Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 071031 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 531 AM EST Thu Dec 7 2017 .SYNOPSIS... Mostly cloudy skies and dry conditions will continue today as an old frontal boundary remains stalled to our south. Later tonight and on Friday, a weak low pressure area may allow for rain mixing with snowflakes in the Piedmont/foothills areas. Another disturbance may spark snow showers with limited accumulation in western Greenbrier County. A stronger shot of colder air and better chances for mountain snow showers Saturday night into Sunday. Temperatures late in the week through the weekend will be below normal. && .NEAR TERM /THROUGH TONIGHT/... As of 500 AM EST Thursday... An elongated axis of mid to high level moisture extending into south Texas continues to spread across the central/southern Appalachians and into the Piedmont this morning, on the southern extent of broad cyclonic gyre over Hudson Bay. This mid and high-level moisture is running over a remnant stalled frontal zone from the lower Mississippi Valley into the central Carolinas. Our area remains governed by a weak but dry surface high pressure area, in spite of all the cloudiness. Overall, first part of Thursday should be a close facsimile to yesterday from a sensible weather perspective. Aside from some early breaks in cloudiness in southeastern West Virginia, look for a continued generous coverage of mid to high clouds, greatest further south. Clouds should prove enough to really limit how much insolation we receive today, which limits high temperatures today in the upper 30s to the upper 40s, with readings within a few degrees of freezing along the Blue Ridge/Alleghany Mtns. Weather then turns a little more interesting looking into the late afternoon and evening. A shortwave trough aloft over southern IL associated with a reinforcing shot of colder air should move to the Ohio Valley late in the day/into early evening. Though the trough is starved of moisture to work with, there should be enough lift to generate limited chances for snow showers in the western Greenbrier hillslopes, a solution reflected in the 00z 3-km NAM/ARW and in a coarser sense the 00z ECMWF. Given the moisture-starved nature of the front and forcing both being pretty scant & passing to the north, any accumulation here would be limited to a coating, ending before midnight. Other area is across the southern third of the CWA after midnight, as the low-level thermal gradient begins to tilt back northwestward in response to weak surface wave development in the Gulf. Moisture creeps back northwestward with time, supporting initial rain along or south of Route 52. Rain may mix with flakes at times given dynamic/wet-bulb cooling effects that push temperatures across the NC Piedmont/foothills and Pittsylvania/Henry Counties into the lower 30s. The dry surface air that should be in place will also lead to a defined SW-NE gradient in precip along rough line from West Jefferson to Brookneal. That said, precipitation rates are light enough and pavement temperatures likely mild enough to keep anything wintry from sticking to pavement. Grassy, elevated or shaded areas could see some mixed snowflakes stick but did not include any accumulations. In between these areas, look for continued overcast skies with dry conditions. Lows around 20 in southeast West Virginia, the mid 20s in the Alleghany Highlands, NRV and into far southern VA and the NC mtns, and between 30-35 in the Piedmont/foothills, generally warmest further southeast. && .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/... As of 430 AM EST Thursday... The cold front that moved through the area yesterday is now located across the southeast states. Deepening low pressure was anchored over the Great Lakes north to Hudson Bay. Troughing was evident from the upper south to the Gulf, with the axis located through the Mississippi Valley region, several hundred miles yet to our west. The eastern portion of the trough was serving to keep a baroclinic zone anchored along the southeast and Mid- Atlantic coast. This pattern will remain in place until a vigorous Alberta Clipper dives into the main upper trough later Friday into Saturday. The northwest extent of precipitation from the baroclinic zone to our east has been a contention point for the last several days. As of this morning the various short range and longer range models are in general consensus that the associated precipitation will just skirt the southeastern corner of CWA Friday into Friday night. This will primarily occur as the Alberta Clipper begins to approach from the northwest resulting in amplification of the eastern U.S. tough and allowing an area of surface low pressure to ride along the front through the southeast states. This will help to buckle the baroclinic zone slightly west late Friday until the clipper begins to dive into the Ohio Valley, which will finally boot the baroclinic zone out to sea. Any associated precipitation will be light and only a few hundreths of an inch, with the heavier precipitation remaining well south and east of our CWA. However, atmospheric temperature profiles will support a rain/snow mixture, or perhaps all snow in some locations. The warm ground temperatures from the recent extended period of above normal temperatures should preclude much if any snow accumulation late Friday/Friday night. Using a combination of model and WPC QPF along with typical snow ratios for these areas, have arrived at less than 1/2 inch of snow across Caswell into southeast Rockingham, with no accumulation indicated elsewhere. This is a bit lower than WPC`s near 1 inch amounts across this area which I just do not feel the QPF or warm ground temperatures will support. As we move into Saturday, the main concern will be the impact of the approaching Alberta Clipper. This system will have three impacts on the CWA, namely (1) bring in a significant surge of cold air for Sun into early Mon as 850mb temps bottom out around -12C Sunday, (2) potential for strong gusty northwest winds near advisory criteria along and west of the Blue Ridge, and (3) accumulating upslope snow showers Saturday into early Sunday across the Alleghanys and down through the mountains of southwest VA. Snow amounts with this event could approach 3 inches in locations such as western Greenbrier. Even though moisture is limited, strong CAA with the passing clipper will should support at least a brief period of rather robust snow showers. Temperatures will be the main story through the period, remaining cold and trending from just below normal to well below normal as 850mb temps plummet, especially behind the weekend clipper system, toward -12C, while hovering in the 0C to -6C prior to such. Surface temperatures will remain mostly below freezing west of the Blue Ridge Sunday behind the clipper, barely reaching the 40s across the Piedmont. In essence, temperatures will average about 10 degrees below normal over the weekend. && .LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 510 AM EST Thursday... The first clipper will lift out of the area fairly rapidly Monday allowing the flow to briefly become zonal and a slight moderating of the temperatures on Monday as a result. We may even see a few 50-degree readings across the Piedmont and 40s in the west. Meanwhile, the deep polar vortex will remain in place just north of the Great Lakes. Another very significant clipper system is progged to dive southeast into the eastern U.S. during the Tue- Wed time frame bringing another round of very cold air, gusty winds, and western mountain upslope snow showers. This air mass should prove to be the coldest in the series with 850mb temperatures bottoming out around -18C midweek. This could drive temperatures down into the single digits west and teens Piedmont with high temperatures remaining below freezing in many areas for potentially a 1-3 day period midweek, depending on location. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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As of 530 AM EST Thursday... Generally VFR, though may see brief periods of MVFR stratocu at Bluefield and along the western Alleghany Mtns through mid- morning. VFR mid-to-high clouds will continue to spill across the TAFs today, with west winds 6-10 kts. Toward later this afternoon into early evening, may see periods of snow showers along the western Greenbrier hillsides but VFR continues elsewhere. As a disturbance passes to the south later tonight, ceilings begin to lower to a BKN-OVC VFR deck from the southern Blue Ridge into Danville with chances for light rain. Rain may mix with wet snow at times, though precipitation rates are expected to be light enough to preclude development of any sub-VFR visibility. A renewed, though somewhat lower confidence, threat for MVFR stratus at Bluefield would be possible after midnight. Winds veer to northwest- north 3-5 kts for overnight. High confidence in ceilings/visbys today, medium confidence on ceilings late tonight. High confidence in wind directions, medium confidence on wind speeds. Extended Discussion... Conditions trend VFR for Friday afternoon. An arctic cold front this weekend could also bring snow showers to the mountains with potential sub- VFR ceilings and intervals of sub- VFR vsby mainly at Bluefield and Lewisburg, with post- frontal strong northwest winds Saturday night into early Sunday. VFR conditions then return Sunday evening through Monday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...RAB LONG TERM...RAB AVIATION...AL

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