438 FXUS63 KLSX 120439 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1139 PM CDT Wed Sep 11 2019 .UPDATE... Issued at 920 PM CDT Wed Sep 11 2019 Will continue with a dry forecast the rest of the night. Satellite imagery is showing only high clouds over western and northern Missouri with active thunderstorms focused farther north over Iowa and the Wisconsin/Illinois border near a quasi-stationary front. This is where the focus for thunderstorms should remain overnight. Going lows look good and made only minor changes based temperature trends. Britt && .SHORT TERM... (Through Friday) Issued at 259 PM CDT Wed Sep 11 2019 An upper level trough continues to dig into the intermountain west while a weak surface low over the High Plains is slowly deepening in response. The paired influence of this low and surface high pressure over the eastern CONUS continues to advect warm, moist air into the region, pushing this afternoon's heat index readings to near or just above 100 degrees. The upper trough will deepen into a closed low as it pushes over the northern Plains by tomorrow morning, reaching the northern Great Lakes by Friday. The surface low will likewise intensify and move east in response to the upper levels. The low is progged to move into northern Minnesota by Friday morning, pushing a cold front through the Midwest. Model consensus shows the front remaining to the north and west of the mid-Mississippi Valley through peak heating tomorrow, keeping the region within the SSW flow. This will again push maximum heat index values into the mid to upper 90s, with areas around St. Louis possibly topping 100. The primary source of uncertainty with the heat forecast is high clouds streaming in ahead of the front. A secondary concern is the potential for isolated diurnal convection, but chances for this appear rather low. Despite these factors, even guidance on the cooler end of the spectrum pushes the heat index to near-criteria levels. With that, I'll let the Heat Advisory remain as-is. Models are in good consensus showing thunderstorms pushing into the area along the front tomorrow evening. With mid-level lapse rates in excess of 6 C/km, ample moisture, and 20-30 kts of 0-6km wind shear, there is a marginal risk that a few of these initial storms become severe, with damaging winds as the primary threat. The front will continue to the SE as we move into the night and storms are expected to diminish in intensity as convective inhibition builds. Current guidance is handling this situation in a variety of ways; synoptic models generally show just a few hundredths in the early morning and CAMs are showing only isolated storms (in different locations depending on the model) along the front. Given this uncertainty, have kept the forecast limited to chance thunderstorms from 06-12z Friday. By Friday morning, the front will likely have reached our southern counties, and isolated convection may persist or redevelop before the front clears the area on Friday evening. Again, model consensus is not great depicting precip potential on Friday morning, and will be going with just a slight chance of storms for the time being. The northwest winds in the wake of the front will allow for highs on Friday to be a good deal cooler than what we've seen the last few days. BSH .LONG TERM... (Friday Night through Next Wednesday) Issued at 259 PM CDT Wed Sep 11 2019 The front will briefly stall to the south of the region on Friday night as the upper flow becomes zonal. Unfortunately, with the front stalling out just to our south, it will not have enough momentum to eject the surface anticyclone out of the SE US. As this high lingers and intensifies into next week, and a thermal low begins to redevelop across the western CONUS, southerly flow will return to the region this weekend, again boosting temperatures back above normal. Contrasting the high confidence in above normal temps, model guidance shows poor consensus regarding storm chances into next week, so have little more than a few periods with a slight chance of storms. BSH && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1127 PM CDT Wed Sep 11 2019 Dry and VFR conditions are expected through 00Z. Then showers and thunderstorms will move into central Missouri and west central Illinois including at KUIN and KCOU during the evening. Any showers and thunderstorms will have the potential to bring MVFR/possible IFR ceilings and visibilities. Winds will remain out of the south during the period. SPECIFICS FOR KSTL: Dry and VFR conditions are expected through 06Z on Thursday night. Then scattered showers and thunderstorms are expected to move into the vicinity of KSTL. Winds will remain out of the south during the period. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 7 PM CDT Thursday for Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO. IL...Heat Advisory until 7 PM CDT Thursday for Madison IL-Monroe IL- Saint Clair IL. && $$ WFO LSX