FXUS63 KLSX 161130 AFDLSX Area Forecast Discussion National Weather Service St Louis MO 630 AM CDT Fri Oct 16 2026 .KEY MESSAGES... - A Frost Advisory is in effect Friday night across portions of northeastern Missouri and west-central Illinois where confidence is highest in widespread temperatures in the mid 30s. && .SHORT TERM /THROUGH SATURDAY NIGHT/... Issued at 215 AM CDT Fri Oct 16 2026 A highly amplified trough can be seen in water vapor imagery currently digging equatorward into the Midwest. The cold front associated with this trough and the one responsible for yesterday's rain has cleared the CWA to the southeast, with deepening northwesterly flow ushering in a fall-like air mass. This air mass will bring notably cooler temperatures compared to what we've been experiencing as of late, with a muted diurnal curve today leading to temperatures topping out in the low to mid 50s; some 20-30 degrees cooler than values on Thursday. Dry conditions are expected for most locations today, with the axis of the upper-level trough only producing a 20- 30% chance of light rain for areas east of the Mississippi River as it pivots overhead during the afternoon. Tonight, high pressure at the surface will settle into the Central and Southern Plains; nosing into the Middle Mississippi Valley. As a result, the pressure gradient will slacken and winds become light to calm as skies gradually clear from north to south. This will make for ideal frost conditions across portions of northeastern Missouri and west-central Illinois, where radiational cooling will allow for widespread lows in the mid-30s - the temperature threshold generally needed for frost development. Where confidence is highest in these conditions occurring, a Frost Advisory is now in effect late Friday night through early Saturday morning. Thanks to the upper-level trough moving further eastward of the region Saturday, low to mid-level temperatures will bounce back by a few degrees C compared to their Friday minimum. Between this and clear skies allowing for abundant insolation, afternoon temperatures will warm to around 60 degrees, which is still about 10 degrees below seasonal normals. There is a low threat (40%) for additional frost across portions of southeastern Missouri and southwestern Illinois Saturday night, and will be dependent on where the aforementioned surface high is relative to the CWA then. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Issued at 215 AM CDT Fri Oct 16 2026 Thanks to consensus among ensemble clusters and deterministic guidance, confidence is high in a benign and uneventful pattern through the upcoming workweek. Continuous troughing over the West Coast and ridging to our east-southeast will keep upper-level flow weakly southwesterly through this portion of the period. This supports warm air advection, though subtly given the weakness of the flow. As a result, daily temperatures will only gradually warm through the workweek. Values closer to seasonal normals are at least expected through the first half of the workweek, but how much warmer we climb toward the end of the week is uncertain. The NBM pushes afternoon highs to around 80 degrees Thursday and Friday, though that is roughly 5 degrees warmer than the 75th percentile of ensemble guidance. This is due to the bias- corrected guidance composing the NBM, and this guidance is several degrees warmer than their non-biased counterparts. Given the bias factor applied here may be too warm given our seasonal cooling, confidence is low in the NBM's late-week temperature forecast. Given the weakness of the flow aloft and lack of any notable disturbances in deterministic guidance, confidence is high in dry conditions prevailing through this portion of the period. This is supported by nearly all ensemble members showing this stretch being rain-free. && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 622 AM CDT Fri Oct 16 2026 In the wake of yesterday evening's front, a stratus deck is departing the region to the southeast. However, additional stratus is rotating into the region from the northwest. Early in the period, this stratus is expected to start off roughly 3.5-4kft before gradually lowering through the mid to late morning to MVFR flight restrictions at all local terminals. Dry air moving into the region this evening into the early overnight hours will push this stratus away from the local terminals and return VFR flight conditions to the region. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Frost Advisory from 1 AM to 9 AM CDT Saturday for MOZ018-019- 026-027-034>036-042-052. IL...Frost Advisory from 1 AM to 9 AM CDT Saturday for ILZ058>060- 095>099. && $$ SHORT TERM...Elmore LONG TERM...Elmore AVIATION...Elmore