802 FXUS63 KLSX 170455 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1155 PM CDT Wed Oct 16 2019 .SHORT TERM... (Through Late Thursday Night) Issued at 139 PM CDT Wed Oct 16 2019 Cool, dry northwest upper level flow will persist through Thursday as high pressure continues to build into the region. NW winds ahead of the surface high will continue to advect the clouds currently across the Midwest out of the area, and the mid-Mississippi Valley should be mostly clear by midnight as the high becomes centered over southern MO. The combination of mostly clear skies and calm winds should allow overnight temperatures to fall off quite a bit, with morning lows falling into the mid to upper 30s across the area. The high will shift to our east during the day tomorrow, swinging surface winds around to the south. These weak southerly winds and mostly clear skies will help temperatures moderate a bit, though with the cool start to the morning, temperatures will still struggle to get much above 60. An upper level ridge will build into the region on Thursday night. High clouds riding atop this ridge appear likely into Friday morning, and should help keep overnight temperatures a bit warmer than what we're expecting tonight. BSH .LONG TERM... (Friday through Next Wednesday) Issued at 139 PM CDT Wed Oct 16 2019 A broad trough will begin establishing itself across western North America by the end of the work week, setting the stage for a period of active weather across the Midwest. On Friday, the first of a series of shortwaves will push into the Great Plains before sweeping through the Great Lake on Saturday. In response to this wave, a surface low will move across the northern Plains on Friday and Saturday. Southerly winds will pick up ahead of the low, finally boosting temperatures to near normal levels on Friday. The warm up will be short lived however, as the low is expected to drag a cold front through the mid-Mississippi Valley on Saturday. Moisture return ahead of this front is progged to be rather weak, raising questions about precip likelihood along the front. While guidance shows good consensus with precip reaching NW MO on Saturday morning, I'm less certain that precip will sweep across the entire area. Therefore have introduced likely PoPs in NE MO on Saturday morning and stuck with just chance wording for the rest of the area. Given poor moisture return ahead of the front, there looks to be little to no instability, so I'm not expected thunder at any time on Saturday. We'll briefly will dry out behind the front on Saturday night and Sunday. Meanwhile, a more robust trough will dig into the Plains before deepening into a cutoff low and curling into SW Ontario by Tuesday morning. Models are in remarkable consensus with the timing and placement of this feature, though the operational GFS does show the low getting quite a bit deeper than the majority of deterministic or probabilistic guidance. Regardless, a strong surface low is likely to track across the northern tier of the central US on Sunday and Monday. The first local impact of this stout low will be brisk southerly winds on Sunday, boosting temps back into the low 70s while ushering surface dew points in the 60s across portions of the region. The cold front trailing behind the surface low is then progged to move through the region on Sunday night or Monday, likely bringing showers and thunderstorms. While showers and storms are likely, guidance differences in fropa timing have resulted in an extended period of chance PoPs. The very strong dynamics forecast with this system are worth noting and are reflected by 0-6km shear in excess of 50 kts and 0-1km shear in excess of 30 kts as the front sweeps across the region. With these strong shear values, there is some concern for severe storms despite the limited instability forecast to be in the vicinity of the region. CIPS severe probability guidance depicts this threat with a 15% probability contour extending from the lower Mississippi Valley into southern Lake Michigan. However, with this event still 4-5 days away, fropa timing differences, and the potential for the front to sweep through when instability is at its nadir, I'm not yet confident enough in severe storms to mention the threat beyond this discussion. That said, this will be an event worth watching as we move into and through the weekend. Deep NW flow will return to the region behind the early week cyclone. This cool, dry flow should help keep things dry, while pushing temperatures back towards or perhaps below seasonal normals by midweek. BSH && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1142 PM CDT Wed Oct 16 2019 VFR conditions are expected at all of the terminals for the majority of the valid TAF period. The only exception may be at KCOU and KSUS where river fog is possible between 09-13z based on the latest satellite trends which depicted fog already starting to develop in river valleys. Otherwise, the remnant stratocumulus deck near KUIN and KSTL/KSUS/KCPS should continue to dissipate due to the influence of an approaching high pressure center. Winds will become light/variable as the high pressure center approaches, then turn southerly to southeasterly after the high has shifted farther eastward. Kanofsky && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX