897 FXUS63 KLSX 120439 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1139 PM CDT Thu Oct 11 2018 .SHORT TERM... (Through Late Friday Afternoon) Issued at 303 PM CDT Thu Oct 11 2018 A somewhat active period is expected across the area Friday into early next week. Tonight however looks tranquil and rather chilly. High pressure will slide into the area from the west this evening bringing light winds. This combined with clear skies should lead to a good temperature drop. As we head into the overnight hours however we will see mid-high clouds overspreading the area from the Plains, occurring in advance of our next storm system associated with a upper trof moving into the north central Plains. The temperature fall overnight should level out from west to east attendant with the thickening cloud cover. Present thinking is the coldest temperatures falling into the middle 30s will be across northeast MO and west/central and south central IL. There is potential for some patchy frost within this area. I think Friday is going to be a big weather shock to alot of folks. The area will see a cold rain and for many areas along and north of Interstate 70, the highs will we be in the 40s and it will be more reminiscent of November. As far as the sensible weather, an upper trof initially centered in the north central Plains will advance quickly eastward and into the Upper-Mid Mississippi Valley by early evening. Large scale ascent associated with the upper trof along with low level WAA and low-mid level frontogenetic forcing will generate a large shield of rain which will spread from western MO into central MO during the first part of the morning and across the remainder of the CWA by early afternoon. The combination of the clouds and rain and diabatic cooling will result in temperatures running 20-25 degrees below average. The latest guidance is a bit faster with the progression of the upper trof and associated forcing, and by late afternoon the rain band should be centered across southwest IL into southeast MO. Glass .LONG TERM... (Friday Night through Next Thursday) Issued at 303 PM CDT Thu Oct 11 2018 Rain should come to an end by mid-evening across southwest IL and southeast MO as the upper trof and associated forcing advances quickly into the Ohio Valley region. The main question then for Friday night and into early Saturday morning is the potential for frost. I am not a frost expert but the conditions don't appear all that favorable. Temperatures will fall into the 30s during the night and winds will be light. The negating factors would be how much clearing of the clouds will occur and the ground will be wet, both of these inhibit decent radiational cooling. Presently we are forecasting overnight clearing in northeast MO and west central IL and I have mentioned some patchy frost, however confidence is low. Temperatures will remain below average through the weekend. Saturday will be the pick day of the weekend and then we will see several chances/waves of rain Saturday night into Monday. None of the rain looks heavy and preferred areas for precipitation vary in location during this time frame. A broad and positively tilted upper trof stretching from the upper Mississippi Valley into the Great Basin will result in southwest flow aloft. Within this flow we will see a series of migratory low amplitude disturbances. The first of these disturbances along with low level WAA and moisture transport will bring a threat of rain, predominately to southern MO and southern IL Saturday night into Sunday morning. Another disturbance along with a cold front advancing from the north and frontogenetic forcing will bring yet another chance of rain to central and northern sections of the CWA Sunday afternoon and evening. By Monday morning, the main focus will then shift again to predominately southeast MO and southern IL as the next disturbance and increasing frontogenesis focus rainfall in that area. More tranquil weather is expected Tuesday through Thursday. Monday night into early Tuesday morning looks like the coldest time period of the next 7 days, possibly a freeze. A pattern shift which results in ridging aloft and a return to a low level WAA regime and southwest low level flow is forecast by Thursday. Translated, temperatures gradually return to near normal by late next week. Glass && .AVIATION... (For the 06z TAFs through 06z Friday Night) Issued at 1107 PM CDT Thu Oct 11 2018 Clouds continue to stream east across KS/wrn MO heading toward terminals late this evening. These MVFR cigs will spread across the region thru sunrise. Guidance continues to show showers developing and spreading into the area, impacting KCOU around sunrise. This area of rain will continue to spread east across the region thru the day, moving south or east of the terminals during the mid to late afternoon hours. Lower cigs shud also move into the region accompanying the rain, eventually lowering to IFR except at KUIN where low end MVFR is expected. That said, there is some indication that IFR cigs are possible at KUIN as well, but confidence is lower at this time. SPECIFICS FOR KSTL: Still expect rain to impact the terminal around 18z. Activity may be more sct/showers before this time and can not rule out a brief shower impacting before 17z. Still expect rain to move out of the area around 22z. With confidence increasing, have added IFR cigs for Fri afternoon. Low cigs shud linger thru the evening hours and into Sat morning. Tilly && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX