670 FXUS63 KLSX 171148 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 548 AM CST Thu Jan 17 2019 .SHORT TERM... (Through Late Friday Afternoon) Issued at 333 AM CST Thu Jan 17 2019 A surface frontal boundary was draped across our forecast area, nearly bisecting it, extending from just south of Indianapolis, IN through the STL metro area and near Joplin in southwest MO. Surface temperatures range from at or just below the freezing mark several miles north of the front, including all of the current Winter Weather Advisory area, to the lower 40s for far southern MO. A developing wave of low pressure was located over eastern OK. Regarding the precipitation moving through our area currently, there is good model agreement on timing the upper level disturbance responsible, driving it east of the forecast area during the early afternoon. Already by 12z today, the main swath of precipitation is expected to be from the STL Metro area and areas to the south and east with more spotty precipitation elsewhere. Even this spotty precipitation is expected to end for areas north and west of the STL metro area by late morning and elsewhere in the forecast area by midday. Precipitation-types continue to look like a little of everything in the current Advisory area, with maybe a slight preference overall to freezing rain or drizzle. Surface temps are either right at the freezing mark or just a degree or two below across the Advisory area with a couple ice accretion sensors showing up to a tenth of an inch so far on elevated surfaces. The wave of low pressure in eastern OK will track northeast along the frontal boundary this morning and the effects of this will be to edge surface temperatures a bit higher. For areas in the current Advisory area, this should edge most locations just above the freezing mark, although there are indications that some areas in northeast MO will hang on around 32F for the entire morning. Fortunately, precipitation will be coming to an end much sooner and with this new timing, we are going to end the Winter Weather Advisory at 15z today. Regarding areas of fog currently across the forecast area, the lowest visibilities are presently around a half mile at several sites scattered across the CWA and there is some support that a Dense Fog Advisory may be needed for much of the morning across a wide section of the forecast area. Conceptually, there is good support with the wave of low pressure riding up through the southeast half of the CWA which should at least lower cloud ceilings and likely visibilities along with it and the timing is right as well, with early morning favorable for this. Will monitor this closely. Otherwise, clouds look to go nowhere through Friday afternoon with any breaks or sightings of the sun likely to be very short lived. Went below MOS on max temps the next couple of days and leaned heavily on the warm side of MOS for nighttime mins for tonight. Precipitation chances for the next system are already edging into the western CWA (sections of northeast and central MO) late Friday afternoon with PoPs ramping up significantly after sunset. TES .LONG TERM... (Friday Night through Wednesday) Issued at 333 AM CST Thu Jan 17 2019 We've still got quite an active pattern in the extended forecast, including a potential winter storm on Friday/Saturday, bitter cold on Sunday/Monday, and another chance of snow by mid week. Attention remains primarily focused on the upper level trough forecast to push into Great Plains on Friday and through the Mississippi Valley on Saturday. This will result in strong cyclogenesis, driving a surface low from the OK Panhandle on Friday to western AR by 12am Sat. The associated warm front will become established over the mid-Mississippi Valley on Friday night, kicking off precipitation. By Friday evening, most guidance shows the surface freezing line draped roughly along I-70, delineating rain to the south and wintry precip to the north. With the warm front overhead, guidance continues to show above freezing temperatures atop the freezing surface temperatures across a portion of the area. Fortunately, maximum wet bulb temps aloft in this area are generally progged to be less than 3C, which would result in primarily a snow/sleet mix, limiting the freezing rain potential. The upper level trough will then swing through the Mississippi Valley on Saturday, through some differences remain in the timing of the trough passage. The Euro is a bit slower and cooler, which if correct, would result in a longer-lived deformation band on the back side of the surface low. The Euro is also more in line with ensemble means, so have leaned on that for QPF and the thermal profile for this storm. Snowfall amounts have trended downward a bit from the previous forecast, and this illustrates the uncertainty with this system. The storm track has only changed slightly, but this has resulted in myriad changes. Compared to previous forecasts, the area that will see snow through the duration of the event has shrunk. Additionally, we'll also see more wintry mix within the area of heaviest QPF, thus reducing our highest snowfall amounts. We'll likely see additional changes closer to this weekend as forecast guidance adjusts to upstream observations. Cold air will rush in behind the low, resulting in lows falling into the teens and single digits on Sunday morning. Additionally, gradient flow behind the low will remain relatively strong, pushing wind chills to near 0. Beyond that, guidance remains a bit out of whack on the upper level pattern. The GFS flattens the ridge and pushes the arctic air out a bit faster than other guidance and therefore shows lows on Monday morning 10-15 degrees warmer than the Euro and GEM. Nonetheless, it still looks to be quite cold on Monday morning as well. Finally, attention turns to next week when yet another cyclone is forecast to push through the Midwest. Unsurprisingly, the GFS remains more progressive than other guidance, bringing the upper trough through the area on Tuesday morning, about 12-24 hours earlier than other guidance. Nonetheless, all guidance does show a low moving through the region, so precipitation will likely occur at some point. With cold air in place behind the arctic high, temps will likely be cold enough for at least a portion of this precip to fall as snow. BSH && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 548 AM CST Thu Jan 17 2019 IFR conditions and light surface winds will prevail at the TAF sites for much of the valid period. The IFR conditions for most of the period will come from ceilings, but for this morning, it will also be from visibilities in fog and drizzle. Visibilities are expected to improve during the late morning. TES && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Dense Fog Advisory until 10 AM CST this morning for Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Jefferson MO-Moniteau MO-Montgomery MO-Osage MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. Winter Weather Advisory until 9 AM CST this morning for Audrain MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Monroe MO-Pike MO- Ralls MO-Shelby MO. IL...Dense Fog Advisory until 10 AM CST this morning for Bond IL- Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe IL- Randolph IL-Saint Clair IL-Washington IL. Winter Weather Advisory until 9 AM CST this morning for Adams IL- Brown IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL- Montgomery IL-Pike IL. && $$ WFO LSX