062 FXUS64 KMRX 141447 AFDMRX AREA FORECAST DISCUSSION National Weather Service Morristown TN 947 AM EST Mon Dec 14 2020 .UPDATE... MORNING UPDATE. && .DISCUSSION... The axis of the vigorous mid/upper shortwave trough and H5 vort max is quickly moving E this morning with the surface low moving across central and eastern VA. This is allowing the deformation band of precip to move E at a good clip. Scattered snowfall reports have been received this morning on the northern Plateau in Scott County as well as around Lonesome Pine (LNP) in SW Virginia. The Newfound Gap webcam also shows snow falling. Based on these reports, current temps in the low/mid 30's across several higher elevation sites, and the radar melting layer, I think it's safe to assume that precip above 3000 feet is primarily snow this morning. The back edge of the precip is now roughly along the I-75 corridor, so current snowfall amounts look reasonable. Will allow the Winter Weather Advisory for elevations above 3000 feet in SW Virginia to expire as scheduled at 18Z since that area still has a couple hours of precip left. Adjusted PoPs to end slightly faster from W to E. Drier air as shortwave ridging and surface high pressure build in from the W will allow most areas to dry out after 17Z except for a few lingering flurries in the E TN mountains. Garuckas && .AVIATION... 12Z TAF DISCUSSION. Rain continues to spread across the area this morning, making for messy aviation conditions. Cigs across the region are highly variable, but the predominant condition through most of the day will be MVFR at the TAF sites. A northerly flow is expected to be gusty at times today, and the feed of colder low level air against the mountains will help maintain a stratus deck at TRI and TYS into the evening, perhaps through most of tonight at TRI. CHA is expected to return to VFR late in the afternoon. DGS && .PREV DISCUSSION... /ISSUED 336 AM EST Mon Dec 14 2020/ SHORT TERM (Today and tonight)... A low pressure center currently over northern AL near the GA border will track NE along a baroclinic zone across western NC this morning. As the midlevel frontogenetic forcing zone over Middle TN now moves east and closer to this frontal boundary, the northerly low level flow will strengthen and draw colder low level air into our area. Current obs in SW VA and eastern KY have temps mostly in the upper 30s, with some KY Mesonet sites in higher elevations showing mid 30s. Dual-pol radar products show the freezing level around 6500 ft in SW VA at the moment, a little higher than depicted by models. But as the aforementioned cold advection kicks in, the freezing level will drop below the higher elevations of SW VA, likely around 12Z. Precip rates at that time will still be fairly high due to strong QG forcing, so snow may accumulate quickly for the first few hours, with light snow continuing until around noon when midlevel dry air builds in. Uncertainty remains about how quickly the freezing level will drop - obviously a faster drop will result in more snow accumulation. In any case, it appears likely that any accumulation will be confined to elevations at or above 3000 feet this morning, with expected amounts in the 1-4 inch range. A Winter Weather Advisory has been issued for locations above this level in SW VA, including High Knob. The temperature drop will occur later in the East TN mountains, reaching the mountaintop level between 12-15Z, so less snow accumulation is expected there. Will let the ongoing SPS continue for that area for 1-2 inch amounts above 3300 ft. The highest peaks of the northern Cumberland Plateau may also see an around an inch. With a northerly flow and cold advection continuing, low stratus clouds will likely remain dammed up against the mountains in the northern and central Valley into this evening, with partial clearing in the southern Valley. A lighter and more northeasterly flow develops late tonight that should allow for at least some partial cloud dissipation. DGS LONG TERM (Tuesday through Sunday Night)... Tuesday and Tuesday night looks to remain dry as a storm system organizes to the west. Highs on Tuesday will remain near average with average lows Tuesday night. A h50 trough digs into the central plains and middle Mississippi River Valley late Tuesday and into Wednesday. Moisture will increase ahead of this feature early Wednesday as jet dynamics begin to approach the area. Rain will increase from west to east on Wednesday. A low level jet will ramp up ahead of this system which will drive a warm nose north into much of the area. Guidance is in good agreement with the low-level thermal profile with 925-700mb max temperatures between 2-3 degrees across much of the area Wednesday morning with an area of 0C to 1C likely across portions of southwest Virginia and extreme northeast Tennessee. The better chance of mixed precipitation will be across portions of southwest Virginia and extreme northeast Tennessee with sleet, and freezing rain a possibility. Enough warm air is expected to advect northward to change most of this to rain by the afternoon. Much cooler lower tropospheric temperatures move in on the backside of this system late Wednesday night and into Thursday. This would result in any lingering precipitation to change over to light snow. Amounts look limited as most of the QPF will have lifted out of the area but the current forecast does have up to an inch of snow across much of southwest Virginia with higher amounts of up to 2 inches across the highest elevations. By Thursday, most of the precipitation should have lifted out of the area however the GFS and CMC still depict light QPF possible on the back side of the system. Temperatures on Thursday will drop to below average with a northwest flow regime aloft along with increased cloud cover. Surface high pressure underneath building heights aloft will bring much drier conditions to the region. Southerly flow on the southwest periphery of the surface high will allow for temperatures to moderate with highs on Friday and Saturday back to near normal. This weekend...another system will quickly dive south into the region with a cold front extending south from the Great Lakes. Moisture and precipitation will move into the area on Saturday with the highest PoP's right now between 00Z Sunday and 12Z Sunday when Q- vector convergence becomes maximized. Temperatures look warm enough for this to be mostly rain across a good portion of the forecast area, however, colder air on the back side of the system could allow for a brief window of rain or snow mainly across southwest Virginia, and extreme northeast Tennessee. Temperatures Saturday and Sunday will run near to slightly below average. Diegan && .PRELIMINARY POINT TEMPS/POPS... Chattanooga Airport, TN 49 31 52 39 47 / 40 0 0 50 70 Knoxville McGhee Tyson Airport, TN 47 27 49 35 45 / 70 0 0 50 80 Oak Ridge, TN 47 28 49 34 45 / 70 0 0 50 70 Tri Cities Airport, TN 43 23 49 30 43 / 90 0 0 40 80 && .MRX WATCHES/WARNINGS/ADVISORIES... NC...NONE. TN...NONE. VA...NONE. && $$