Area Forecast Discussion
Issued by NWS Blacksburg, VA

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608 FXUS61 KRNK 222342 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 742 PM EDT Thu Mar 22 2018 .SYNOPSIS... High pressure over the Mississipppi River Valley will move east tonight through Friday night. Low pressure will develop in the Central Plains friday night and track southeast into the Tennessee Valley by Sunday morning. A wedge of high pressure will remain over the Mid Atlantic region Sunday and Monday. && .NEAR TERM /THROUGH FRIDAY/... As of 630 PM EDT Thursday... 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 /FRIDAY NIGHT 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 /23Z THURSDAY THROUGH TUESDAY/... As of 730 PM EDT Thursday... High pressure moving into the region has scattered out/moved most of the cloud cover from the forecast area. Some fair weather cumulus could develop during the afternoon hours tomorrow, but nothing of any consequence. High confidence that VFR conditions will persist through the TAF period. Winds are beginning to die down as the sun sets across the region creating an inversion to prevent mixing. When the inversion breaks in the morning, should see a brief period of gusty winds but otherwise should remain steady for most of the day. Extended Aviation Discussion... Late Friday night into Saturday there will be a return to sub- MVFR conditions along a warm front as low pressure approaches the Tennessee Valley. Snow or a winter mix will spread into the region from the southeast and continue into Sunday. 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 or lower clouds and fog. && .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 NEAR TERM...AMS/JR SHORT TERM...AL LONG TERM...AL AVIATION...AL/JR EQUIPMENT...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.