019 FXUS63 KLSX 240537 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1237 AM CDT Thu Jul 24 2025 .KEY MESSAGES... - Another hot and humid day is forecast for most of the area on Thursday with peak heat index values of 105-110F. The exception could be parts of northeast Missouri which may be cooler due to clouds and thunderstorm chances. - There remains quite a bit of uncertainty with respect to Friday's high temperatures due to chances of showers and thunderstorms. Confidence for extreme heat however increases again late this weekend into early next week. && .SHORT TERM... (Through Late Thursday Afternoon) Issued at 158 PM CDT Wed Jul 23 2025 Isolated showers and a couple rumbles of thunder have developed near the Mississippi River in southeast Missouri and southwest Illinois along a subtle surface trough over the past two hours. This activity has gradually pushed eastward and should continue over the next few hours before dissipating close to 0Z. Otherwise, dry weather is forecast overnight with very mild temperatures. Lows in the 70s are expected, with near 80 degree readings in the urban heat island of metropolitan St. Louis. Widespread showers and thunderstorms are expected overnight tonight across the mid-Missouri Valley and Upper Midwest. This activity should slide gradually southeast into Thursday morning as the low- level jet veers more to the southwest and the mid/upper level ridge shifts more into the mid south/Ohio Valley. There remains some uncertainty as to how much of this activity makes it into parts of northeast Missouri and west-central Illinois, but I think that least least isolated showers/weak thunderstorms will sneak in there between within a couple of hours of 12Z. The loss of stronger low-level moisture convergence should lead to a lull in activity late morning toward midday, though look for scattered showers and thunderstorms to redevelop across northeast Missouri and west-central Illinois along the cold front or outflow boundary from the earlier morning convection. High temperatures in the upper 80s to low 90s are forecast in these locations, but actual highs could be a couple of degrees cooler or warmer than forecast depending on how far southeast overnight activity is able to reach. Because of this uncertainty, did not cancel any counties in the advisory in northeast Missouri or west-central Illinois, but it is definitely possible that peak heat index values stay below 100 degrees (i.e., below duration advisory criteria). Further southeast, there is higher confidence in high temperatures reaching back into the mid to upper 90s. Highs tomorrow should be very similar to today across much of the area (outside of northeast Missouri and west-central Illinois detailed in the paragraph above), though they may be a touch (1-2F) warmer in spots due to slightly increased mixing (and more veered surface winds). However, this increased diurnal mixing also portends to lower dewpoints (upper 60s to mid 70s instead of 70s to near 80 degrees). That means that overall similar peak heat index values (105-110F) are expected across much of the area. Gosselin && .LONG TERM... (Thursday Night through Next Wednesday) Issued at 158 PM CDT Wed Jul 23 2025 (Thursday Night - Friday) Deterministic guidance, including extended CAMs, all show quite a bit of shower and thunderstorm activity late Thursday night. Given a west-east frontal boundary draped across northeast Missouri/west central Illinois, increasing moisture convergence on the nose of a 25-30+ kt low-level jet, and a midlevel shortwave moving across the region, this makes a lot of sense. CAMs are not too organized with this activity right now, but the setup looks conducive for at least a low chance for flash flooding. Thunderstorms may tend to train along/near the surface boundary with very weak steering flow. In addition, thunderstorms may be pretty efficient given precipitable water values above 2.00" (~99th percentile of climatology). Some gusty winds may also be possible as cold pools may be able to remain balanced given 20-25 knots of deep-layer shear. Speaking of cold pools, there may be a southward component to them, which could serve as a focus for additional development Friday afternoon. Given the potential for well-organized convection at least in northern sections of the area, will not extend any heat headlines into Friday. The best chance for dangerous heat and humidity to continue is in southern sections of the area as things look right now, but things could still shift over the next 24-36 hours. (Friday Night - Next Wednesday) Friday likely is the only day where the heat may be abated by showers and thunderstorms (or at least convective debris). Guidance has made a couple of shifts over the past 24 hours: 1) building the mid/upper level ridge quicker back into the area Friday night/Saturday and 2) centering the 597+ dm 500-hPa anticyclone further to the east late this weekend into early next week. This means dangerous heat is likely areawide Saturday through at least Tuesday. High temperatures in the mid to upper 90s with peak heat index values of 105-110+F are forecast each afternoon. There remains some hope of a real pattern change heading into midweek next week (and a real cold frontal passage). Ensemble guidance is in pretty good agreement in a retrogression of the longwave pattern across the CONUS, with the center of the mid/upper level ridge moving westward into the Plains/intermountain west. Anomalous troughing is also expected to dig into southeast Canada/Hudson Bay. Unsurprisingly, there is some spread on both of these features, but there may be some more substantial/long lived heat relief in the offing later next week if this pattern comes to fruition. Gosselin && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1236 AM CDT Thu Jul 24 2025 Largely dry and VFR flight conditions are expected through Thursday afternoon across the region. Dissipating overnight showers and thunderstorms will approach KUIN Thursday morning, but it is uncertain if they will reach the terminal. Another round of showers and thunderstorms is expected Thursday evening into overnight, mainly across northeastern MO and west-central IL. Confidence is higher in this round impacting KUIN, sufficient for a PROB30 group. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 7 PM CDT this evening for Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-Gasconade MO-Iron MO- Knox MO-Lewis MO-Lincoln MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO- Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Shelby MO- Warren MO-Washington MO. Extreme Heat Warning until 7 PM CDT this evening for Franklin MO- Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO. IL...Heat Advisory until 7 PM CDT this evening for Adams IL-Bond IL- Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Montgomery IL-Pike IL- Randolph IL-Washington IL. Extreme Heat Warning until 7 PM CDT this evening for Monroe IL- Saint Clair IL. && $$ WFO LSX