154 FXUS64 KMRX 122038 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 338 PM EST Sat Dec 12 2020 .SHORT TERM (Today into Tomorrow)... Showers are currently moving through the eastern Tennessee Valley this afternoon, as a cold front moves through the region. Coverage of showers remains scattered, especially in areas north of the Interstate 40. Expect these light showers to continue through the rest of the day, and QPF forecast remains relatively unchanged with most locations expected to see less than 0.25 inches of rain, and closer to 0.15 inches in northeast Tennessee and southwest Virginia. Expect the front to move east of the mountains by midnight tonight, and behind it winds will take on a more westerly component overnight into tomorrow. This will help to start clearing out some of the cloud coverage overnight which should present an opportunity for fog formation tomorrow morning. Expect the best chances for fog to be in areas south of Interstate 40 where more rain occurs, and west of Interstate 75 where skies should be more clear before sunrise. Still opportunities for fog formation in other areas, but expect southeast Tennessee to see the most widespread fog overnight. Expect the drier air to continue to be in place tomorrow which will lead to mostly sunny conditions across the area, and allow for more sun to make it to the surface. Temperatures tomorrow are expected to be similar to today, and maybe just a few degrees cooler in some spots. ABM .LONG TERM (Sunday Night through Saturday)... Key Messages: 1. Rain Sunday night into Monday morning with snow in the mountains of far E TN and SW VA mainly above 5000 ft, but snow amounts have decreased. 2. A second system will bring a rain/snow mix Tuesday night into Wednesday, but confidence on snowfall amounts is low. 3. Mainly dry late week into the weekend with below normal temps warming to near normal by the weekend. Discussion: Sunday Night and Monday... The mid/upper trough that will be the weather maker for this period is currently crossing the Intermountain West. This feature will reach the MS Valley by Sun night with phasing of upper jets between the northern and southern stream producing a 130-140 kt H3 jet streak over the OH Valley Sun night. The mid/upper trough will tilt negative as it lifts NE late Sun night into Mon, and this combined with the strong upper divergence beneath the right entrance region of the upper jet streak will promote CVA resulting in a strengthening surface low moving NE from the deep South through the Carolinas. Snowfall Guidance has trended warmer overall with the NAM and SREF continuing to be the farthest NW with the surface and 850 lows (over extreme SE TN) compared to the GFS, ECMWF, and Canadian which take the surface low from GA into NC/SC. This results in the temp fields on the NAM and SREF being much warmer with 850 mb temps as high as 8-9C advecting across the Smokies and central and southern valley late Sun night, which would keep all areas rain, even the mountain peaks. Despite the other guidance being farther SE with the surface/850 low tracks, they have even trended warmer as previously stated. All guidance has 850 mb temps above 0 and thicknesses well above 540 prior to 12Z Mon, so feel confident that all lower elevations will see rain through the night with a change to snow over the far E TN and SW VA mountains generally above 5000 ft toward sunrise as colder air begins to wrap in behind the system allowing 850 mb temps and wet bulb temps to cool enough for snow in these higher elevations. The surface low will move off the VA coast Mon afternoon with the parent upper dynamics shifting E as well. The loss of upper support will quickly weaken low and mid level frontogenetic forcing, so expect precip to end quickly as the cold air filters in Mon morning. This will greatly limit snow potential. HREF members are on-board with this idea suggesting a rapid decrease in widespread, heavy precip Mon morning, so could see a few flakes in the valley during the morning, but no accumulation is expected outside of the mountains. Lowered snow totals to 2-4 inches across Smoky Mountain peaks above 5000 ft with generally a trace to 2 inches across the rest of the E TN and SW VA mountain peaks above 5000 ft. Rainfall Amounts In terms of rainfall totals, the aforementioned forcing combined with strong moisture advection will lead to periods of moderate to heavy rain Sun night into early Mon with amounts of 1.25 to 1.75 inches before the precip exits by mid morning Mon. Winds As the mid/upper trough axis pivots ENE, a 50-70 kt southerly 850 mb jet in the 09-12Z timeframe will lead to a brief period of strong winds gusting over 50 mph in the E TN mountains, but the flow not well aligned perpendicular to the terrain for a long period of time should limit this to an advisory event. Monday night through Tuesday... Mid/upper heights will quickly rebound in this period ahead of the next trough crossing the Rockies leading to dry conditions, but a cold surface high ridging in from the Great Lakes will maintain below normal temps. Tuesday night through Wednesday... The aforementioned Rockies mid/upper trough will reach the MS Valley Tues night and progress into the OH Valley and Great Lakes Wed while phasing with the northern stream, with additional shortwaves behind it leading to broad troughing taking hold of much of the CONUS late in the week. At the surface, the old baroclinic zone/front pushed south behind the early week system will serve as a focus for a developing surface low, with a general track consensus from the Ark La TX Tues night across the S or central Appalachians Wed as a new coastal low forms off the Mid Atlantic coast by Wed evening. This looks like a classic Miller B Northeast U.S. storm set up. In terms of impacts for us, colder air and lower thickness values at the onset of the isentropic lift/jet dynamic forced precip Tues night may allow most areas to start as snow or rain/snow mix, but GEFS and ECMWF ensemble members show large spread in how quickly WAA pushes the rain/snow line N and E early Wed, with a cold wedge over the Carolinas further complicating the temp fields. For now, kept rain/snow mix Tues night gradually transitioning to rain Wed, but did not dive into any details of snow or any mixed precip. Wednesday Night through Saturday... A large surface high building into the OH and TN Valleys in the wake of the system Wed night through Fri with troughing aloft will bring dry weather and below normal temps, rebounding to near normal Sat as the surface high shifts E and shortwave mid/upper ridging builds ahead of the next system. Garuckas && .AVIATION... 18Z TAF DISCUSSION. Showers will continue to move eastward through the day, leading to scattered showers. Rain won't be continuous, so went with a VCSH and a TEMPO group to account for the highest chances of when rain occurs. We have a good chance to see a round of fog in the morning, but didn't mention these in the TAFs to keep the length of them to a minimum. && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 46 61 42 50 31 / 20 20 100 20 0 Knoxville McGhee Tyson Airport, TN 48 60 41 48 29 / 30 10 100 40 0 Oak Ridge, TN 44 59 40 47 28 / 20 10 100 30 0 Tri Cities Airport, TN 48 58 39 46 25 / 30 10 100 60 0 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$