Area Forecast Discussion
Issued by NWS Blacksburg, VA

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646 FXUS61 KRNK 081059 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 559 AM EST Thu Dec 8 2016 .SYNOPSIS...
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A strong push of cold air will overtake the region today and tonight behind the passage of an arctic front. This cold airmass will stick around through Saturday before beginning to moderate ahead of a cold front that will approach late Sunday Night.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 500 AM EST Thursday... Hope everyone is ready for a little polar plunge. Weather change is underway. An Arctic front will cross the forecast area from west to east today, moving through the mountains this morning, and across the piedmont this afternoon. Aside from cloud cover, moisture depth associated with the front is pretty scant, so no more than a sprinkle or flurry at best with the actual passage of the front. Low level forcing from increasing northwesterly upslope wind flow, provide a better chc for mountain flurries tonight and early Friday with some light accumulation of snow for the WV Highlands where 1 to 3 inches of snow is possible for northwestern Greenbrier County, northwest of Lewisburg. Elsewhere only trace amounts expected, with no accumulation east of the Blue Ridge were downslope flow and drying will result in clear skies tonight. Highs today will occur early, then steadily fall after the frontal passage. Warmest readings will once again be across southside VA and the piedmont of NC with highs around 50. Mountain counties will make an early effort to test 40, but then begin pulling back toward and then below freezing this afternoon. Winds will also increase behind the front, wind chills becoming a factor, especially at the higher elevations. Strong cold air advection and blustery conditions will be in full swing tonight, 850 mb temperatures falling to -6 to -12C. This will result in surface temperatures in the lower to mid 20s east of the Blue Ridge, near 20 for the Shenandoah, New River and Greenbrier Valleys, and teens for elevations above 2500 feet, and potential for upper single digits for elevations above 4500 feet. Add the blustery NW wind, and wind chills will range from the teens east of the Blue Ridge to the single digits west of the Blue Ridge, and sub-single digits (zero to 10 below) for elevations above 3500 feet.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
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As of 430 AM EST Thursday... Modified Arctic airmass will still be in place Friday with cold temperatures (some 10-15 degrees colder than 1980-2010 climo normals) and even colder wind chills especially in the western half of the forecast area. Moderation to temps then begins later Friday into the weekend with a frontal system beginning to reach our area later Sunday. Details follow... Friday: Abundant sunshine for will help raise temps from very cold lows somewhat, but it won`t feel that way as northwesterly winds 10- 15 mph lead to daytime wind chills in the teens west to the 20s/low 30s out in the Virginia piedmont. Will have some lingering clouds and snow showers in the hilly terrain in western Greenbrier County, though with the snow growth region only partially saturated that will tend to limit any accumulations in that area to an inch or less. I`ve shown highs ranging from the mid/upper 20s west of the Blue Ridge into the North Carolina mountains to the mid/upper 30s in eastern sections. Friday Night: Ridging continues to build into the region allowing for winds to generally slacken to light west. 850 mb temps still will be in the -7 to -12C neighborhood. Clear skies, light winds and dry air mass should allow temperatures to plummet to lows in the teens to near 20, coldest west of the Blue Ridge. Saturday/Saturday Night: Start to begin a moderating trend with surface high pressure becoming centered over our area Saturday and continuing to shift east Saturday night. Mid-level flow turns zonal during this period and should be watching a weak 500 mb trough and associated weak surface low pass to our north over the Ohio Valley. South-southwest 850 mb winds of around 25-30 kts advect in warmer air and increasing high clouds, with our 850 mb temps rising to around -2 to -4C into overnight. Looking at highs in the 30s to low 40s falling back to lows in the 20s Saturday night. Sunday/Sunday Night: Southwest flow continues to gradually warm the column in advance of a 500 mb shortwave trough over the Upper Midwest, that should induce a surface low to develop/strengthen near the mid-Mississippi Valley and advance to the Ohio Valley Sunday night. The 00z GFS and ECMWF are at odds as to how strong the 500 mb trough with the GFS being stronger and a little faster than the ECMWF. Though timing shortwaves in fast zonal flow is a challenge, I opted to lower PoPs down into the slight chance range until Sunday evening. I focused the Chance PoPs more into Sunday night but that still may be too fast. Temps should be warm enough for rain at onset except mixing with snow across the higher elevations above 3000`. Highs Sunday in the upper 30s to low 40s, falling only slightly to lows at or a few degrees above freezing.
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&& .LONG TERM /MONDAY THROUGH WEDNESDAY/...
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As of 400 AM EST Thursday... Aforementioned frontal system is depicted generally by most of the global models to lift over or just northeast of the central Appalachians on Monday. PoPs were raised into the high Chance/low Likely range. Compared to the 12z guidance, it appears that thermal structure has warmed enough aloft to support a p-type of rain given surface temps above freezing, except perhaps at the highest elevations, but a general transition to rain is anticipated in those areas as well. There still remains less clarity regarding to the system moving in for Wednesday. Where the GFS has a substantial amount of coverage of precipitation from the Gulf to the Great Lakes in association with the passage of another shortwave trough, ECMWF has a much weaker system traversing our region. The GFS wants to tie both a northern stream wave with a piece of energy ejecting out of the western Gulf. The ECMWF keeps any wave in the Gulf disconnected, the thus has the drier solution for our area. Our forecast will reflect a rain versus snow forecast with more weighting placed on the wetter GFS solution. As a whole, this time period will average a few degrees plus or minus of normal temperatures for this time of year.
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&& .AVIATION /10Z THURSDAY THROUGH MONDAY/... As of 1210 AM EST Thursday... VFR conditions to be the rule through overnight and into most of Thursday. High clouds will continue to stream northeastward across the TAFs through the overnight. However, it`s worth mentioning obscured mountains across far southwest Virginia and northwest North Carolina due to a layer of stratus that has advected northeast from the Tennessee Valley, but this will stay out of the TAFs. WInds to remain light to calm through overnight. Into Thursday, modified Arctic front to move across the TAFs through early afternoon hours. Once the front clears, northwest wind speeds and gusts to increase, with sustained winds 6-12 kts with gusts up to 25 knots in cold advection. Confidence remains high in all aspects of the forecast. Extended aviation discussion... Continued cold blustery conditions Thursday night into Friday, with upslope snow showers along areas west of BLF/LWB though some snow may tease BLF. MVFR cigs will be found across the mountains with VFR east of the Blue Ridge. West to northwest winds behind the front will be strong into Friday with gusts likely topping 25-30 kts at times. Winds and any residual low level cloud cover should quickly fade by Saturday as high pressure builds overhead making for overall VFR conditions this weekend. Next weather system to follow will be a cold front, models suggesting a Sunday Night arrival. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM/WP SHORT TERM...AL LONG TERM...AL/DS AVIATION...AL

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