674 FXUS63 KLSX 091754 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1254 PM CDT Tue Oct 9 2018 .SHORT TERM... (Through Late Tonight) Issued at 416 AM CDT Tue Oct 9 2018 Main concern during this period will be the potential for a few severe thunderstorms across central and northeast Missouri as well as west central Illinois. Weather is currently quiet across the CWA early this morning as a surface front extends from eastern Iowa into central Kansas. Upper air analysis continued to show an anomalously strong high off the East Coast and a longwave trough over the Rockies. Nearly southerly flow up to 500mb was over Missouri and Illinois with the best focus for thunderstorm development staying closer to the front early this morning. Will keep chances of showers and thunderstorms today to just central and northeast Missouri and west central Illinois. This is as the RAP/NAM is beginning to show the upper trough beginning to slowly move east toward the Midwest which will allow some ascent into these areas. These same areas will also some increase in weak low level moisture convergence today. By this afternoon, MUCAPES will be around 1000 J/kg with little CINH. The best chance for showers and thunderstorms including the potential for severe storms will be tonight. The GFS/NAM are showing an upper trough lifting out and moving across the Plains which will increase large scale ascent across Missouri and Illinois tonight. The attendant cold front will move into western Missouri late tonight. Showers and thunderstorms will develop ahead of this front over western Missouri this afternoon and move into central and northeast Missouri late this evening into the overnight hours. There will be sufficient instability and shear late into the evening for a few strong to severe thunderstorms over parts of central and northeast Missouri as well as west central Illinois. Low level and deep level shear profiles suggest that damaging winds will be the primary threat, with a tornado or two and hail also possible. Highs today will warm back into the 80s given the current warm temperatures and the some solar insolation. Lows tonight will be relative warm before the cold front moves through. Britt .LONG TERM... (Wednesday through Monday) Issued at 416 AM CDT Tue Oct 9 2018 A strong cold front will move quickly eastward through our forecast area on Wednesday with showers and storms expected along and just ahead of the front. Strong upper level divergence will be over our area Wednesday morning as the upper level trough over the Plains takes on a more positively tilted orientation. By Wednesday afternoon most of the precipitation will be across IL and southeast MO, and by evening it will shift east of our forecast area as a large and strong surface ridge builds southeastward into MO behind the cold front. Cooler temperatures can be expected on Wednesday, particularly across northeast and central MO due to cold air advection behind the cold front as well as cloud cover hindering diurnal solar insolation, plus cooling due to precipitation. Below normal temperatures can be expected across the entire area Wednesday night and Thursday. By late Thursday night the 850 mb temperatures will drop to around 2-6 degrees C across our forecast area, with low temperatures about 10 degrees below normal which will feel quite cold after the unseasonably warm conditions so far for most of October. A quick moving northwest flow shortwave may bring a brief period of light rain or showers to parts of our area on Friday, although past model runs have been very inconsistent with their QPF during this period. Dry air in the boundary layer will limit precipitation amounts and may lead to mainly just sprinkles over much of the area. Much more substantial precipitation is expected later in the weekend as a southern stream shortwave and associated surface low over the southern Plains pass south of our forecast area, while a northern stream shortwave and associated cold front drop southeastward into our area Sunday night. Showers will spread eastward into much of the forecast area Saturday night, northeast of the surface low. The operational GFS shifts most of the precipitation east of the forecast area by Sunday afternoon, but the ECMWF model is slower and deeper with the upper level trough and movement of the surface low keeping the rain going across much of our area Sunday night. For now will drop off pops from northwest to southeast by Monday. Colder temperatures can be expected late Sunday night and Monday after passage of the cold front, and also due to deep upper level troughing over the central US. GKS && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1214 PM CDT Tue Oct 9 2018 Diurnal cu have developed in a region of moderately unstable conditions with surface based CAPES in excess of 1000 J/kg and effective bulk shear approaching 30kts especially over portions of central/northeast Missouri. So will see showers and thunderstorms develop and track north through this area. Added vicinity thunderstorm mention at KCOU after 20z Tuesday and around 00z Wednesday at KUIN. Rest of TAF sites to remain dry and VFR through first half of forecast period. After initial activity diminishes, will see better chances of showers and thunderstorms with a prefrontal trof that will slide across the region late this evening and into the overnight hours. MVFR ceilings and visibilities can be expected with the storms. Showers and storms will persist through the morning hours on Wednesday as main cold front slides through region. Otherwise, main forecast issue will be the winds. Gusty south winds will begin to veer to the southwest by mid morning on Wednesday. The front will move through KUIN by 16z Wednesday with winds veering to the west and picking up. SPECIFICS FOR KSTL: Even though diurnal cu have developed across the STL metro area, expect VFR ceilings and dry conditions through 07z Wednesday. Prefrontal trof to approach KSTL after 07z Wednesday with increasing chances of showers and thunderstorms. MVFR ceilings and visibilities can be expected with the storms. Gusty south winds will veer to the southwest and diminish a bit after 14z Wednesday. Cold front will slide through KSTL around 20z Wednesday with winds veering to the northwest and becoming gusty once again. Byrd && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX