FXUS63 KLSX 170838 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 338 AM CDT Fri Apr 17 2026 .KEY MESSAGES... - Strong/severe thunderstorms are expected from late afternoon into the evening, mainly in central/northeast Missouri and west- central Illinois. Damaging wind gusts are the most likely hazard, followed by brief tornadoes. Occasional large hail and localized flash flooding are secondary threats. - Weather hazards are minimal Saturday through mid week, but a modest potential (30-50%) for patchy frost exists Sunday morning. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 247 AM CDT Fri Apr 17 2026 With yesterday's showers and thunderstorms concluded, attention turns squarely onto the potential for severe thunderstorms late this afternoon through the early overnight hours, which remains on track for a substantial portion of the area. Strong to severe thunderstorms continue to be expected in portions of northeast/central Missouri and west-central Illinois at a minimum, with damaging straight line-winds the primary threat, followed by a more limited potential for large hail and tornadoes. As we approach sunrise, dry but relatively humid conditions have developed throughout the area after a round of scattered thunderstorms across the Ozarks yesterday. As the day progresses though, strengthening southerly low level flow will transport increasingly rich moisture (surface dewpoints in the mid-upper 60s) northward into much of Missouri ahead of an advancing cold front, and underneath a plume of steepening mid-level lapse rates (7-8C). By mid afternoon, the combination of these factors along with afternoon heating is expected to generate a broad corridor of substantial instability, with high confidence in 3000-4000 J/kg of mixed-layer CAPE just ahead of the advancing front by mid- afternoon. The strongest instability is expected to initially be located just to our east across northwest and north-central Missouri, but still- impressive values of 2000-3000 J/kg are also projected to extend farther east into much of northeast/central MO and west-central IL. Meanwhile, guidance continues to project around 30-40kt of 0-6km bulk shear, with long straight-line hodographs and perhaps some slight curvature in the low levels. To put it more succinctly, this environment will have more than enough of the baseline parameters to set the stage for severe thunderstorms across a broad area. Exactly what materializes in our portion of this event will depend on a few more subtle factors. High resolution guidance continues to suggest that rapid overturning and widespread convective initiation will occur along and just ahead of the cold front during the mid- late afternoon period, thanks to both the front and cooling aloft/increasing divergence from an approaching shortwave trough. However, exactly where this occurs will depend largely on the timing, as earlier initiation would likely keep the vast majority of initial storms to our west. Meanwhile, CAMS continue to depict a few open warm-sector storms developing in our area earlier in the day in our area, mainly central/NE MO, but without an obvious forcing mechanism, along with recent failures of such activity to materialize during other recent events, confidence is low that this will occur. However, confidence remains high that strong/severe storms will reach our area at some point in the late afternoon/early evening given the ample instability present, along with a steadily increasing low level jet during this period. As for storm mode, initial storms are likely to feature a mix of modes, with supercells and multicells favored initially. However, given the strong forcing along the front, numerous updrafts/cold pools interacting with each other, and bulk shear vectors largely parallel to the front, we expect that storms will develop upscale into one or more QLCS' before they arrive or shortly afterward. As such, damaging straight line wind continues to be the primary hazard with this event, with perhaps some more significant straight line winds approaching 70 mph (or more). The potential for QLCS tornadoes will also exist, particularly along bowing segments during the early evening that can become more favorably oriented with 0-3km shear vectors. This will be most favored during a window in the early evening when strong instability is still present ahead of a strengthening low level jet and low level shear/SRH, and LCLS can lower slightly, and while the advancing line is still strong. Shear parameters are not particularly strong (0-3km of 30-40kt), but with the right orientation there is enough there to support a couple of brief tornadoes. As the evening progresses into the overnight hours, model guidance continued to suggest that instability will quickly wane, which should limit the overall severe threat with southeastward extent. Exactly when and where this occurs, though, is somewhat uncertain. While we do expect at least some strong/severe storms in our area during this event, the full geographic coverage is less certain this far east than it is for our western and northern neighbors. Still, there is a reasonable potential for severe storms as far southeast as the I-44 corridor until roughly midnight or so, with quickly decreasing confidence later and farther southeast. For the remainder of the event, a weakening convective line with widespread stratiform rain will be more likely, albeit with lingering lightning and a limited potential for rogue strong/severe gusts throughout the night. Finally, while the advancing line of storms should have enough forward motion to limit the potential for training and flooding in most areas, the large parallel component of the wind shear with the evolving line will maintain at least a limited potential for this, particularly along the southwestern flank of the line in central Missouri where forward motion may be a bit slower. LPMM output does paint around 1.5-2.5 inches of rain in this area, and there are a few locations around here and closer to I-44 that have already seen 1-3 inches of rain over the past couple of days. While there isn't a high probability for widespread flash flooding, we can't rule out some localized issues if storms can orient themselves favorably in these areas late in the evening and overnight. As lingering showers and thunderstorms move southeast late tonight through early tomorrow morning, the cold front will filter in behind with breezy northwest winds and quickly dropping temperatures, to the tune of 30 to 45 degrees colder from this afternoon to tomorrow morning. Persistent northwest winds will maintain these chilly temperatures through the day, with highs in the mid 50s to mid 60s Saturday afternoon (about 10 degrees below average). Most areas are also likely to remain dry throughout the day, but a few showers may linger across our far southeastern border into the afternoon. BRC && .LONG TERM... (Saturday Night through Thursday) Issued at 247 AM CDT Fri Apr 17 2026 After today's thunderstorms and tonight's cold front, a relatively quiet period of weather is expected through mid-week as northwest flow aloft slowly transitions into a building ridge. This will lead to a gradual warming trend through mid-week and mostly dry weather, with perhaps a return to more active weather late in the period. Before we get to our warming trend, though, we will need to monitor the potential for patchy frost Sunday morning, as surface high pressure finally allows winds to weaken. Current temperature forecasts suggest morning lows may dip into the mid 30s in some areas, and thus may not be fully capturing protected valleys where cool air drainage could lead to some low 30s in a few spots. This may also be true Monday morning as well, but more likely in our eastern areas. Otherwise, the main story through Wednesday will be steadily increasing temperatures as southerly and southwesterly low level flow resumes, and an upper ridge reasserts itself. Ensemble guidance suggests that temperatures will likely climb back to near seasonal averages by Monday afternoon and slightly above Tuesday and Wednesday, with relatively narrow ensemble spreads. Meanwhile, precipitation chances remain low throughout this period, with only a few hints of isolated showers by mid-week. Until southerly flow can become more firmly established, moisture return will be quite limited, thus keeping precipitation chances relatively low through this period. However, this may change over the latter half of the week, as southerly flow continues to bring increasing moisture northward, and some variation of an upper trough approaches from the west. While the upper flow pattern becomes more amplified in the extended as this trough approaches, the timing and strength of this trough remain highly variable among ensemble members and clusters. Still, precipitation probabilities do increase substantially Thursday onward, signaling perhaps a return to a more active period. BRC && .AVIATION... (For the 12z TAFs through 12z Saturday Morning) Issued at 331 AM CDT Fri Apr 17 2026 Thunderstorms, including some severe, are the primary threat during the 12Z TAF period. VFR conditions are expected this morning through early afternoon, with breezy south winds. Widespread thunderstorms will begin to impact UIN/JEF/COU late in the afternoon through the evening. A few earlier storms are also possible, but with much lower confidence. With the main round of storms, very strong wind gusts will be possible, along with a brief tornadoes and frequent lightning. Large hail is also a possibility but this is a secondary threat. Visibility and ceiling category reductions are also likely at times, with some low clouds potentially trailing storms for a few hours. Storms are likely to weaken as they approach STL/COU/SUS/CPS later in the evening and overnight, but gusty winds and visibility/ceiling reductions are also possible. A cold front will bring breezy northwest winds overnight and showers will diminish, eventually leading to VFR conditions after sunrise. && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX