721 FXUS63 KLSX 070838 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 338 AM CDT Fri Oct 7 2022 .SHORT TERM... (Through Late Saturday Night) Issued at 330 AM CDT Fri Oct 7 2022 A series of shortwaves continue to dig through an upper level trough over the eastern CONUS. One of these shortwaves brought last evening's cold front through the area, while another disturbance is shunting a reinforcing shot of cool and dry air through the Northern Plains and Upper Midwest this morning. This airmass will continue southward today and tonight, helping to clear morning clouds and will keep afternoon temperatures cool. Tonight, high pressure associated with this cooler airmass will settle over the region, resulting clear skies, light winds, and by tomorrow morning, the coolest temperatures of the season thus far. Even the 75th percentile of NBM minimum temperatures (min Ts) on Saturday morning fall to or below 32F across much of northeast MO and portions of west-central IL, meaning that at least 75% of model guidance is forecasting freezing temperatures in these areas. The combination of the above meteorological setup typical of very cool nights and strong model consensus for freezing temperatures has bolstered confidence that many areas in our north will see their first freeze of the season. We've therefore gone ahead and upgraded the Freeze Watch to a Freeze Warning in these areas. Further south, the 75th percentile of NBM min Ts drops below 36 for most areas in our Missouri counties outside of St. Louis, and just a degree or two warmer across Illinois. These temperatures, along with light winds, will lead to conditions favorable for frost. With widespread frost expected across our rural Missouri areas and more patchy frost expected around St. Louis and into Illinois, we've issued a Frost Advisory for the rest of the forecast area. We'll remain beneath northwesterly flow aloft on Saturday while the center of the surface high slides east across the region. Clear skies will allow temperatures to rebound 25-30 degrees from the morning lows, but given the cool starting point, temps will remain roughly 10 degrees below normal. Temperatures will again be quite cool from Saturday night into Sunday morning, though more westerly winds on the back side of the high will keep temperatures a bit warmer than what we're expecting tonight. Another freeze warning is unlikely, but an additional frost advisory may be needed, especially over our SW IL counties where the light northerly winds will hang on a bit longer. BSH .LONG TERM... (Sunday through Thursday) Issued at 330 AM CDT Fri Oct 7 2022 The eastern CONUS trough will persist into Sunday and Monday, keeping northwest flow in place a top the region. The surface high will continue to very slowly drift further to our east, keeping weak west to southwest winds in place over the Mid-Mississippi Valley. With little in the way of moisture return, skies will remain mostly clear through Monday. The combination of clear skies and S-SW winds will push temperatures back to normal by Sunday and above normal by Monday. On Tuesday, an upper trough will begin to dive into the Canadian Plains and northern High Plains, with the southern periphery of the trough reaching the Mid-Mississippi Valley on Wednesday. In response, low level low pressure/troughing will develop to our west and support increasing SSW flow across the region. This will further boost temperatures before a cold front moves into the region on Wednesday, perhaps stalling across the region on Thursday. The front will bring precipitation chances back into the region, though this portion of the forecast is far from certain. First of all, there are differences in frontal timing and evolution amongst the guidance. Secondly, moisture return does not appear to be all that impressive ahead of the front, limiting precipitation potential. Nevertheless, the majority of members in the latest run of the NBM do show at least measurable precipitation across our northern CWA at times on Wednesday and Thursday. Chances for 0.1", 0.25", and 0.5" fall off from there, topping out across our north at around 40%, 25%, and 15% respectively. That's all to say that while some light precipitation is likely midweek, especially in NE MO and west-central IL, it seems unlikely that we'll see enough precipitation to put much of a dent in the increasingly dry conditions we've got across the region. BSH && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1031 PM CDT Thu Oct 6 2022 VFR flight conditions are expected to continue through the remainder of the night. The cold front has made it south of the St. Louis and Columbia metro terminals, and winds are expected to remain elevated overnight. Gusts have dropped off after the initial frontal passage and upstream obs do not indicate that significant gusts will continue overnight, though a stray gust in the upper teens cannot be ruled out. Cloud cover associated with the front has been limited and remained at VFR flight conditions. Ceilings are expected to be at or above 25,000 ft across all terminals once some of the straggling thunderstorm cloud debris near the St. Louis metro terminals diminishes or exits the terminal space. As previously mentioned, winds are expected to remain elevated overnight, even in the sheltered and "calmer" TAF sites like KJEF, KSUS, and KCPS. Fog development at these sites is therefore unlikely, but if a site were to drop to 3 kt winds or less for several hours overnight, the risk for fog at KJEF, KSUS, and KCPS would increase. Given the large amount of uncertainty, have kept fog and visby reduction out of the TAF for now. MRM && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Frost Advisory from 1 AM to 9 AM CDT Saturday for Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO- Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. Freeze Warning from 1 AM to 9 AM CDT Saturday for Audrain MO- Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO- Shelby MO. IL...Frost Advisory from 1 AM to 9 AM CDT Saturday for Bond IL- Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL- Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL- Randolph IL-Saint Clair-Washington IL. Freeze Warning from 1 AM to 9 AM CDT Saturday for Adams IL-Brown IL-Pike IL. && $$ WFO LSX