Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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239 FXUS63 KLSX 260526 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1226 AM CDT Wed Jun 26 2024 .UPDATE... Issued at 1224 AM CDT Wed Jun 26 2024 Earlier storms have greatly reduced instability for most of the area, although a Severe Thunderstorm Watch remains just to our west. The best instability, and best chance of getting a strong to severe storm is along the western periphery of the forecast area in central Missouri over the next 2-3 hours. The rest of us are likely to just see rain with embedded lightning. Locally heavy rain is possible. The trends have been to clear this activity out by morning as well, with a much nicer day in store for Wednesday. Kimble && .KEY MESSAGES... - Scattered to numerous thunderstorms are expected overnight. A few of these storms may become strong to severe, with the main threat being damaging wind gusts. && .SHORT TERM... (Through Wednesday) Issued at 332 PM CDT Tue Jun 25 2024 A cold front is currently stretched west to east across Iowa and far northern Illinois. Convection just ahead of the front from this morning has maintained itself into the afternoon and formed several outflow boundaries that are moving southward over the CWA. South of the front and outflows is a hot airmass with widespread temperatures in the mid to upper 90s, with locations within the St. Louis metro currently around 100 degrees. With dew points in the upper 60s to low 70s, this heat has yielded 3,000+ J/kg of SBCAPE per recent SPC mesoanalysis. While this amount of instability ahead of outflow boundaries and among ongoing convection is concerning, 0-6 km bulk shear remains relatively weak (20 kts or less), leading to updrafts being mostly short-lived baring an organized cold pool. Given the inverted "V" seen in local soundings among models and ACARS out of Lambert Airport (KSTL), damaging wind gusts continue to be the primary concern with any convection this afternoon. While severe hail (1" in diameter) is possible, the freezing level and warm low- level temperatures will likely make it quite difficult for severe- size hail stones to reach the surface. This threat will be mostly confined to west-central Illinois in the near this afternoon. A complicated convective scenario remains for this evening and tonight across the area. Currently, a weak shortwave/MCV is drifting eastward toward the CWA across western Missouri. There is a low chance that this feature may produce isolated convection across portions of central and eastern Missouri late this afternoon and evening, and if this occurs, the primary threat would be damaging wind gusts. Additional convection is expected along the cold front later tonight as it sags further southward toward the area. Instability will have decreased, but still be on the order of around 1,500 J/kg for MUCAPE. Shear will remain weaker at around 20 kts, leading to multicell convection that CAMs show congealing across Iowa and moving south-southeastward toward the CWA as an MCS. This round of convection has the greatest chance to be strong to severe where it tracks, with the main threat being damaging wind gusts. CAMs have varied with the track and timing of this MCS, though at least have been consistently trending slower with the system. Current thinking is that it will move into the CWA around 3am and exit through Wednesday morning. The front will slowly move through and exit the CWA in the wake of the convection tomorrow morning, with northerly low-level flow advecting cooler air into the CWA. This, paired with lingering cloud cover for portions of the area, will keep temperatures relatively cooler CWA-wide, with afternoon temperatures remaining in the mid to upper-80s. Elmore && .LONG TERM... (Thursday through Next Tuesday) Issued at 332 PM CDT Tue Jun 25 2024 The consensus among ensemble clusters and deterministic guidance is that upper-level ridging will gradually build back into the Middle Mississippi Valley on Thursday. As a result, high pressure at the surface will shift eastward, causing low-level flow to become increasingly southerly-southeasterly. While the more southeasterly component of the winds does not support robust warm- air advection, a majority of global ensemble members show 850 mb temperatures rising by a couple of degrees through the day, climatologically favoring a slight bump in temperatures at the surface compared to Wednesday mainly along and south of I-70. The upper-level ridge builds further into the region on Friday as a trough digs across the Northern Plains. Upper-level flow over the Middle Mississippi Valley will become increasingly southwesterly, and the previously mentioned surface high will continue to shift eastward. Low-level flow will become more southerly, advecting warmer air into the region and pushing 850 mb temperatures to at least 20 degrees C (roughly the 25th percentile of ensemble guidance). The result will be a return of surface temperatures in the low to mid-90s. However, a potential limiting factor to this heat is convection during the day associated with a shortwave traversing the northern periphery of the ridge. If this convection and/or convective debris passes through the CWA, we could see resulting temperatures 5-10 degrees cooler than currently forecast. Guidance consensus is that a cold front associated with the main upper-level trough will move through the region sometime Saturday as the trough traverses the Upper-Midwest. At this lead time, spread is understandably large in the timing of this front. If the front is quicker, temperatures on Saturday may only top out in the low to mid- 80s. If the front is slower, temperatures may reach the low to mid- 90s. Regardless, quasi-zonal to northwesterly flow aloft and northerly surface flow behind the front will keep temperatures cooler for at least Sunday before ridging builds back in to the region early next week, marking the return of a period of heat. Elmore && .AVIATION... (For the 06z TAFs through 06z Wednesday Night) Issued at 1221 AM CDT Wed Jun 26 2024 Main concern for the TAF forecast is timing of thunderstorms at each location. Confidence has grown relative to prior forecasts that the final round of storms will be moving through overnight. The best instability, and most confidence for thunderstorms, is in central Missouri, but most of the area will see rain and some embedded thunder. Winds are variable as downdrafts are coming from many directions with all the convection around, but the general trend will be toward northerly winds by morning. There`s a low chance we could develop some MVFR ceilings behind the main line of convection, but so far this has not materialized and confidence has decreased in it occurring. We should see a trend toward VFR clearing during the day Wednesday. Kimble && .CLIMATE... Issued at 353 AM CDT Mon Jun 24 2024 With hot temperatures forecast, here are some of the daily high temperature records: MON JUN 24 TUE JUN 25 ST LOUIS 102 in 1988 102 in 1954 COLUMBIA 103 in 1988 102 in 1988 QUINCY 99 in 1988 102 in 1931 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX