190 FXUS63 KLSX 012344 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 544 PM CST Fri Mar 1 2019 .SHORT TERM... (Through Late Saturday Afternoon) Issued at 347 PM CST Fri Mar 1 2019 A weak ridge of high pressure centered over eastern Arkansas and extending up the Mississippi Valley will drift southeast tonight. Another, much stronger high centered over southwest Canada will begin nosing into Missouri and Illinois tonight, bringing the leading edge of the cold air down into southern Illinois and southeast Missouri by early Saturday morning. Upstream temperatures in Iowa, Nebraska, and South Dakota are generally in the 20s at this time on the leading edge of the high, and this agrees well with MOS forecast lows for Saturday morning across our area. There is light precipitation showing up on regional RADARs over northern Missouri and Iowa up into Minnesota and Wisconsin associated with a shortwave that's moving across the Midwest. There haven't been many reports of the precipitation reaching the ground in Iowa and none in Missouri, but I can't rule out a few sprinkles or flurries this evening across northeast Missouri and west central Illinois. The cold advection weakens a bit on Saturday as the low level flow veers a bit from the northwest to the north-northeast. This will allow for a least a modest diurnal rise in temperatures into the afternoon. However the guidance looks too warm (especially the MAV) given the northerly flow and likely overcast sky. I leaned toward the colder guidance for highs Saturday which yielded highs around 30 in northeast Missouri to the low and mid 40s from St. Louis and further south. There may be some sprinkles or flurries in the afternoon due to increasing low and mid level frontogenesis ahead of Saturday night's storm system. I do not expect any of this precipitation to be heavy enough to accumulate as forecast soundings show a dry layer around 850mb as well as a dry boundary layer. Carney .LONG TERM... (Saturday Night through Next Friday) Issued at 347 PM CST Fri Mar 1 2019 Our main focus this forecast remains centered on the winter storm for Saturday night and Sunday, followed by very cold weather and well below normal temperatures through the first part of next week. Overall the new guidance suite is a tad further south and not quite as robust with QPF with the upcoming winter storm system for Saturday night into Sunday. It also looks like the onset of precipitation may be a bit slower. There is really a lack of much organized forcing on Saturday evening and thus for most areas I think it will remain dry. The possible exception is for perhaps the extreme southern portion of the CWA - southeast MO and far southwest IL. Across that area both the ECWMF and NAM are showing some light QPF for a portion of the evening, and that appears to be primarily in response to low level WAA. Thermal profiles suggest it could be a wintry mix on the NAM while the GFS and ECWMF would suggest a rain/snow mix. The real thrust of the winter storm still looks on track for overnight Saturday into Sunday morning or 06-18z Sunday. Forcing/ascent associated with the fast-moving migratory upper trof, increasing low-mid level frontogenetic forcing, and divergence with the upper level jet exit region should result in a large west-east oriented band of snow overspreading spreading the area from the west, entering central MO between 06-09z and reaching south central IL just before daybreak. While there could be a period of accumulating sleet across southeast MO and southwest IL where there are suggestions of an eroding elevated warm layer, diabatic cooling should change any precipitation over to snow by 12z. The snow should come to an end as the fast-moving upper trof zips by with measurable snowfall ending by early afternoon along the MS River and in our eastern CWA by 21z. A climatological snow band study by Gravelle indicates the median width of the 4+ inch snow band in these types of events is on the order of 85 nm. Meanwhile applying techniques of Browne and Younkin, the composite track of the 850 mb low would place the axis of heaviest snowfall from roughly the Lake of the Ozarks to around Mount Vernon, IL. This region is also where omega is briefly maximized within the dendritic growth zone on Sunday morning. Overall liquid to snow ratios will increase during the event, primarily due to CAA. This currently looks like a widespread winter weather advisory level event with snowfall amounts ranging from 2-3 inches across northeast MO/west central IL to 4-5+ inches along and south of Interstate 70 with isolated 6+ inch amounts possible - likely within the aforementioned KAIZ-KMVN corridor. We are leaving the winter storm watch in place for now after collaboration, and to help with the uncertainty in the forecast for neighboring offices to the west. Hazardous travel is expected after the onset of snowfall/wintry precipitation late Saturday night and especially on Sunday morning with snowfall rates maximized at 1/2 to 1 inch per hour and due to the cold ground temperatures. North-northeast surface winds will also be increasing and become gusty during the morning and into the afternoon associated with the tightening pressure gradient and deepening surface low to the south-southeast of the region. These winds could produce some blowing and minor drifting of snow, adding to the hazardous travel. The period from Sunday night into Monday still looks exceptionally cold for early March with temperatures around 30 degrees below normal. While the winds will be diminishing late Sunday evening into Monday morning, they will still be high enough when combined with temperatures falling to near zero to produce dangerous wind chills across parts of northeast MO and west central IL. A deep long wave trof centered through eastern NOAM will keep it cold with below average temperatures into midweek. The GFS and ECMWF suggest there is potential for more wintry precipitation Thursday into Thursday night although the pathways getting there are a bit different. Glass && .AVIATION... (For the 00z TAFs through 00z Saturday Evening) Issued at 521 PM CST Fri Mar 1 2019 Main question will be cigs tonight. With the approach of a cold front, expect cigs to raise. However, MVFR cigs may redevelop Sat morning with what little heating is available. These cigs should lift through the morning, although, KUIN should remain lower but these low clouds may become scattered sooner than currently forecast. SPECIFICS FOR KSTL: Expect high MVFR or low VFR cigs to remain in place tonight. These lower cigs should move out of the region Sat morning and be replaced with mid clouds. Otherwise, light winds through the period. Tilly && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Winter Storm Watch from Saturday evening through Sunday afternoon for Audrain MO-Boone MO-Callaway MO-Cole MO- Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO- Lincoln MO-Madison MO-Moniteau MO-Monroe MO-Montgomery MO- Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO- Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. IL...Winter Storm Watch from Saturday evening through Sunday afternoon for Bond IL-Calhoun IL-Clinton IL-Fayette IL- Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL- Washington IL. && $$ WFO LSX