337 FXUS63 KLSX 210831 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 331 AM CDT Sat Aug 21 2021 .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 06z TAFs through 06z Saturday Night) Issued at 1110 PM CDT Fri Aug 20 2021 Complex of thunderstorms over northwest/west central missouri will continue move east-southeast tonight. The storms look like they are continuing to weaken as they move toward our area, but gusty winds to 30kts primarily over central Missouri will still be possible. MVFR ceilings and IFR visibility will also be likely with these storms. Also, some low MVFR/IFR ceilings have cropped up over the past couple of hours across parts of southeast Missouri and southwest Illinois, along with patchy dense fog. These ceilings and fog should eventually be washed out by the thunderstorms as they move through the area tonight. Some MVFR ceilings are likely along with lingering showers in the wake of the main body of storms on Saturday morning, but low clouds should lift to VFR by late morning/early afternoon. Some scattered storms are again possible during the late afternoon, mainly over southeast Missouri and south central Illinois. Carney && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX