849 FXUS63 KLSX 211305 AFDLSX Area Forecast Discussion...Updated National Weather Service Saint Louis MO 805 AM CDT Sat Aug 21 2021 .MESOSCALE DISCUSSION... Issued at 752 AM CDT Sat Aug 21 2021 Key Messages... 1) Storms have quickly dissipated across most of eastern Missouri and western Illinois. The morning is now expected to be largely dry areawide. 2) Redevelopment of widely-scattered storms is still expected ahead of a cold front this afternoon, mainly in far eastern Missouri and western Illinois. Technical Discussion... Latest radar imagery depicts a rapidly-deteriorating line of remnant showers from earlier convection, and if this trend continues, we should be dry for at least a few hours during the morning. Surface analysis places the cold front, which will be the focus for convection this afternoon, across northwestern Missouri. An axis of more pronounced surface dewpoints is evident along and east of the Mississippi River, where more robust convection did not occur. The front will continue to advance southeast through the day, interacting with approximately 1000-1500J/kg of SBCAPE this afternoon collocated with the axis of pronounced moisture, to spark widely-scattered storms. In the absence of appreciable shear and upper-level support, the only threats these storms will likely pose are lightning, gusty winds, and heavy downpours. MRB && .SHORT TERM... (Through Late Sunday Night) Issued at 328 AM CDT Sat Aug 21 2021 Currently, a shortwave can be seen in water vapor imagery moving out of the Northern Plains into southern Canada. Its associated surface low is currently centered over the U.S.-Canada border near the Minnesota-North Dakota state line, with the low's cold front draped southward into the Central Plains. A complex of showers and thunderstorms ahead of this front is currently pushing into portions of eastern Missouri and west- central Illinois. The storms in this complex have been sub-severe so far, and with effective bulk shear forecast to remain 20 kts or less and better instability remaining further south, I expect storms to stay sub- severe. That being said, I can't rule out wind gusts up to 20-30 mph with the leading gust front, and heavier cores will be capable of a quick burst of heavy rainfall. With the progressive nature of these storms, flooding is not anticipated, but ponding on roads is possible. This complex will continue southeastward through the morning, and is expected to clear the CWA by approximately 9-10 am if it doesn't diminish sooner. The front will be making its way into northeastern portions of CWA around this time and be roughly oriented along I-40 in Missouri and I-70 in Illinois during peak heating. Convective debris in the wake of the morning storm complex will likely inhibit robust recovery, limiting instability for the frontal passage. However, the front will still likely force scattered showers and storms this afternoon along and ahead of it. Effective bulk shear is forecast to be 20 knots or less, and with a lack of upper-level forcing, updrafts will tend to be weaker and unorganized. The front and associated convection is expected to clear the CWA later this evening. Sunday will be a relatively calm day thanks to the front clearing the CWA and northerly low-level winds in its wake. This will help keep temperatures in the mid-80s up to around 90 for most of the CWA. Low 90s are possible in central and southeastern Missouri due to an upper-level ridge building into the Midwest through the day. This will gradually cause mid-level winds to become southwesterly, advection warmer air over this portion of the CWA. Sunday night, this changing pattern will cause the cold front to lift northward as a warm front, marking the beginning of our next heat wave. Elmore .LONG TERM... (Monday through Friday) Issued at 328 AM CDT Sat Aug 21 2021 Ensemble guidance continues to be in good agreement in a broad upper-level ridge building across the southern half of the country early in the work week. Beneath this ridge, 850 mb temperatures are still on track to reach 22-23 degrees C Monday and Tuesday per both the NAEFS and ECMWF ENS. This correlates well with the mid to upper-90s seen in surface temperatures produced by most members of both ensembles. With these temperatures and dew points forecast to reach the mid-70s, heat index values of 100-105+ appear likely Monday and Tuesday afternoon. I can't completely rule out an isolated shower or thunderstorm during peak heating, but the forecast these days look mostly dry. Confidence in the forecast wanes starting Wednesday through the end of the period due to differences in the strength of the ridge and shortwaves passing along its periphery through the Midwest. A passing shortwave on Wednesday and its associated convection could mitigate hotter temperatures across portions of the CWA. However, depending on the timing and placement of convection, temperatures could reach values seen on Monday and Tuesday thanks to similar 850 mb temperatures. Both 850 mb and surface temperatures decrease a couple of degrees beyond Wednesday, and rain chances are more widespread in the NBM, but that is most likely due to the spread in solutions composing the blend. I suspect there will still be plenty of dry time, allowing for temperatures to still climb into the low to mid 90s for afternoon highs. Elmore && .AVIATION... (For the 12z TAFs through 12z Sunday Morning) Issued at 631 AM CDT Sat Aug 21 2021 Currently, a complex of showers and weak thunderstorms is moving across portions of eastern Missouri and western Illinois, impacting all local terminals to varying degrees. This complex is weakening, so confidence is not high in additional thunderstorms, and the showers/stratiform rain is expected to diminish over the next few hours as the complex moves away from the terminals. Low ceilings associated with this convection are roughly oriented along the Mississippi River spreading eastward, impacting the KSUS, KSTL, and KCPS terminals. These ceilings are expected to move eastward this morning, allowing for flight conditions to gradually improve. While confidence is greatest in the times of the flight category changes currently in the TAFs, I would not be surprised if conditions took roughly an hour longer to improve than what is currently forecast. Confidence is high that VFR flight conditions will prevail from roughly midday through the remainder of the period as a cold front moves over the terminals. Additional showers and storms are possible along this front this afternoon, but they are expected to form south of the terminals. Elmore && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX