944 FXUS63 KLSX 111119 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 619 AM CDT Thu Oct 11 2018 .SHORT TERM... (Through Late Tonight) Issued at 409 AM CDT Thu Oct 11 2018 Main concern during this period will be the potential for some areas of frost tonight. Widespread low clouds were found across the entire CWA early this morning in the low level cyclonic flow. These clouds are expected to gradually become scattered later this morning as the flow becomes nearly neutral. In addition, the RAP forecast soundings are showing the moisture that is producing this cloudiness is in a relatively shallow layer that will diminish through the morning into the afternoon. Temperatures this morning are much cooler than in recent days, with current readings ranging from the mid 40s to the mid 50s. Temperatures will likely fall into the lower to mid 40s for lows the morning before they begin to rise again today. While there will be some sunshine by late this morning and this afternoon, it will be offset by cold air advection. Expect highs this afternoon to be only in the 50s as a result. Tonight will be the first in a string of cool nights. A surface high currently centered over the central High Plains will move into the Midwest by tonight. This will cause winds to turn light by late tonight. At the same time, an upper trough currently over the upper Rockies will drop southeast into the Central Plains which will cause an increase in clouds in areas west of the Mississippi River late tonight. The net effect will be for temperatures to fall off quickly this evening, but then level off late tonight as the clouds move into the area. There still will be the potential for some frost to develop over parts of northeast Missouri and west central Illinois late tonight where lows will drop in the the middle to upper 30s. With the aforementioned upper trough moving into the Plains tonight, ascent and low level moisture convergence will increase late tonight ahead of it over central Missouri resulting in a chance of rain. Britt .LONG TERM... (Friday through Wednesday) Issued at 409 AM CDT Thu Oct 11 2018 Rain will spread eastward through much of our forecast area Friday due to increasing upper level divergence and low-mid level warm air advection ahead of an approaching upper level trough. Although the surface/boundary layer will be initially dry with a surface ridge over our area, it appears that it will eventually moisten enough to allow at least a period of light rainfall. With low level cloud cover, precipitation, and relatively cold 850 mb temperatures particularly across northeast MO and west central IL, colder than previous model runs, will forecast well below normal high temperatures on Friday, about 20 to 25 degrees below normal. This rain will shift southeast of our forecast area by late Friday night with well below normal temperatures continuing Friday night. Although partly cloudy skies and recent rainfall may hinder frost formation there may still be patchy light frost across parts of northeast MO and west central IL late Friday night and early Saturday morning. Warmer temperatures, albeit still below normal can be expected on Saturday with at least partial sunshine, and as surface winds become southerly on the backside of the retreating surface ridge. Rain will spread east-northeastward into at least southeast MO and southwest IL late Saturday night and Sunday as a southern stream shortwave and an associated surface low move eastward through the southern Plains and into the Gulf Coast states. The models are a little further south with this feature and have the better QPF further south. A northern stream shortwave will also move eastward through the northern Plains and into the Great Lakes region and send a cold front southeastward through our area late Saturday night and Sunday with light rain possible along and just behind this front. This front will bring another shot of colder air into our region with colder temperatures forecast Sunday night and Monday. The chance of rain will continue Sunday night and Monday, especially across southeast MO and southwest IL as a positively tilted upper level trough slowly approaches with a nearly stationary front south of our area leading to post frontal rain. GKS && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 610 AM CDT Thu Oct 11 2018 Widespread clouds with ceilings between 2000-3000FT AGL are expected to scatter or move out of the area between 14-18Z. Then dry and VFR conditions are expected through 06Z. Increasing clouds will move into the area from the west and some rain will move into central Missouri with some chance of rain at KCOU after 09Z. This chance is too low to include in the TAF at this time. Otherwise, gusty northwesterly winds will decrease after 00Z. SPECIFICS FOR KSTL: Clouds with ceilings near 3000FT AGL with improve to VFR around 15Z. Then dry and VFR conditions are expected through 12Z. Then rain will move into the terminal from the west on Friday morning. Otherwise, gusty northwesterly winds will decrease after 00Z. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX