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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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288 FXUS63 KLSX 132030 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 330 PM CDT Tue Aug 13 2024 .KEY MESSAGES... - An active weather pattern continues across the region, with locally heavy rainfall and flash flooding possible, including across central and southeastern MO. However, it is uncertain where the axis of heavy rainfall occurs. - Additional rounds of showers and thunderstorms are forecast Wednesday night into Thursday morning and then again sometime late Thursday, which lower confidence. Confidence is low on whether or not thunderstorms are severe. - Lower chances (20 to 40 percent) of showers and thunderstorms persists through the weekend, but dry conditions are most likely. && .SHORT TERM... (Through Late Wednesday Night) Issued at 329 PM CDT Tue Aug 13 2024 A slowly weakening MCS continues to track eastward across the southern third of MO and into southeastern MO. Although gusty winds may accompany this MCS, it should eventually dissipate through late afternoon as it encounters lower instability and moisture. A few showers could linger across central and southeastern MO through the rest of the afternoon and early evening as an remnant MCV also arrives. A front, temporarily pushed into southern KS, northeastern OK, and western AR by the ongoing MCS will lift northeastward this evening through overnight. The cool (east) side of this front as it becomes oriented from north-northwest to south-southeast will be the focus of another round of showers and thunderstorms somewhere from northwestern MO to southern MO, potentially including central and southeastern MO, between late this evening through Wednesday morning as a 30-kt southwesterly LLJ develops. The resulting pattern and environment (e.g., PW >2", deep warm cloud depths, deep-layer flow paralleling the front) are favorable for locally heavy rainfall from an MCS with training showers and thunderstorms, but CAMs have been struggling to decipher where the heavy rainfall axis will originate and be placed largely due to the influence of the ongoing MCS. The latest HREF LPMM highlights a corridor of 3 to 7+" from west-central to southern MO, which is a slight southwest shift from the previous run. Between dry antecedent conditions and this trend in the HREF, confidence has not increased in locally heavy rainfall and flash flooding occurring within the CWA in central and southeastern MO. That being said, the variability in the CAMs suggests the the placement of the heavy rainfall axis is not yet decided and a shift back northeast cannot be ruled out. Showers and thunderstorms are expected to gradually shift northeastward toward the Mississippi River Wednesday morning into midday as the front also attempts to shift northeastward, but intensity and coverage will gradually decrease as the LLJ wanes and mid-level heights rise with the arrival of an upper-level ridge axis during the afternoon. Only a few isolated showers and thunderstorms are possible by the evening. Uncertainties in how quickly showers and thunderstorms dissipate and lingering clouds break also has implications for the high temperature forecast on Wednesday with the NBM interquartile range spanning 5 to 7 F, spread from below to near average. Pfahler && .LONG TERM... (Thursday through Next Tuesday) Issued at 329 PM CDT Tue Aug 13 2024 Another round of showers and thunderstorms is expected again overnight Wednesday night into Thursday morning with origins across northeastern KS, eastern NE, and southwestern IA during the evening as an upper-level trough approaches. A veering LLJ and a reservoir of at least elevated instability across much of MO indicated by model guidance, suggest that these showers and thunderstorms will tend to move east-southeastward into the CWA along and north of I-70 overnight into Thursday morning as they grow upscale into one or more MCSs. A few of these thunderstorms could produce gusty winds, but there is question on how unstable the BL will be by that time and how balanced thunderstorms will be with their cold pools. The environment will still be favorable for heavy rainfall, but the pattern itself will not be as favorable as tonight/Wednesday morning due to less of a parallel component of deep-layer flow with the frontal boundary that continues to lift northeastward to somewhere near the Mississippi River by Thursday morning. Concern is increasing for redevelopment of showers and thunderstorms later in the day on Thursday to be influenced by an MCS that will likely be ongoing during the morning. These influences include where and how much airmass/instability recovery takes place and if there will be any remnant outflow boundaries. Synoptically, an upper-level trough will approach the Mid-Mississippi River Valley, spreading broad ascent over the region, but model guidance generally agree that an associated cold front will not arrive until Thursday night or Friday morning. As a result, confidence is low on where and when shower and thunderstorm development occurs and what the coverage will be, with even lower confidence on whether or not thunderstorms will be severe. However, with 30 to 40 kt of deep- layer shear advertised, Thursday will need to still be monitored for severe thunderstorms, if other factors can contribute constructively. High temperatures warming to above average could also be jeopardized on Thursday if morning showers and thunderstorms linger long enough. There is still uncertainty in how quickly the cold front will clear the CWA on Friday, with the chance (20 to 30 percent) of showers and thunderstorms lingering through the day. Upper-level northwesterly flow will prevail through the weekend into early next week over the Mid-Mississippi River Valley with the front settling just south and southwest of the CWA. With the front possibly remaining nearby and several upper-level shortwave troughs navigating the northwesterly flow, some ensemble model members depict showers and thunderstorms at times through the period. A slightly larger majority has dry conditions. Temperatures will also depend on the proximity of the front, with NBM interquartile temperatures ranges spanning 4 to 7 F between near and slightly below average temperatures. Pfahler && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1219 PM CDT Tue Aug 13 2024 A decaying complex of showers and thunderstorms will continue to track across southern and southeastern MO early this afternoon, with only a short period of rain expected to reach KJEF. Otherwise, improvement in flight conditions are expected through the afternoon, reaching VFR this evening. Late this evening through Wednesday morning, another elongated complex of showers and thunderstorms is forecast to develop, but there is uncertainty in its exact placement/track, which has the potential to include central and southeastern MO. MVFR flight conditions are included with this TAF package, but greater reduction in flight conditions and heavy rainfall are possible. Elsewhere to the north and east, VFR flight conditions are likely to persist. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX