Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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281 FXUS63 KLSX 121753 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1253 PM CDT Sun May 12 2024 .KEY MESSAGES... - After another mostly dry day today, showers and thunderstorms are expected, numerous at times, from early Monday morning into Tuesday evening. - There is another opportunity for showers and thunderstorms around the Thursday-Friday timeframe. && .SHORT TERM... (Through Late Monday Afternoon) Issued at 311 AM CDT Sun May 12 2024 An upper-level ridge, ahead of an eastward-ejecting closed low over the Front Range, will dominate the Mid-Mississippi River Valley through much of the day today with largely dry conditions. Departure of a surface SLP ridge this morning and a transition to weak low- level southerly flow/WAA with strong insolation will lead to a large diurnal jump in temperatures today, from the upper 40s to mid-50s F this morning to the 80s F this afternoon. Isolated showers and thunderstorms are possible (20 percent chance) in northeastern MO and west-central IL as early as late afternoon as low-level moisture and modest diurnal instability increase, but large-scale forcing remains nebulous until the crest of the ridge shifts east tonight. The upper-level closed low will advance into the Central Plains behind the ridge tonight, with showers and a few thunderstorms gradually increasing across the CWA as southerly flow strengthens and moisture and broad large-scale ascent increase. Showers and thunderstorms are expected to become more numerous and pivot northward through the CWA Monday morning into early afternoon as a lead low-amplitude upper-level shortwave trough passes in tandem with a band of mid-level WAA. Confidence is high that showers and thunderstorms will persist across portions of the CWA Monday afternoon into night as the closed low slowly traverses the Mid-Mississippi River Valley, but coverage and placement are a little less clear as forcing/ascent becomes fairly broad. Some breaks in widespread clouds are expected during the afternoon and evening, supporting a marginal increase in instability, with the latest HREF indicating probabilities of SBCAPE over 750 J/kg greater than 60 percent in a corridor through central, southeastern MO into east-central MO. With 20 to 30 kt of deep-layer wind shear also forecast in those areas, loose organization/ aggregation of thunderstorms into multicell clusters is possible during afternoon and evening. Some deterministic model guidance indicate localized MUCAPE just over 1000 J/kg, which could be sufficient for stronger thunderstorms to produce small hail. However, confidence in an overlap of instability and deep-layer wind shear sufficient for severe thunderstorms is still greater south and southwest of the CWA on Monday. Widespread clouds and periods of precip will keep temperatures much cooler on Monday than today and near to slightly below average. Pfahler && .LONG TERM... (Monday Night through Saturday) Issued at 311 AM CDT Sun May 12 2024 Late Monday night into Tuesday, a weak surface cyclone is progged to traverse the CWA, but there is uncertainty in its exact speed and track along with whether it will be deepening. Regardless, lingering showers and a few thunderstorms are expected on Tuesday mainly north and west of the the cyclone within weak large-scale deformation. From this evening through the ending of showers and thunderstorms Tuesday evening, ensemble model probabilities of total rainfall over 0.5" are higher than 70 percent across the entire CWA, suggesting that most locations will see at least 0.5" of rainfall. Probabilities of over 1" are 50 to 60 percent generally along the I- 70 corridor, but rainfall amounts over 1" will be largely dependent on where thunderstorms and heavier rainfall are most frequent. As with Monday, temperatures will remain cool on Tuesday with clouds and precip. Global model guidance have come into better agreement on the evolution of the upper-level wave pattern across the CONUS, at least through the end of the week. An upper-level ridge will briefly pass over the Mid-Mississippi River Valley on Wednesday, providing short- lived CWA-wide dry conditions, before an upper-level trough is advertised to traverse the central CONUS Thursday into Friday. The characteristics of this trough still differ across deterministic and ensemble model guidance, especially its amplitude and tilt. These differences aside, the majority of ensemble model members depict showers and thunderstorms across the CWA between Thursday and Friday. NBM interquartile temperature ranges Wednesday through the end of the week show a slight warming of temperatures from around average to slightly above average. Pfahler && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1246 PM CDT Sun May 12 2024 VFR conditions are expected to prevail for the remainder of the day and well into the evening. Overnight, ceilings will gradually lower as precipitation arrives from the west. Ceilings will likely remain at VFR levels until early tomorrow morning, but will fall to at least MVFR, possibly IFR throughout the morning and through the end of the TAF period. Showers are also expected to impact all local terminals during this time frame, with at least some potential for a thunderstorm or two. Confidence in widespread precipitation is high, but low regarding the potential for thunderstorms. Visibility reductions to MVFR levels will be possible due to this precipitation, potentially lower within stronger convective cores. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX