Area Forecast Discussion
Issued by NWS Blacksburg, VA

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485 FXUS61 KRNK 230511 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 111 AM EDT Fri Mar 23 2018 .SYNOPSIS...
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High pressure extending south across the region will linger to the north of the area into Saturday. Low pressure will track east into the Tennessee Valley Saturday before redeveloping offshore the Carolinas by Sunday. This system looks to result in widespread wintry weather during the weekend before a wedge of high pressure brings drier weather later Sunday into Monday.
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&& .NEAR TERM /THROUGH TODAY/... As of 1130 PM EDT Thursday... Clear skies tonight will allow temps to drop into the 20s, to lower 30s. Some increase in high clouds toward dawn possible in the west. Have a special weather statement out to cover refreeze of snowmelt mainly in the mountains. Still looking at new incoming data for upcoming snow threat this weekend. Latest NAM continues to advertise a more northern track from the Greenbrier Valley to north of Roanoke, while the early afternoon GFS and late morning ECMWF have it further south over the New River Valley into southern WV. No changes to the watch at this time, but changes are likely by early Friday morning. Previous early evening discussion... Not much adjusted needed for the near term. High pressure clearly taking over based on Satellite showing skies clearing over the region. Winds are beginning to slow down across the region so dialed back the forecast wind gusts during the nighttime hours. Adjusted the low temp up slightly per short range guidance however will need to monitor hourlies to see how quickly we radiate out. As of 403 PM EDT Thursday... Persistent northwest flow over the eastern United States tonight and Friday as pieces of compact short wave energy rotate through the upper trough. Models showed an increase in humidity above 850mb Friday afternoon. Locations that in in sheltered valleys that will have lighter winds and still have snow cover will be the cold spots tonight. Planning on staying on warm side of guidance for maximum temperatures on Friday with ample sunshine during the morning. This still keeps highs 10 to 15 degrees below normal. && .SHORT TERM /TONIGHT THROUGH SUNDAY/... As of 403 PM EDT Thursday... There is increasing confidence in a potentially impactful winter storm to affect a large part of the Blacksburg forecast area Saturday into early morning hrs of Sunday. Significant accumulations of heavy, wet/dense snow are becoming more likely along/west of the Blue Ridge. A frontal system in the Saturday through Saturday night period appears to skirt to our southeast. 12z NWP guidance generally showed consistency with earlier forecast model runs in terms of liquid-equivalent QPF. The primary difference that exists in today`s runs is in terms of timing - that being forecast guidance generally has slid onset timing of QPF back by about 3 to 6 hours. This lends to somewhat lowered confidence on start time and accumulation potential especially east of the Blue Ridge/in the Piedmont where temperatures may rise some into part of Saturday before hovering/slowly cooling due to wet-bulbing. A sharp NW to SE oriented thermal gradient will exist across the forecast area tied to the warm front, and associated 925-700 mb frontogenetic forcing spreading slowly east should force potentially moderate- intensity banded type snow Saturday late morning into the afternoon especially in a corridor bounded between Grayson and Alleghany Counties northward into the Virginia Highlands/Greenbrier County. This is late March and it is often quite difficult to get significant accumulation on roads/sidewalks, unless you can generate significant precipitation rates. This very well be one of those instances. The leading edge of the warm nose (850 mb temps nosing into the +1 to +2C range) may also force periods of sleet south of I-77, but this warm nose doesn`t appear to make it that much further north/east. Using snow to liquid ratios between 9 to 11:1, in the western mountains I`ve shown forecast storm total accumulations between 4 to 9 inches, localized amounts in the mountains approaching a foot. This is lower than WPC`s Days 2-3 accumulations mainly due to the potential for rain to mix in at times, but still hint at the potential for substantial accumulations of wet snow. For that reason and with increased confidence on significant banded accumulation, I`ve opted to expand the Winter Storm Watch into Summers and Mercer Counties a large part of the New River Valley, Roanoke County, Grayson Highlands, and into Alleghany County NC. Further expansion east or southeast may be needed in later shifts. Into the Piedmont of VA and NC and the foothills...accumulations here during Saturday will be nil or negligible as temperatures should prove too warm, and any precipitation that does fall will go to cool temperatures. Precipitation should begin to transition from rain to rain/snow or pure snow from north to south Saturday evening. Amounts here were lower, on the order of 1 to 4 inches in the VA Piedmont, with a slushy inch or two across the NC foothills/Piedmont into Henry and Patrick Counties in VA. Where there is the greatest amount of uncertainty is into Watauga/Ashe Counties up into Smyth and Tazewell, where amounts here are lower due to the warm nose changing precip over to rain soonest and changing back to snow latest. Though eastern sections of these counties may near Watch criteria type snows, it may be difficult for a large part of these counties to reach Watch criteria accumulations given the warming. It would behoove residents to pay close attention to the forecast and updates for this weekend. With that being said, confidence is increasing in the potential for a significant accumulation of snow, especially in areas along and west of the mountains. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 403 PM EDT Thursday... Heavily used a forecast guidance blend in this period given active weather in the short term period. In the wake of the frontal system passing to our southeast, wedge conditions begin to take hold for much of the early to midweek period. While generally dry, this will lead to lower than normal temperatures and a generous amount of clouds. It`s not until late in the week when forecast guidance begins to show erosion of the wedge with southerly flow returning and temperatures beginning to trend closer to normal. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/...
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As of 105 AM EDT Friday... High pressure should maintain overall VFR conditions through the TAF period. However lingering mountain wave clouds under northwest flow aloft could result in high end MVFR cigs across the mountains overnight into Friday morning. Northwest winds should continue to slowly diminish over the mountains through the overnight before increasing again Friday morning as another upper impulse slides across. This may again result in winds gusting to 20-25 kts during much of Friday. Clouds increase Friday night in advance of a warm front heading in from southwest. However appears bases will remain VFR at all sites at this point. Extended Aviation Discussion... Conditions will deteriorate from west to east Saturday as snow and/or mixed precip advances east during the day. Should see most drop to sub-VFR during the afternoon with areas of IFR or worse in heavier snowfall. This will continue Saturday evening before precipitation decreases from north to south overnight. Models indicate a bit faster drying as well by Sunday when spots across the north could return to VFR Sunday afternoon with lingering sub-VFR to the south for much of the day. Dry weather will return on Monday and Tuesday with high pressure to the north wedged along the mountains. But the wedge may hold in MVFR lower clouds.
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&& .EQUIPMENT... As of 1050 AM EDT Thursday... The Mount Jefferson NWR remains off the air. The earliest this system will again be operational will be Friday afternoon. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Winter Storm Watch from Saturday morning through Sunday morning for VAZ010>020-022-023. NC...Winter Storm Watch from Saturday morning through Sunday morning for NCZ002. WV...Winter Storm Watch from Saturday morning through Sunday morning for WVZ042>044-507-508. && $$ SYNOPSIS...AMS/JH NEAR TERM...AMS/JR/WP SHORT TERM...AL LONG TERM...AL AVIATION...AL/JH/JR EQUIPMENT...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.