954 FXUS64 KMRX 081533 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 1033 AM EST Sun Dec 8 2019 .UPDATE... MORNING UPDATE. && .DISCUSSION... No real changes to the forecast for today. Biggest changes were to tweak hourly temps and dewpoints to match up with latest obs. Areas along the foothills of the eastern mountains are currently warmer than most valley locations. This is due to the light southeasterly surface winds in place causing some minor downsloping. Clouds will slowly increase from southwest to northeast this afternoon. SR && .AVIATION... Southerly winds will develop in the low levels above the surface and then work their way down to the surface beginning this afternoon. Clouds will be increasing and lowering through the forecast period, but thinking VFR will persist at the sites for the period. Winds aloft will be increasing at 2kft AGL from 09/08Z thru 09/12Z, but will not yet include any mention of LLWS in that time window. && .PREV DISCUSSION... /ISSUED 342 AM EST Sun Dec 8 2019/ SHORT TERM (Today and Tonight)... H5 zonal flow begins to become more southwesterly as troughing develops to the west. Southerly low level flow by tonight will bring up increased moisture and broad lift that will produce scattered showers tonight. Temperatures today to continue mild, but the increased moisture tonight will be reflected in min temps 10-15 degrees above normal mins. May get some mountain wave winds developing overnight but will still not introduce any advisories at this time as ingredients are still marginal. LONG TERM (Monday through Sunday)... By daybreak Monday the forecast area should be firmly in the warm sector of a maturing mid latitude cyclone, moving northeast through Illinois and into Michigan. Southwesterly flow through the low levels of the atmosphere will mean plenty of cloud cover and unseasonably warm temperatures. Model soundings reveal plenty of low level moisture as well on Monday, and while some showers will likely be seen in favorable south-facing upslope regions of the Appalachians, expect much of the area will see a dry day on Monday as the best isentropic ascent will be to our north and west. Significantly lowered PoPs to account for this persistent trend in rain chances over the last couple of nights. Monday night into Tuesday however, the incoming cold front associated with this system over the Great Lakes will be approaching from the west and rain chances will increase dramatically. By this time, a +150kt jet streak will be pivoting eastward from the central plains and upper midwest. Stronger forcing for ascent will mean a notable increase in rain chances, with categorical PoPs across the CWA by Tuesday morning. The cold front will sweep into the Tennessee Valley by mid Tuesday morning, leading to a non-diurnal temperature trend for Tuesday. Highs on Tuesday will likely occur during the overnight hours, with temperatures falling through the afternoon behind the front. Despite the frontal passage, precipitation will linger well behind the front for an anafront type setup. Model soundings show a strong surface-based cool down behind the front, but keep an elevated warm nose around until well into Tuesday evening or perhaps Tuesday night. Had previously been using a straight surface temperature threshold for precipitation type, but now that we're withing 48 hours of this event, decided to add in some detail. The gist of it is, expect a mix bag of ptypes Tuesday evening and Tuesday night, with rain changing to freezing rain, to sleet, and finally to snow in fairly quick succession. Don't get too hung up on all the winter precipitation types - given how warm air and ground temperatures will be during the days preceding this event, and how much QPF is expected in the sub-freezing air, this should not be an impactful event. That said, would not be surprised if some higher elevations locales pick up an inch or possibly two before all is said and done. For Wednesday onward, no significant changes were made. By Thursday models continue to diverge with their handling of the upper pattern, with the ECMWF taking a shortwave trough very far south into the western Gulf (seemingly to the Bay of Campeche) while the GFS is notably quicker and further north towards the Gulf coast. Cut back PoPs a bit for Thursday night, and also cut back on the mention of any wintry weather there as well. Otherwise, left much of the rest of the forecast the same. Not sure how much value there is to be added by making drastic changes given the uncertainties at hand. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 56 49 64 58 59 / 10 40 30 70 80 Knoxville McGhee Tyson Airport, TN 58 46 63 58 58 / 0 50 40 70 80 Oak Ridge, TN 58 46 61 56 57 / 0 50 40 70 80 Tri Cities Airport, TN 54 41 61 55 56 / 0 60 40 50 80 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$