Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 081822 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 122 PM EST Fri Dec 8 2017 .SYNOPSIS... Low pressure will develop along a frontal boundary over the Southeast United States today and move northeast and off the Mid- Atlantic Coast into Saturday. This will produce an initial mix of rain and snow this afternoon, changing to all snow tonight into Saturday where the cold air interacts with the system over parts of the Mid- Atlantic States. Greatest snowfall accumulations are expected across the southern Piedmont in Virginia and North Carolina, and the Blue Ridge and southern foothills in North Carolina. A stronger front with a shot of colder air and better chances for mountain snow showers will arrive Saturday night into Sunday. && .NEAR TERM /THROUGH TONIGHT/... As of 1200 PM Friday... Digesting new 12z model guidance which have trended higher with respect to snowfall amounts along the Blue Ridge from Floyd into the NC Mountains. Headlines will likely need some adjusting from Floyd-Watauga with respect to amounts that may push warning criteria. Looking at webcams...very little accumulation is occurring east of the Blue Ridge where some mix of both rain/snow is occuring Danville and Vicinity and ground temps still warm. Will make overall adjustments this afternoon. Only change to forecast so far is to raise amounts along Blue Ridge from Floyd to Boone. Reports so far indicate 1 to 3 inches has occurred within that corridor. Low pressure is organizing over northern Gulf of Mexico, aided by a parent mid-level trough evident in water vapor imagery over southwest Texas. Stream of subtropical moisture continues to emanate along the northern periphery of a stalled SW-NE oriented frontal zone over the Deep South. Nearly all 00z guidance trended much wetter, nearly by a factor of 2 compared to 12/18z solutions from Thursday. Key in that wetter trend is the faster digging northern-stream trough making headway in the Canadian Prairies, offering the potential for at least some (though not fully complete) phasing of this northern stream with the aforementioned southern stream trough expected to be in the Tennessee Valley later tonight. Though the Advisory takes effect at noon, the most impactful period based on current guidance indications is overnight tonight into early Saturday. There`s still a considerable deal of uncertainty on snow accumulations, involving multiple angles: (1) likely warm pavement temperatures limiting impactful accumulations until better, frontogenetically-enhanced precip rates can force more snow (2) when snow actually ends (3) Snow to liquid ratios (SLR) values should be initially low due to rain/snow mix falling in marginally supportive temperatures and isothermal thermal structure aloft, before increasing to approximate 10:1 values overnight and (4) the westward extent of the QPF shield, given likelihood of dry air potentially eroding significant snow accumulations from the Blue Ridge westward than guidance consensus QPF presently indicates. So with that stated, for today we`ll be looking at precipitation moving northward into VA from the Carolinas with precipitation type initially a cold rain across the piedmont. Wet-bulb effects should produce a mix of rain and wet snow during the afternoon across the piedmont and all snow the closer you get to the Blue Ridge. As mentioned, I`m highly dubious that wet snow will accumulate on anything other than grassy, elevated or shaded surfaces through the daylight hours over the Piedmont where ground temperatures too warm to permit accumulations on pavement/asphalt. It will be a different story along the Parkway and elevations above 1500 feet where both ground and air temperatures are colder. As the afternoon and evening progresses, a northwestward expansion of the precipitation shield is anticipated, while the strongest 850-700 mb frontogenesis begins to pivot northwestward from the central Piedmont of NC. Precipitation rates should begin then to increase, and rates should be sufficient enough to overcome warm pavement and allow for greasy accumulations on pavement beginning tonight in the Advisory area. In the Advisory area, several inches of snow may accumulate, roughly from one to three inches in the Piedmont/foothills and 3-6" along the southern Blue Ridge in NC and into Grayson, Carroll, Floyd,and Patrick Counties...although these numbers may need to be adjusted upward pending model trends. Much of this will fall tonight into Saturday (into the Short- Term period). From an impacts perspective, it appears that the evening period would be the beginning of the most adverse of impact with the likelihood of a wet, greasy snow as temperatures fall after sunset. && .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... As of 100 PM EST Friday... As the coastal low pulls away from the region late Saturday, a cold front will move through the region Saturday evening. This will bring a reinforcing shot of cold air to the Appalachians and central mid Atlantic region and set the stage for upslope snow showers west of the Blue Ridge with a few inches of fluffy snow expected at the higher elevations. The low level wind field will also amplify considerably with very blustery conditions leading to areas of blowing snow and low wind chill values. Accumulating snow showers will taper off to flurries Sunday morning but blustery conditions look to continue through the first part of next week as the wind field remains energized by numerous short waves zipping through fast upper level flow. Temperatures will remain well below normal into the first part of next week. Many locations will see departures of 10 to 15 degrees below seasonal norms. && .LONG TERM /TUESDAY THROUGH THURSDAY/... As of 100 PM EST Friday... Amplified upper pattern featuring a western ridge and eastern trof will persist through next week, though some deamplification looks to occur toward the end of the week. The main storm track will keep areas of low pressure moving from the Great Lakes into New England with trailing cold fronts pushing reinforcing shots of cold air into the Appalachians with occasional bouts of snow showers west of the Blue Ridge. The fast moving pattern will also keep a good amount of energy in the low level wind field with blustery conditions through the period. With little change to the cold pattern in place, expect temperatures to remain well below normal through the upcoming week. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... As of 1245 PM EST Friday... Radar indicates bands of light to locally moderate snow falling across northern North Carolina and portions of Virginia east of the Blue Ridge this afternoon, with visibilities within the snow fluctuating between 1/2SM to 3SM. Further north & west, a sharp gradient exists between the IFR conditions and VFR conditions, as moisture associated with the approaching storm system slowly intrudes into the dry air that was in place across the mid Atlantic. Snow will move across the area in waves Friday afternoon through shortly after dawn on Saturday. Most heavily affected airports will be those east of a HSP to MKJ line, with impacts expected to decrease further west, areas further removed from the path of the storm. Greatest hazards will include up and down visibilities in the heavier bands of snow, as well as ceilings falling below 1kft. Expect improving conditions after 09/18Z from west to east, as low pressure begins to drift away from the region and winds shift more northwesterly. Medium confidence on aviation elements and timing. Extended Discussion... After the passage of low pressure on Saturday, a trailing arctic cold front is expected to bring another round of snow showers to the mountains, with potential sub-VFR ceilings and intervals of sub-VFR visibility, mainly to Bluefield and Lewisburg. Strong northwest winds are expected behind the front on Saturday night into early Sunday. VFR conditions then return Sunday evening through Monday. Next chance for sub-VFR conditions is with a clipper system Tuesday, particularly in the western mountains. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Winter Weather Advisory until 4 PM EST Saturday for VAZ017-033- 045>047-059. Winter Weather Advisory until 4 PM EST Saturday for VAZ015-016- 032-043-044-058. NC...Winter Weather Advisory until 4 PM EST Saturday for NCZ001>006- 018>020. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...RAB LONG TERM...JH/RAB AVIATION...AL/NF

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