Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 061126 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 626 AM EST Wed Dec 6 2017 .SYNOPSIS... A cold front will progress across the Piedmont though this morning, stalling to our south later today into tonight. Noticeably cooler temperatures compared to yesterday can be expected today as high pressure builds in. Cooler and dry conditions will continue through the rest of the workweek as a weak low pressure area rides the stalling front, passing along the Southeast U.S. coast. The upcoming weekend features an even stronger shot of colder air with periods of mountain snow showers. && .NEAR TERM /THROUGH TONIGHT/... As of 330 AM EST Wednesday... Cold front is still making its way through a rough Buckingham- Pittsylvania-Rockingham county line. Advertised cold and dry advection underway to the west of this line, with rather breezy conditions at elevation this morning (ref: TNB METAR peak gust of 35 kts around 07z) given rather robust pressure rises. Radar mosaic indicates only lingering light showers across the far eastern third of the forecast area. By sunrise, would expect most areas to be dry. Should see moderate northwesterly breezes through around late morning boosted by low-level cold advection especially west of the foothills, with less gusty conditions into the afternoon as pressure gradient slackens and mixed-layer winds become weaker. Gusts as much as 40 mph could be possible at elevation along the Blue Ridge from Floyd through Carroll and Watauga Counties. I sided high temperatures today on the colder side of guidance, as southern and eastern counties have a tougher time fully shaking free of at least mid cloud cover given the proximity to the baroclinic zone. Meanwhile in southeast West Virginia to the Blue Ridge, potential for heating through cloud breaks are offset by strongest cold advection. Highs range from the upper 30s-mid 40s in these western locales, and into the upper 40s in the VA/NC Piedmont, foothills and Roanoke metro areas. With broad troughing being carved out into the Ohio/Tennessee Valleys, and west-southwest 700-400 mb winds tapping into expansive area of mid-/high cloud cover that trails back to New Mexico, expect any cloud breaks to be short-lived. Though it`s likely clouds hang tough through the day in Southside into the Piedmont/foothills as baroclinic zone stalls to our south, look for an increasing trend to clouds across all areas again tonight. With indications of poorly organized/fast vort maxima running atop west-east frontal zone, how far south the frontal zone sags appears key to PoP chances across our southern third of counties (along/south of Rt 52). 00z NAM is the furthest north, and if taken verbatim would bring PoPs as far north as Lynchburg. Official forecast will side closer to 03z SREF and 00z GFS/ECWMF trends which keeps any measurable precip far enough south. Wouldn`t discount a few sprinkles after 06z from Surry County northeast to Halifax County, but these may really go to only evaporatively cool the low-level temperature profile overnight. With a good amount of clouds expected tonight, kept temperatures more milder than 850 mb temps of -1 to -4C would otherwise suggest. Called for lows in the mid 20s/around 30 in the western mountains and New River Valley, with middle 30s in the Roanoke Valley, Piedmont and foothills areas. && .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... As of 425 AM EST Wednesday... The entire forecast period will be dominated by a deep polar vortex or upper low pressure area anchored across eastern Canada under a Greenland block. It appears this pattern will more or less likely remain in place for the next couple of weeks. Each surface of Arctic/Canadian air will become progressively colder. For now we are looking at 850mb temps bottoming out in the -8C to -10C range with the first wave, around -12C with the weekend surface, and as cold as -18C with the third surge mid part of next week. Consequently, we can expect an extended period of much more wintery weather than we have been experiencing recently as well as below to much below normal temperatures more often than not. The good news is that our area remains on the southern end of the deep polar vortex upper low, allowing somewhat warmer air intrusions to visit from time-to-time instead of being entirely locked into a permanent deep freeze in this pattern. For Thursday, the main concern will be monitoring a baroclinic zone lingering across the southeast states to northeast just off the NC/VA coastline. With the axis of the parent upper trough still to the west of the area, this baroclinic zone will linger in place with very little eastward shift until an Alberta Clipper dives southeast into the upper trough over the weekend. The models have been at odds over the past few days as to how far northwest precipitation would reach with this system, with the GFS the most northwest and the ECMWF least northwest. Latest consensus is that the bulk if not all of the precipitation associated with the baroclinic zone will remain just south and east of our area. Will show some slight chance pops into the far southeast corner of the CWA, mainly in the late Friday time as the Alberta Clipper begins to sharpen the meridional flow within the eastern U.S. trough, briefly helping to pull the deeper moisture back to the northwest. This will then shift quickly out of the CWA by Saturday morning as the clipper dives into the Ohio Valley. Through this time frame, temperatures are expected to be just below seasonal normals, mostly highs in the 30s west to 40s east and lows primarily in the 20s. Given the lack of expected QPF, any amounts of winter precipitation would be inconsequential and thus will remove the mention of such from the HWO. Upslope at this point will be limited as well due to lack of decent orthogonal flow. The clipper will move through the region late Saturday into Saturday night. This will bring a more significant surge of Arctic air into the region dropping temperatures to well below normal for Sunday and early Monday. With respect to potential for winter precipitation/snow with this clipper, while a period of light snow and perhaps some snow squalls across the western mountains, especially eastern WV, appears likely given the strength of CAA and dynamics with the sharp upper trough/clipper, moisture is limited and the trough will sweep quickly through the CWA in about 12 hours. Some snow accumulation certainly appears to be a possibility for areas like western Greenbrier and other typical western upslope areas. A bigger concern for the majority of the CWA, especially west of the Blue Ridge would be strong, gusty northwest winds. Yesterday, MAVMEX guidance suggested that we were approaching levels where our local northwest wind headline program will need to be examined. && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 450 AM EST Wednesday... Sunday will be a very cold day with temperatures running about 10-15 degrees below normal, likely remaining below freezing for most areas west of the Blue Ridge. Monday morning will be quite cold, but the daytime Monday will bring a brief but notable warmup as the Alberta Clipper quickly lifts northeast out of the area and the flow goes zonal for about 24 hours into Monday as the persistent deep upper low/polar vortex across the Great Lakes reloads with another strong clipper like system slated to reach our area midweek. As the upper trough begins to sharped Tuesday in advance of the next clipper, enough moisture could be drawn back into the area for precipitation. For now, given the lack of any deep moisture and general dry Arctic air mass in place, precipitation appears to be light, but this could be a period of light snow or winter precipitation that will need to be monitored more closely as this forecast period approaches. At the moment, this again does not appear to be a significant winter weather concern. Then as we move into Wednesday, some of the coldest air we have seen in quite some time will spread across the region with 850mb temps approaching -18C during this surge. It should be noted that with a predominant northwest flow and a series of clippers tracking across the region, moisture will be limited, but periods of strong CAA and upslope wind flow will result in several chances for accumulating snow across the western mountains, especially those in eastern West Virginia/Alleghanys, but even to some extent in southwest VA and northwest NC. At this point, any individual snow event appears to fall within the realm of minimal advisory level or less. However, residents of these areas should be prepared for an extended period of cold weather and at least minor snow accumulations. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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As of 530 AM EST Wednesday... Any lingering MVFR stratus mainly southeast should trend VFR shortly. VFR conditions should prevail for much of the day today. Will see variable cloudiness, generally lowest and thickest furthest south today, closer to the position of where the cold front will sag and stall. Period of breezy northwest wind conditions expected at Roanoke until about noon (gusts up to 25 kts), with sustained winds generally 8-12 kts through this morning. Winds should then abate to 5-10 kts by this afternoon, further diminishing to light/variable for the evening. Another increase in cloud cover expected late tonight as a weak wave rides along stalled frontal zone, but should be VFR regardless. High confidence in ceilings and visibilities. Medium to high confidence in wind speeds and gusts, high confidence in wind direction. Extended Discussion... Should see continued VFR Thursday and Friday with light north to northwest winds, as a disturbance rides along a frontal zone well south of our region. A period of more notable impact to aviation interests is the strong Arctic cold front for the weekend. Potential exists for sub-VFR ceilings and intervals of sub-VFR snow showers along and west of the Blue Ridge Saturday, with post-frontal strong northwest winds Saturday night into early Sunday. VFR conditions then return Sunday evening into Tuesday.
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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 is the U.S. government's official web portal to all federal, state and local government web resources and services.