Expires:No;;337658 FXUS63 KLSX 170817 AFDLSX Area Forecast Discussion National Weather Service St Louis MO 317 AM CDT Mon Aug 17 2026 .KEY MESSAGES... - Temperatures more typical of late summer are forecast this week. - Multiple rounds of showers and thunderstorms are possible Tuesday night through Thursday. && .SHORT TERM /THROUGH TUESDAY/... Issued at 317 AM CDT Mon Aug 17 2026 The latest surface analysis shows that the cold front has pushed well off to the southeast, with winds veering to the north. Scattered showers and thunderstorms lag behind the surface front, and seem tied to the 850-hPa frontal zone. This boundary should also push off to the south and east over the next 1-2 hours, taking with it any lingering chance of rain. The remainder of the day will be dry with partial sunshine. Strong low-level cold air advection will continue through the day, helping keep this afternoon's highs to be mainly in the low to mid 80s. These readings would be a few degrees below normal for the date. In addition, more typical dewpoints (humidity) levels will advect southeastward behind last night's cold frontal passage. After a largely hot and humid past 7+ days, you will certainly notice the air mass change if you go outdoors today. Tonight should be the coolest night in at least 8-9 days (and possibly the entire month of August at some locations!), with a weak ridge of surface high pressure across the region. The combination of light/variable winds near this anticyclone and at least a partially clear sky should lead to favorable conditions for radiational cooling. Lows are forecast to bottom out in the low to mid 60s across the bi-state region, with the exception of the urban corridor of St. Louis where upper 60s are expected. A few spots may even flirt with the upper 50s within favored valley terrain across portions of east central and southeast Missouri. Increasing mid/high level clouds are forecast on Tuesday beneath northwest flow aloft. There may even be a few isolated rain showers late in the afternoon across parts of northeast Missouri/west central Illinois, but largely dry weather is expected. Temperatures across central/southeast Missouri should rebound back into the upper 80s to low 90s as surface winds turn out of the southwest to the west of the surface high moving into the Ohio Valley. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Issued at 317 AM CDT Mon Aug 17 2026 (Tuesday Night - Thursday) The coverage of showers and thunderstorms should increase across the area quickly Tuesday night as the nocturnal low-level jet strengthens and a fairly potent midlevel shortwave trough approaches from the northwest. Storms look pretty progressive along/north of a weak cold frontal passage, which should limit rainfall totals. There is a conditional threat of a few strong to severe thunderstorms, but there is low confidence that the area will be unstable enough to sustain severe convection. Moisture return ahead of this boundary is the big question mark, and therefore, instability. Recent trends have been more toward limited moisture return, with SBCAPE probabilities of at least 1000 J/kg late Tuesday night from the GEFS/GEPS dropping into the ~10% range. The machine-learning CSU guidance also has trended downward with its severe probabilities across the region Tuesday night. However, individual deterministic models (e.g., NAM/ECMWF) show at least the potential for higher instability, so this will be a period we will need to watch over the next 12-24+ hours. What is not a question mark is the anomalously strong deep-layer shear, which should be in the 35-45+ knot range. Therefore, if there is sufficient instability, thunderstorms would likely organize into a line capable of producing damaging wind gusts. Another (slower) moving midlevel shortwave trough is forecast to cross the Mississippi Valley between Wednesday night and Thursday. This feature should also help yield another round of showers and thunderstorms, with the best chances (50-70%) along/south of I-70 near the 850-hPa frontal zone. This frontal zone is nearly quasi- stationary, only oozing southward with time. Given the potential for a long(er) duration of forcing for ascent and the potential for showers and thunderstorms, there may be a low threat for heavy rainfall and flash flooding across the eastern Ozarks during this time period. (Thursday Night - Sunday) A surface ridge is likely to move into the mid-Mississippi Valley to end the work week heading into the weekend. Near to below normal temperatures and mostly dry weather is forecast Thursday night through Saturday. Ensemble guidance shows a broad mid/upper level trough digging into the Great Lakes/northeast by Sunday, though there are differences with the strength of the trough. Two of the four clusters of the 500-hPa pattern on the LREF are pretty sharp with this feature, which would likely bring a shot of much cooler temperatures into the region. These two clusters comprise about half of the total LREF members. Meanwhile, the other half shows a much weaker trough. What is interesting is that a pretty large majority of both the EPS/GEPS favor the stronger trough and cooler air mass making into our region while the GEFS has an even larger percentage in the weaker trough/cold front scenario. Most typically, when 1 of the 3 ensemble suites does not match the other two, it is not correct. I would therefore at least slightly favor the cooler temperature scenario to end the weekend heading into early the following week. && .AVIATION /00Z TAFS THROUGH 06Z TUESDAY/... Issued at 1118 PM CDT Sun Aug 16 2026 Scattered showers and thunderstorms will continue moving through eastern Missouri and southern Illinois tonight. While the strongest storms will likely be through our area into eastern Illinois by 06-07Z, a few lingering weaker showers or storms will likely continue to move across parts of south central Illinois through 09-12Z. Thunderstorms will be capable of reducing the visibility to 2SM or less in locally heavy downpours. Wind gusts to 35-40kts will also be possible with the strongest storms. There's still a signal for MVFR stratus across the eastern Ozarks into southern Illinois before sunrise. Stratus may stretch as far north as the St. Louis Metro area, and up to I-70 or I-55 in Illinois. With that in mind, the signals aren't particularly strong, so confidence in MVFR ceilings isn't high at this time. Any low clouds that do develop should lift and scatter by 17-18Z with VFR flight conditions prevailing thereafter. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ SHORT TERM...Gosselin LONG TERM...Gosselin AVIATION...Carney