Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200601 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 201 AM EDT Sun Aug 20 2017 .SYNOPSIS... A front will remain stalled over the eastern Carolinas tonight into Sunday before dissipating. High pressure works in from the Ohio Valley during the weekend, then overhead Monday resulting in mainly dry weather across the region. Another cold front will approach the area by the middle of next week with the next chance for showers and storms. && .NEAR TERM /THROUGH TODAY/... As of 736 PM EDT Saturday...Per water vapor trends, mid/upper- level trough axis now appears to be clearing our far southwestern counties; this feature lying atop weak surface ridge building in from the west. Through next several hours, trend will be for dissipating fair weather cumulus along with generally clear skies. There may be some convective debris high clouds that approach our northern and northwestern counties later in the overnight from convection across PA, northern WV and into southeast OH, convection that should remain north or northwest of the forecast area. Look for good radiational cooling and drying through tonight. Should see nocturnal patchy radiation fog develop later in the overnight in and adjacent to the Greenbrier, New and Holston/Clinch river valleys. Lower confidence on dense fog in these areas due to expected dry advection overnight, but vis could be as low as one mile at times. All in all though, a fairly comfortable mid- August evening, even in southeastern counties as dewpoints start to drop to more comfortable levels. Lows appear on track with no other changes needed attm. Previous near-term discussion issued at 230 PM follows... Upper trough will pass across the region this evening followed by flat ridging overnight into Sunday. Most moisture remains to the north closer to the cool pool aloft, so keeping things dry this evening per latest model consensus. Next round of weak cool advection to follow this impulse overnight as weak surface high pressure builds in from the northwest under increasing subsidence. However may see some lingering strato-cu/mid deck in the wake of the trough this evening before clearing overnight. Otherwise expect somewhat lower dewpoints under mostly clear skies late to allow for more comfortable lows in the 60s, with perhaps a few 50s valleys. Appears some patchy fog again possible late tonight espcly valleys although should be less than last night per drying from this afternoon. High pressure remains just north on Sunday with a gradual transition to low level southeast flow espcly southern/east sections in the afternoon. Models attempt to nudge a bit more low level moisture up against the southern Blue Ridge late where instability appears highest. A few solutions even hinting at the potential for isolated convection far southwest ridges late but looks overdone per dryness aloft off forecast soundings. Thus leaving out any pop mention with overall mostly sunny skies for now. Given little temp change aloft and the flow turning more southerly, bumped highs up a bit more but not as warm as the latest Met mos which pushes mid 90s southeast. && .SHORT TERM /TONIGHT THROUGH TUESDAY/... As of 240 PM EDT Saturday... High pressure will be centered along the Appalachian Sunday night, then drifts off the VA/NC coast on Monday. With some weak southerly upslope flow and marine moisture, scattered showers and thunderstorms are possible along the Blue Ridge Monday afternoon. Any afternoon storms will then drift east in zonal flow and fade over the foothills during the evening. Cloud cover Monday afternoon should not be enough to hamper viewing of the eclipse. The eclipse, coming over the region during peak heating, may hamper afternoon convection to isolated coverage. Temperatures during the eclipse may drop 3F to 5F, especially south of a line from Richlands VA to Mount Airy NC. Temperatures will rebound following the eclipse with daytime highs ranging in the 80s with near 90F across Southside. High pressure will lose its hold on the region as a cold front approaches from the west Tuesday. Models have backed off on diurnal afternoon convection Tuesday, likely due to the lack of jet dynamics and increasing heights aloft. Some prefrontal showers may enter SE WV mountains late in the day, however confidence is low. Since we are expecting more sun and less rain, temperatures will warm above normal with 80s across the mountains and lower 90s east. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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As of 255 PM EDT Saturday... Despite a very warm environment across the region, models are pushing a cold front across the area on Wednesday. Most models are running fast and actually have the front coming across SE WV during the early morning hours Wednesday. Both the GFS and ECM have the front hung across the mountains Wednesday morning then jumping it to the piedmont in the afternoon. This may leave a minimum or shadow of rainfall along the Blue Ridge and foothills. The best low level jet is north towards DC and with the winds coming around to the west quickly, the severe threat will be limited. The exception to the rule will be across NW NC piedmont and Southside VA late in the afternoon and into the evening, where instabilities will be high. Following the front Wednesday night, cooler drier air will move into the region and remain into next weekend. Temperatures will likely be cooler than normal by 5F or so. Humidity levels will also be comfortable as dew points drop into the 50s.
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As of 200 AM EDT Sunday... Satellite imagery indicates clear skies across the forecast region this morning, though a few showers are drifting just to the north. With light winds in place, expect another round of river valley fog which will drop visibilities locally to LIFR, including at LWB. Outside of that, VFR conditions will persist. Strong heating will cause fog to burn off quickly after 20/13Z. Scattered CU will develop during early afternoon, with bases between 4 to 6 kft. Models hinting at isolated to widely scattered, short-lived showers developing along the Blue Ridge during the 20/18Z to 20/23Z timeframe, but not enough coverage to mention in TAFS. Expect to enter Sunday evening under mostly clear skies. With winds shifting southerly on Sunday evening, expect dewpoints to be on a gradual rebound. Another night of mostly clear skies and light winds are expected to support increased coverage of river valley fog. Extended Aviation Discussion... Moisture will increase Monday and Tuesday as high pressure weakens and moves offshore ahead of another cold front that will arrive from the northwest around midweek. However still appears it should remain mainly VFR Monday into Tuesday with localized MVFR under isolated to widely scattered convection. Late night/early morning fog will also be possible each day. Better potential for sub-VFR will come Wednesday when possibly more widespread showers and storms arrive with the next cold front.
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&& .EQUIPMENT... As of 400 AM EDT Friday Aug 18th... KFCX doppler radar expected to be down for the rest of the month due to a failing bull gear. Technicians are working on the radar, through next week and the radar will most likely stay down completely as the repairs are being made. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...AL/JH SHORT TERM...RCS LONG TERM...RCS AVIATION...JH/NF EQUIPMENT...RAB/WERT/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.