Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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446 FXUS63 KLSX 110817 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 317 AM CDT Sat May 11 2024 .KEY MESSAGES... - After a tranquil weekend, a relatively long period of showers and thunderstorms, widespread at times, is expected late Sunday night through Tuesday. - There is another opportunity for showers and thunderstorms late in the week. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 315 AM CDT Sat May 11 2024 Upper-level northwesterly flow will remain in place over the Mississippi River Valley today, with a trough traversing the Great Lakes. A weak cold front associated with this trough is currently passing the CWA, but will advance to the east this morning. This front will remain dry and post-frontal low-level CAA will be weak and brief, giving way to WAA by afternoon. The transition to WAA and abundant insolation is expected to yield warmer, closer to average temperatures from yesterday and in the 70s F CWA-wide. A surface SLP ridge will arrive to the region this evening, favoring respectable radiational cooling with light/calm winds and clear skies. As a result, low temperatures in the upper 40s to mid-50s F are forecast. As an upper-level closed low begins ejecting eastward from the Intermountain West through the Central Plains on Sunday, a transition from northwesterly to quasi-zonal flow is anticipated. Following departure of the SLP ridge, low-level flow will become southerly, strengthening WAA and supporting a return to above average high temperatures in the upper 70s to low-80s F. Through Sunday night, the strongest large-scale ascent associated with the closed low will remain to the west of the CWA, but largely diurnally driven isolated to scattered showers and thunderstorms are possible (20 to 40 percent chance) in central and northeastern MO by Sunday evening. Pfahler && .LONG TERM... (Sunday Night through Friday) Issued at 315 AM CDT Sat May 11 2024 The upper-level closed low is progged to continue eastward and reach the Mid-Mississippi River Valley late Monday before finally exiting on Tuesday. There are still slight differences in the exact timing and evolution of the trough as well as timing and maturity of an attendant surface cyclone tracking generally through the southern half of the CWA. Slow movement of the entire system will lead to a relatively long duration of showers and thunderstorms in the CWA late Sunday night through Tuesday, but coverage of showers and thunderstorms should generally be greatest with a warm front lifting northward into the CWA on Monday and behind the surface cyclone on Tuesday within large-scale deformation. Ensemble model guidance probabilities of total rainfall over 0.5" are higher than 60 percent across the entire CWA with a broad corridor of probabilities of total rainfall over 1" at 40 to 60 percent now centered on I-70, where rainfall is indicated to be most persistent. Currently, confidence in instability and deep-layer wind shear sufficient for severe thunderstorms is greatest in the Lower-Mississippi River Valley on Monday, decreasing with northward extent toward the CWA. Therefore, confidence is still low in any strong to severe thunderstorms occurring in the CWA. NBM temperature interquartile ranges have slowly closing with each iteration toward cooler and near to below average temperatures on Monday and Tuesday with widespread cloud cover and periods of precip. From Wednesday through the remainder of the week, global model guidance become increasingly divergent in their depictions of the upper-level wave pattern across much of the CONUS. That being said, there is consensus for upper-level shortwave ridging to dominate the Mid-Mississippi River Valley with a period of dry conditions and moderating temperatures around Wednesday, before another upper-level trough passes around Thursday into Friday. This trough will drive the next opportunity for showers and thunderstorms, but there are large differences in the structure, amplitude, and timing of the trough. Temperatures are also expected to remain near to above average through the end of the week. Pfahler && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1120 PM CDT Fri May 10 2024 Dry and VFR flight conditions will persist through the TAF period. A weak cold front passing overnight will veer winds to the northwest, strengthening and gusting to 15 to 20 kt occasionally during the day Saturday. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX