385 FXUS63 KLSX 250935 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 335 AM CST Mon Nov 25 2019 .SHORT TERM... (Through Late Tonight) Issued at 335 AM CST Mon Nov 25 2019 Early this morning, NW flow aloft prevailed with weak upper level disturbances over the High Plains. These upper level disturbances were generating mainly just high cirrus cloudiness and very little way in the precipitation. A much stronger storm system was coming onshore in the Pacific Northwest near Oregon. At the surface, SW flow prevailed over the forecast area. A cold front was moving through northwest Iowa and southeast Nebraska. Skies were clear over our region with temperatures ranging from the mid 30s where decoupling can occur and winds become near calm to the mid 40s where SW winds were more stout. The weak upper level disturbances are expected to track eastward and drive the area of thin cirrus clouds across our region during the day today, and should result in mostly sunny skies. SW surface winds will likewise continue as the surface cold front enters northeast MO late in the afternoon--too late to have any impact on temperatures. Temperatures on Sunday overperformed over the MOS guidance and leaned strongly to the high-end members of the SREF. Using this as a guide to forecasting max temps today, we should see readings peaking in the upper 60s in east-central MO and the STL metro area, mid to upper 60s in central MO and around 60 elsewhere. No precipitation is expected with the front through early this evening as it stalls in the far NW suburbs of the STL metro area with the column too dry to act on. The strong storm system coming onshore in Oregon will be busy, digging towards the Four Corners region and backing our flow aloft more from the SW heading into tonight. The SW flow aloft will increase WAA and combined with upper level disturbances being stripped from a storm system in the eastern Pacific will eventually attain enough moisture and lift in the column to result in scattered shower development late tonight. The best chances are focused for areas south of Interstate 70 through 12z/Tue. TES .LONG TERM... (Tuesday through Sunday) Issued at 335 AM CST Mon Nov 25 2019 Guidance remains fairly steady in showing a strong short wave digging across the eastern Rockies into the Western Plains on Tuesday morning. This forces strong lee-side cyclogenesis over southeast Colorado, and the surface low pressure eventually drops to around 985mb according to the GFS/NAM/ECMWF as it ejects northeast through the central Plains into the Midwest and Great Lakes Region Tuesday afternoon through Wednesday morning. Deterministic guidance is all in pretty good agreement now on timing the resulting cold front as it passes through our forecast area. We should stay in the warm sector all day on Tuesday which will keep temperatures mild in the mid 50s to low 60s even with a mostly cloudy sky and showers likely. The front should sweep through central and eastern Missouri between about 04Z and 07Z, and then through south central Illinois into eastern Illinois by 09Z. All deterministic guidance prints out several tenths of an inch of QPF ahead of the front, so high PoPs still look to be in order. The GFS and ECMWF show 200-400 J/Kg MUCAPE building ahead of the front late Tuesday afternoon into the evening...and the NAM substantially more between 1000-1200 J/Kg. All models show 60-80+ kts of 0-6km bulk shear to go along with that instability, so thunderstorms continue to be a distinct possibility...and I'm increasingly concerned that some of the storms could be severe. The only real question in my mind is whether the low level convergence ahead of the front will coincide with the instability and be strong enough to force convection rooted in the boundary layer. If that happens, we could have a few low-top supercells to deal with Tuesday evening. As the deep surface low moves northeast early Wednesday morning, the strong pressure gradient between the low to our north-northeast and a 1027mb surface high building into the Great Plains will produce strong winds across the mid Mississippi Valley. Current guidance is showing sustained winds in excess of 25kts with gusts in excess of 40kts generally along and north of I-70 with somewhat lower wind speeds further south. It will certainly feel much colder on Wednesday between the wind and highs close to or slightly below normal in the mid 40s to around 50. Winds should diminish by late Wednesday afternoon as the Great Plains high continues building east into the Midwest. The precipitation type forecast for Thanksgiving Day continues to be uncertain. Cold air in place due to high pressure moving across the Midwest will allow overnight temperatures to drop into the mid 20s to low 30s. However, deterministic models have been trending drier Thursday morning when temperatures will be supportive of wintry precipitation. GFS and ECMWF keep a long wave upper level ridge over the Mississippi Valley which suggests broad scale subsidence. The 850mb ridge axis is now forecast to be just entering central Missouri at 12Z Thursday, so the 850mb flow will remain out of the northwest during the morning, and will not be turning back to the south until 18Z or later. This keeps the corresponding low level moisture convergence and the lion's share of the QPF in the models well south-southwest until Thursday afternoon. Probabilistic guidance is also mostly dry...although a few members still do spit out a little precip. With that in mind, have backed off PoPs Thursday morning, though I'm not quite ready to pull PoPs entirely. If there is any precip Thursday morning, forecast soundings suggest it could be a mix of sleet and snow, and then change over to rain as low level warm advection ramps up. If there is any precip, all indications are that it will be light and accumulations will be minor. Friday through Sunday still looks wet as the next strong storm system develops over Colorado and moves across the Great Plains into the Midwest. Currently it looks like this system will take a similar track as Tuesday's storm, and that it will be at least as strong. ECMWF and GFS are in good agreement with the warm front moving through Missouri and Illinois Friday night, and then the cold front moving through Saturday afternoon and evening. All deterministic and probabilistic guidance shows a solid period of likely precipitation Friday through at least Saturday afternoon. Guidance isn't showing much in the way of instability yet, so have left mention of thunder out for now. There may be lingering rain on Sunday as well as the stacked low pressure system tracks through Iowa and northern Illinois. Carney && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1120 PM CST Sun Nov 24 2019 VFR flight conditions through the forecast period with light south to southwest winds veering to the west as weak cold front sinks through by Monday afternoon. Front will then stall out over St. Louis metro area with winds becoming light and variable Monday evening. Byrd && .PRELIMINARY POINT TEMPS/POPS... Saint Louis 68 45 60 42 / 0 20 70 80 Quincy 59 37 55 38 / 0 0 80 80 Columbia 65 42 59 36 / 0 20 70 60 Jefferson City 66 42 61 37 / 0 20 60 50 Salem 61 44 58 42 / 0 10 90 90 Farmington 62 46 61 39 / 0 40 80 70 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX