Area Forecast Discussion
Issued by NWS Blacksburg, VA

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062 FXUS61 KRNK 232307 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 707 PM EDT Wed Aug 23 2017 .SYNOPSIS... A cold front will continue to move southeast tonight. A large high pressure system will move out of central Canada and build eastward behind the front Thursday, bringing significantly cooler temperatures to the region into the weekend. && .NEAR TERM /THROUGH THURSDAY/...
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As of 706 PM EDT Wednesday...Ridge of high pressure from sprawling surface high centered over the upper Midwest is building in rather quickly from the northwest. Much drier air already is making its presence known across the northwestern two-thirds of the forecast area. Post-frontal regime still in place across the Southside and into the NC Piedmont/foothills areas where dewpoints are presently the greatest but even here, it`s expected that dewpoints will decrease rather markedly as the nightttime hours progress. I did decrease PoPs a bit earlier based on recent hourly-updated guidance runs through midnight. While these do show some lingering light QPF/showers overnight into the Southside feel that this probably is more of a lower stratus as indicated in RAP model RH profiles. Best chance for patchy fog given good radiational cooling conditions is in the Greenbrier and New River Valleys where winds will go calm/decouple quickest. Combination of at least potential scattered clouds and enough northerly gradient flow to the south and east should mitigate such development in our southern and eastern counties. No changes made to lows. More refreshing conditions tonight given expected dry advection areawide. Previous near-term discussion issued at 238 PM follows... KFCX WSR-88D is back in service ! A cold front with associated showers and thunderstorms will move southeast this afternoon into tonight. Solar heating has created some surface based instability ahead of the boundary reinvigorate convection with the front mainly east of the Blue Ridge mountains. The northwest winds behind front are hindering showers in the west. The best low level winds and dynamic support will be moving by to our north which will greatly limit the potential for any thunderstorms to become severe. The Day One Convective Outlook has most of the forecast area in general thunderstorms, with marginal risk of severe clipping only portions of extreme southeast counties. A large area of high pressure will move out of central Canada and build eastward behind the front, bringing an end to any lingering showers early tonight. Slackening winds west of the Blue Ridge and a cooler airmass should help generate some late night valley fog. Low temperatures tonight will range from the lower 50s in the mountains to the mid 60s in the Piedmont. A broad upper trough will slide east from the Great Lake regions to the East Coast Thursday. This afternoon`s cold front will continue to push south into the Southeast States. During Thursday afternoon, a shortwave embedded within the broad upper trough will rotate eastward into the Mid Atlantic Region. This feature may create clouds to upslope areas of the Alleghanys, with a few showers possible during peak diurnal heating. The NAM was the furtherest south with the southern extent of the isolated showers compared to ECMWF and GFS. High temperatures Thursday will vary from the upper 60s in the northwest mountains of Greenbrier county to the lower 80s in the Piedmont.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 230 PM EDT Wednesday... Some low level moisture in southeast West Virginia along with a weak short wave so some upslope clouds expected. Short wave tracking through the upper northeast trough on Thursday night and early Friday morning will provide some lift. But by Friday morning the 850MB winds forecast to be from the north which is not as favorable for upslope. Slightly cooler temperatures at 500 MB will result in larger lapse rates and instability Friday. NAM and GFS showing an increasing amount of moisture below 700MB Friday night and Saturday and a wedge develops and strengthens. Have lowered minimum temperatures on Thursday night, trending closer to MET guidance. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 1230 PM EDT Wednesday... For Monday models continued to show separation between a northern stream system approaching the Ohio Valley and the tropical system over the Lower Mississippi Valley. Short wave in the southwest flow over the Mid Atlantic region forecast to cross through the area on Tuesday. Confidence in the synoptic scale weather pattern decreases on Tuesday and Wednesday mainly due to uncertainty and spread in the guidance with the evolution of the Gulf of Mexico and southeast coast tropical systems. Surface high pressure moves into southeast Canada with a strong wedge down the east side of the Appalachians by Sunday. Cool air comes down with the wedge by Sunday with 850MB temperatures on Monday and Tuesday in the +8 to +12 range. Models showing abundant low level moisture, especially in the foothills and eastern upslope areas through Sunday. Will cut back the probability of precipitation on Monday but will leave in clouds. Plan to slightly lower maximum temperates Sunday through Tuesday. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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As of 706 PM EDT Wednesday... VFR conditions to be the rule for much of the TAF period, outside of river valley fog. Skies should generally go SKC from the Blue Ridge foothills westward with sunset. Indication from forecast RH profiles tonight towards Lynchburg and Danville of potential SCT-BKN VFR cloudiness. Northerly winds under 5 kts will also mitigate any overnight fog for these areas. Best chance at IFR to LIFR fog is at Lewisburg and Blacksburg where skies will be clearest and winds the lightest along with proximity to the Greenbrier and New Rivers. Areas along the southern Blue Ridge foothills into Southside/Danville may see lingering VFR stratus deck into the mid to late morning hours. Should see FEW-SCT VFR fair weather cumulus otherwise through the TAF valid period. Winds northerly 4-6 kts. Medium to high confidence in ceilings,visibilities and winds during the taf period. Extended Aviation Discussion... Generally VFR conditions are expected Thursday night into the weekend as high pressure builds over the region. There will also be a diurnal trend for late night fog/stratus especially west of the Blue Ridge with KLWB and KBCB the most likely TAF sites to be affected. Isolated convection may be possible across the mountains Saturday into Monday.
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&& .EQUIPMENT... As of 1205 PM EDT Wednesday... KFCX doppler radar has completed all repairs and is now back in service. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...AL/KK SHORT TERM...AMS LONG TERM...AMS AVIATION...AL/KK EQUIPMENT...KK

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