .KEY MESSAGES... - Dangerous heat and humidity is expected to expand northward on Sunday, peaking across the area on Monday and Tuesday. Widespread relief from the heat is expected between Wednesday and Thursday. - Mainly scattered thunderstorms are forecast tonight through Sunday. A few of the strongest storms may have gusty winds with locally heavy rainfall also possible. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 235 PM CDT Sat Jul 26 2025 Widespread showers and thunderstorms in northeast Missouri weakened late this morning, but an outflow boundary further to the east has been a focus for showers and thunderstorms so far this afternoon. This southwest-northeast axis in the vicinity of the outflow boundary is where the best chances (40-60%) are through mid/late evening, roughly from KJEF to southeast of KPPQ. Further southeast, there has been more purely diurnally-driven convection with a focus across portions of southeast Missouri and southwest Illinois. The latest SPC mesoanalysis shows a very unstable environment (>3000 J/kg MLCAPE), but deep- layer shear has decreased to around 25-30 knots behind the MCV moving into the southern Great Lakes. These values will continue to drop over the next few hours. RAP soundings show some dry air at midlevels and moderately high low- level lapse rates (~ 7 C/km), indicating a low threat for isolated microbursts. This threat should gradually wane after dusk as diurnal instability abates and low-level lapse rates weaken. Overnight tonight, low-level moisture convergence increases on the nose of a ~20 kt low-level jet. At least scattered showers and thunderstorms are expected along/north of the composite outflow boundary from this afternoon/evening`s convection. The low-level moisture convergence is not particularly strong. Therefore, would not expect too much in the way of organization of the convection late tonight. That being said, there may be some training of storms in an environment characterized by deep warm cloud depths and precipitable water values around 2.00" (>95th percentile of climatology). The 12Z HREF hints at this threat as well, showing some isolated pockets of 2-4" of rain. Given that this rain should focus southeast of where it fell last night/this morning and the fact that the low-level forcing for ascent is not particularly strong, did not elect to go with a flash flood watch and think that flash flooding, if any were to occur, would be very isolated in nature. Exactly where in the CWA this activity will focus is uncertain, mainly due to the fact we will not know where the composite outflow boundary will be later this evening. The best guess is that it may get as far southeast as the I-44/I-70 corridors in Missouri/Illinois respectively. That would mean that the best chances (40-60%) of showers and thunderstorms tonight would tend to want to focus from central Missouri east/northeast into south- central Illinois. While there is some variability in CAMs with how things evolve on Sunday, the nocturnal low-level jet does not fade very quickly. This would suggest that isolated to scattered showers and thunderstorms may keep going through the morning hours, before there is at least a slight uptick in coverage by early afternoon as diurnal instability increases. The best chances by Sunday afternoon (30-50%) should shift southeast, more across east central and southeast Missouri as well as southwest Illinois. The threat for any microbursts should be lower than today, mainly due to poorer low-level lapse rates. In terms of the heat, an expansion to the north still appears likely. Even with highs around 90 degrees in northeast Missouri and west-central Illinois, dewpoints in the upper 70s are expected. Therefore, expanded the heat advisory for the remainder of the CWA as peak heat index values of around 105 degrees are expected Sunday afternoon. Further southeast, there actually is a bit more uncertainty with respect to temperatures and associated heat index values given the better chances for showers and thunderstorms Sunday morning/afternoon. However, given that the convection is not expected to be too widespread nor organized, temperatures should be able to get at least to around 90 degrees. With the very high humidity/dewpoints, that should be enough to see maximum heat index values of 100-105F+. That being said, there may be some pockets that stay cooler tomorrow afternoon due to thunderstorms, but confidence in widespread cooler temperatures is definitely not high enough to make any changes with the ongoing heat advisory/extreme heat warning. Gosselin && .LONG TERM... (Sunday Night through Next Saturday) Issued at 235 PM CDT Sat Jul 26 2025 (Sunday Night - Tuesday Night) There may be some lingering showers and thunderstorms Sunday evening across parts of southeast Missouri and southwest Illinois, but this activity should diminish with loss of daytime heating. It appears we may finally get a dry overnight period as the mid-Mississippi Valley begins to feel more direct influences from the amplifying mid/upper level anticyclone just to our south. This should help steer Sunday night/Monday morning`s round of storms further to the north and east. Dry weather is then forecast through Tuesday night. Early next week is all about the heat. The very anomalous mid/upper level anticyclone continues to build northward through Monday night/early Tuesday. A near 600 dm 500-hPa anticyclone centered in the mid south would be record-breaking for that part of the country to put this pattern in perspective. High temperatures will increase early next week, likely peaking on Tuesday afternoon in most areas. Highs in the mid to upper 90s are forecast each afternoon. I think the chances of anyone actually seeing 100+ degree highs has decreased as the 850-hPa thermal ridge is a bit more modest (+21 to +23C) compared to what it looked like 24-48 hours ago. These values are certainly anomalously warm, but more on the order of just above the 90th percentile of climatology and not extremely warm for this time of year. In addition, there is a bit more confidence in dewpoints staying well into the 70s for most locations. This rich low-level moisture also helps with diurnal cumulus development in the afternoon, helping to limit the temperature rise after the cumulus forms. Regardless of the exact air temperatures, very dangerous heat is expected both afternoons. Peak heat index values of 105-110F+ are expected, with the highest values within the urban heat island of metropolitan St. Louis and adjacent portions of southwest Illinois where dewpoints are most likely to be closer to the 80 degree mark. (Wednesday - Next Saturday) A significant pattern change will take place as we transition from mid to late week as anomalous mid/upper level troughing carves into southeast Canada. All signs continue to point toward a cold frontal passage sometime Wednesday/Wednesday night, with significantly cooler temperatures infiltrating the mid-Mississippi Valley. Ensemble guidance has also trended cooler with the strength of this air mass, with mean 850-hPa temperatures anomalies Friday/Friday night in the -2 to -5C range. What remains more impressive is the strength of the surface anticyclone moving into the Upper Midwest. The latest NAEFS has a 1026+ hPa high in Wisconsin Friday afternoon, which would be near-record breaking for the beginning of August. High temperatures in the mid 70s to low 80s are forecast Friday/Saturday, with lows ranging from the upper 50s to mid 60s. Spread on the NBM remains on the low side for 5-7 days out, with only about a 3-5 degree difference for most locations between the 25th/75th percentiles. Even the 75th percentile still shows high temperatures some 5+ degrees below normal, so there remains high confidence in well-below normal temperatures entering next weekend. What does remain uncertain is exactly when the cold front will move through the region. Latest trends have slowed down the front slightly, but there is a signal for a possible MCS moving southeast into the area Wednesday morning. This of course would lead to much cooler temperatures due to the rain and associated cloud cover. However, if the front is delayed enough and it stays dry/sunny, one last day of heat/humidity would be in store at least for parts of the area. Given the uncertainty and the fact that Wednesday is still 4 days out, no change to the end of the heat headlines. The best chances of showers and thunderstorms (40-60%) are accompanied by the cold front in the Wednesday/Wednesday night time frame. Deterministic guidance also shows some northwest flow disturbances behind the front, which could yield some showers and maybe even some elevated convection Thursday/Thursday night. Gosselin && .AVIATION... (For the 00z TAFs through 00z Sunday Evening) Issued at 630 PM CDT Sat Jul 26 2025 The main and likely only concern this TAF period will remain focused on showers and thunderstorms at times across the area through Sunday evening. There is a rather large amount of uncertainty on the location and coverage of showers and thunderstorms through the rest of the evening into overnight with near-term TEMPO and later PROB30 groups included at all terminals for the periods of highest chances. Additional development or redevelopment of showers and thunderstorms are expected to mainly be focused across east-central/southeastern MO and southwestern IL Sunday afternoon and evening. Therefore, St. Louis metro terminals have the highest chance (relative to other terminals) during that time. Thunderstorms will be capable of MVFR to briefly IFR flight conditions and gusty winds. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 7 PM CDT Tuesday for Audrain MO-Boone MO- Callaway MO-Cole MO-Crawford MO-Gasconade MO-Iron MO- Lincoln MO-Madison MO-Moniteau MO-Monroe MO-Montgomery MO- Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Francois MO- Warren MO-Washington MO. Extreme Heat Warning until 7 PM CDT Tuesday for Franklin MO- Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO. Heat Advisory from 1 PM Sunday to 7 PM CDT Tuesday for Knox MO- Lewis MO-Marion MO-Shelby MO. IL...Heat Advisory until 7 PM CDT Tuesday for Calhoun IL-Fayette IL- Greene IL-Jersey IL-Macoupin IL-Montgomery IL-Pike IL. Extreme Heat Warning until 7 PM CDT Tuesday for Bond IL-Clinton IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL-Washington IL. Heat Advisory from 1 PM Sunday to 7 PM CDT Tuesday for Adams IL- Brown IL. && $$ WFO LSX