Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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579 FXUS63 KLSX 100854 CCA AFDLSX Area Forecast Discussion...CORRECTED National Weather Service Saint Louis MO 354 AM CDT Fri May 10 2024 .KEY MESSAGES... - The next opportunity for showers and thunderstorms across much of the area is Monday and Monday night. Strong to severe thunderstorms are unlikely. - Generally seasonable temperatures are expected through next week. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 342 AM CDT Fri May 10 2024 Upper-level northwesterly flow will remain in place over the Mid- Mississippi River Valley through Saturday night, navigated by a pair of troughs accompanied by very limited chances of rain. The first trough is already passing overhead this morning, but associated isolated to scattered showers have dissipated as they entered northeastern MO and west-central IL as marginal instability decreased and low-level forcing remains negligible. Therefore, dry conditions will prevail today as shortwave ridging briefly builds in the wake of the trough. With a seasonably cool airmass in place behind last evening`s cold front and prevalence of scattered to broken stratocumulus/cumulus, temperatures are expected to be slightly below average today with high temperatures in the mid-60s to low-70s F. The next trough will arrive tonight, forcing a weak cold front through the CWA. The strongest large-scale ascent and frontal moisture convergence will pass to the north and northeast of the CWA, but enough forcing may available for isolated showers along the front in northeastern MO and west-central/south-central IL. Post- frontal CAA will be short-lived and insolation will be abundant on Saturday, resulting in a warming trend through Saturday with temperatures near average. Pfahler && .LONG TERM... (Saturday Night through Thursday) Issued at 342 AM CDT Fri May 10 2024 Global deterministic and ensemble model guidance are in general agreement with an eastward ejection of an upper-level closed low from the Rocky Mountains to the Mississippi River Valley on Monday that will both interrupt the northwesterly flow and provide a more robust opportunity for showers and a few thunderstorms. There are differences in the exact structure and timing of the trough along with associated surface cyclone track, but at least a period of showers is expected for most of the CWA between Sunday evening and Tuesday with ensemble model 24-hour probabilities of QPF greater than 0.10" over 80 percent CWA-wide. Probabilities of QPF greater than 0.50" are 40 to 50 percent along and south of I-70, closer to the mean surface cyclone track and associated warm sector with marginal instability and the best chance of thunderstorms. The potential for any strong to severe thunderstorms appears low with probabilities of more than 1000 J/kg of SBCAPE less than 10 percent and deep-layer wind shear not progged to be particularly strong. Ahead of the trough, low-level southerly flow is anticipated to warm temperatures to just above average on Sunday, but the impact of clouds and precip on Monday is uncertain with the NBM high temperature interquartile range nearly 10 F. Disparities in global model guidance increase through the rest of next week, especially with the amount of upper-level ridging across the western CONUS, but there is at least a signal for another trough to pass over the Mississippi River Valley Thursday into Friday before differences upstream become a more significant impact. A large portion of ensemble model guidance have showers and thunderstorms in the CWA with this trough at some point in that timeframe. Otherwise, after temperatures cool again to slightly below average again by Tuesday, but increasing spread in the NBM interquartile range thereafter demonstrates a question in whether temperatures warm to average or above average. Pfahler && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1046 PM CDT Thu May 9 2024 MVFR ceilings rotating in from the north tonight have begun to break up and confidence has decreased in how much of our area will be affected and for how long. It is possible that a brief period of MVFR occurs more broadly shortly after sunrise. If clouds don`t fill in more overnight, then there could be some localized areas of fog developing. Conditions improve Friday morning with VFR expected area wide by late morning. Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX