993 FXUS63 KLSX 191947 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 247 PM CDT Tue Oct 19 2021 .SHORT TERM... (Through Late Wednesday Night) Issued at 247 PM CDT Tue Oct 19 2021 Key message in the short-term period through tomorrow night: A cold front will enter the area tomorrow afternoon, bringing a threat for scattered showers and an isolated thunderstorm or two. While gusty winds will be the predominant threat, severe storms appear unlikely. Current surface analysis places a developing cyclone lee of the Rockies across the northern Plains, placing most of the Mississippi Valley under relatively weak southerly winds. Aloft, increased moisture return on the backside of a mid-level ridge is evident in WV imagery and the high clouds across eastern Missouri and western Illinois. Winds this evening will remain elevated enough to inhibit radiation fog from developing in any notable way. Attention turns to the aforementioned cyclone and associated cold front, which will eject east today through tomorrow into the region along with its upper-level support. The low itself will shunt north as the upper-level system pivots into a negative tilt, and the associated cold front will advance east through tomorrow afternoon. Convergence along the front is meager at best according to all deterministic and short-range ensembles, and upper-level support is confined north of the CWA where more impressive mid-level lapse rates reside. 0-6km bulk shear values of 40kts across most of our region will only have upwards of 500-750 J/kg of MUCAPE to work with in aiding deep convective initiation, thanks to poor moisture return ahead of the front. The higher shear and "higher" CAPE are also not coincident for more than an hour or two in the early evening. Model soundings also depict a strong capping inversion that the cold front's limited convergence would have difficulty overcoming. All of this is to say that it appears that both the coverage and intensity of convection tomorrow is unimpressive. While gusty winds typical of any thunderstorm are certainly in the cards with the few storms that can more fully develop, severe storms are not likely. After sunset, any remaining instability will be elevated and likely won't have much forcing to maintain precipitation beyond tomorrow evening when the front clears the area. MRB .LONG TERM... (Thursday through Next Tuesday) Issued at 247 PM CDT Tue Oct 19 2021 Key messages through the extended forecast period: 1) Dry weather and seasonable temperatures stay in place behind tomorrow's cold front through the end of the work week. 2) Rain chances ramp up late Saturday into Sunday, with most of the area expected to see showers and scattered thunderstorms. Temperatures will rise next week, but how much they will warm is uncertain. Deterministic and ensemble guidance are in good agreement that upper- level northwest flow will bolster seasonable temperatures and dry weather through the next few days. Though subtle shortwave ripples are depicted in the guidance, they will be moisture-starved and will likely not produce anything more than cloud cover. Active weather returns to the region this weekend when longwave troughing and broad diffluent upper-level flow set up just lee of the Rockies. The resultant surface low that develops Saturday afternoon with an upper level shortwave will deepen and move east Saturday night. WPC Cluster Analysis depicts agreement among the EPS/GEFS/CMCE members on a swath of precipitation associated with stronger WAA and broad isentropic ascent oriented perpendicular to a lifting warm front on Sunday. However, subtle differences in the position of the low (and thus the warm front) place the QPF maximum in slightly different locations depending on the cluster. The ECMWF EFI, which measures the extremity of the EPS guidance relative to its model climate, shows values of 0.5-0.6 for QPF in central and east-central Missouri. These values are indicative of unusually-high QPF values appearing in the EPS guidance for this area. NAEFS guidance also depicts IVT values near the 90th climatological percentile on Sunday afternoon, which backs up the potential for heavy rain during this time period. While most of the area could stand to see some rain, attention will be given to the potential for localized flooding if waves of showers repeatedly impact the same locations. A brief respite from rain appears possible following this system, but guidance diverges regarding the next chances for rain. Though this forecast features high temperatures next week that are warm for late October, uncertainty exists over where another warm front sets up and how anomalous temperatures aloft will be. As better agreement on the pattern emerges in the guidance, these temperatures will be refined. MRB && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1129 AM CDT Tue Oct 19 2021 VFR conditions are expected at all terminals through this TAF period. The main concern over the TAF period will be an approaching cold front that brings the threat of scattered showers and isolated thunderstorms. It appears that the onset of any showers/storms that develop will fall just outside the valid TAF period (between 18z Wednesday and 00z Thursday), however thunderstorm activity will be quite limited and terminal impacts are not expected to be widespread or long-lived. Otherwise, a threat for LLWS also exists at KCOU, KJEF, and KUIN overnight tonight. Winds aloft do not appear particularly strong, but with due southerly winds at the surface and west-southwest winds aloft, directional LLWS is possible. Right now, confidence in the LLWS criteria for TAF mention is low. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX