336 FXUS63 KLSX 292341 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 541 PM CST Tue Jan 29 2019 .SHORT TERM... (Through Late Wednesday Night) Issued at 315 PM CST Tue Jan 29 2019 Bottom line up front: Latest high resolution and ensemble guidance is showing slightly colder temperatures tonight than the previous forecasts, so we've adjusted accordingly. With that, the Wind Chill Warning has been extended south to include much of the St. Louis metro area. Additionally, we've expanded the portion of the warning that lasts into Thursday. While the area warned through Thursday should easily reach into warning criteria tomorrow morning, lighter winds on Thursday morning will likely keep the wind chill between 15 and 25 below in many locations. Rather than having both a warning and advisory in effect for the same areas, we instead have gone with only a warning, which for many areas may be adjusted to an advisory tomorrow. Onto the discussion: A deep upper level low is currently centered over northern MN. The associated surface low is over the Great Lakes, with a cold front trailing southwest through roughly St. Louis as of 2pm. Widespread clouds and light radar returns have blossomed along the front in the last few hours in response to slightly better moisture along and south of the Missouri River. Thanks to very dry air at the surface, much of this snow is not reaching the surface, but we have had a few reports of light snow across the region. The front will continue to push south through the afternoon, and snow will likely become more widespread as it moves into even better moisture. However, snow will remain light, and fairly short residence times will limit snow accumulations to under and inch. The bigger impact of this frontal passage will be a reinforcing shot of very cold air, bringing sub-zero temps to much of the area tonight. Furthermore, winds will remain elevated through the night, which coupled with the very cold temperatures will result in dangerously cold wind chills. Very cold temperatures will remain in place through tomorrow as arctic high pressure settles over the area. Fortunately with the high overhead, winds will diminish and boost wind chill readings somewhat. Meanwhile, a shortwave embedded in the NW upper level flow will drop through the Great Plains, spurring weak lee cyclogenesis. A warm front will extend eastward from the weak low, with the upper reaches of the front pushing into central MO by tomorrow afternoon. This may bring on yet another bout of light snow for areas west of the Mississippi through the evening. Despite warmer air arriving in the mid levels, surface temperatures will remain quite cold into Thursday morning. BSH .LONG TERM... (Thursday through Next Tuesday) Issued at 315 PM CST Tue Jan 29 2019 The upper pattern that has yielded our current outbreak of very cold air will continue for the first part of this period: a longwave upper TROF over the eastern CONUS and a longwave upper RIDGE over the western CONUS. The Arctic airmass will be slow to pull away on Thursday and also we are expecting a lot of cloud cover as easterly surface winds begin to pick up some. Factoring in everything, it appears dangerous level wind chills of 15 below or lower will hang on through the morning hours of Thursday for much of northeast MO and central IL, still constituting the single wind chill "event". Thus, the wind chill headlines for these areas have been extended until noon Thursday. One final clipper system is progged to drop down in the resultant NW flow aloft with most of the energy associated with it missing our region to the north. The portion of this system that will impact the forecast area will be the surface frontal boundary being dragged southward through, not unlike the event this afternoon. The precipitation for most of this event is expected to fall as snow, but again, with most of the energy passing to the north of our region, this should also keep any accumulations to a minimum (dusting or less). The frontal boundary will settle and stall just to our south late Thursday night as a weak wave of low pressure slides along it, maintaining the threat for light precipitation for areas south and east of STL metro until close to sunrise Friday morning. However, as cloud ice is expected to be lost by this point, the light precipitation will attempt to briefly transition from light snow to freezing drizzle before ending. At this point, all of the models forecast a major pattern change heading into the weekend and early next week. This pattern will overall feature progressive flatter flow, bottling up the cold Canadian air for a time, and instead, steering milder Pacific airmasses into our part of the country. Weak upper ridging will first build in for the weekend and Monday, and coupled with southerly flow, should result in much milder temperatures. The MEX MOS does have max temps well into the 60s for Sunday and Monday, but concerns at this time revolve around stratus cloud development, limiting any major warmup. If the GFS is right, a great opportunity may avail itself on Monday afternoon as the dry slot of a system pushes through, but that is also 6 days out, so it is difficult to zero in on those little details right now. Did introduce low 60s into some areas for Sunday and Monday for now. Some light rain chances are possible during the weekend, but a better chance should exist on Monday with a stronger system and front affecting the area. Precipitation-types from Friday night through early Tuesday should be all rain. The mild pattern may already be breaking down by next Tuesday as a deep upper TROF attempts to build into the central CONUS on the heels of another storm system rolling through. The initial look at this system favors mostly rain, but it may end as snow for many areas as colder seasonable air returns to the region. TES && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 523 PM CST Tue Jan 29 2019 A small band of flurries and light snow has developed impacting KSUS/KCPS. This snow may linger for up to an hour at KCPS, but is expected to have pushed south of KSUS by 00z. Clouds will also continue to push south this evening, leaving much of tonight clear at terminals. Guidance is suggesting a small band of low VFR cigs will push south late tonight into Wed morning. Confidence is somewhat low regarding coverage of these clouds and have kept at SCT for now. Winds will diminish Wed morning as the surface ridge builds into the region. Another round of light snow is possible late Wed afternoon into the evening and may impact KCOU. Currently expect the bulk of this snow to remain south of the terminal, but small shifts in the location are possible and will continue to monitor. SPECIFICS FOR KSTL: Expect ongoing light snow to push south of the terminal by 00z. Strong NW winds will slowly diminish Wed morning. Remainder of TAF period expected to be VFR. Tilly && .CLIMATE... Record lows and record "low" high temperatures Wednesday (Jan. 30) Thursday (Jan. 31) St. Louis -9 (1936)/ 10 (2004) -9 (1936)/ 10 (1951) Columbia -9 (1936)/ 8 (1965) -12 (1936)/ 5 (1985) Quincy -13 (1965)/ 1 (2004) -14 (1996)/ 3 (1985) && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Wind Chill Advisory until noon CST Wednesday for Cole MO- Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO- Madison MO-Moniteau MO-Osage MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. Wind Chill Warning until 3 PM CST Wednesday for Audrain MO-Boone MO-Callaway MO-Lincoln MO-Montgomery MO-Saint Charles MO- Saint Louis City MO-Saint Louis MO-Warren MO. Wind Chill Warning until noon CST Thursday for Knox MO-Lewis MO- Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO. IL...Wind Chill Advisory until noon CST Wednesday for Monroe IL- Randolph IL. Wind Chill Warning until 3 PM CST Wednesday for Bond IL-Calhoun IL-Clinton IL-Fayette IL-Jersey IL-Madison IL-Marion IL- Saint Clair IL-Washington IL. Wind Chill Warning until noon CST Thursday for Adams IL-Brown IL- Greene IL-Macoupin IL-Montgomery IL-Pike IL. && $$ WFO LSX