Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 201859 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 259 PM EDT Sun Aug 20 2017 .SYNOPSIS... High pressure across the mid-Atlantic will gradually shift east tonight and Monday, allowing winds to turn southerly, resulting in a return of deeper moisture by Monday afternoon. Another cold front will approach the area by the middle of the week, bringing with it a better chance for showers and storms. && .NEAR TERM /THROUGH MONDAY/... As of 240 PM EDT Sunday... Analysis showing only weak instability across the region this afternoon as higher dewpoints remain slow to return north with high pressure lingering across the area. Latest short term guidance still shows some weak convergence and subsequent isolated convection developing mainly Blue Ridge and points north into early this evening but looks iffy attm. May leave in a mention pending release trends, otherwise should see clouds and any showers fade shortly after sunset with skies becoming clear overnight after some evening cloudiness. Fog again likely to develop with dewpoints rising some under clear skies, so beefed up coverage espcly valleys and low spots out east. Lows not quite as comfortable as last night under increasing moisture with most only falling into the 60s, with perhaps only around 70 southeast. Flat upper ridging will linger Monday while the parent surface high pressure shifts east and weakens. This should allow for weaker subsidence by Monday afternoon when more of a low level southwest trajectory starts to return higher PWATS, including some 850 mb theta-e ridging along the mountains. However lift outside of orographics looks weak given little support aloft, with early/mid afternoon instability likely dampened some by the eclipse. Latest Nam again looks overdone with a band of deeper convection along the Blue Ridge, although most solutions develop at least isolated to widely scattered mountain showers/storms as the mid level cap fades. Therefore leaving in some low chance pops mountains per latest ensembles, while delaying the onset and trimming back coverage during mid afternoon. Otherwise clouds the main concern as appears could be some low clouds early on, followed by mainly sunny skies before cumulus development takes place around eclipse time in the afternoon. Right now appears overall 30-50% cloud cover except locally more where any convection does develop sooner. Similar temps aloft to the past few days again supports highs low/mid 80s mountains and around 90 east. Did trim back a few degrees for the eclipse before readings spike back up late in the day where clouds are less. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 240 PM EDT Sunday... High pressure and loss of heating in a weakly capped environment will have any Monday afternoon storms fading in the evening. On Tuesday, high pressure drifts further east into the Atlantic. Return southwesterly flow on the back side of this high will bring more heat and humidity into the region. Upslope flow along the Blue Ridge and a lee trough over the piedmont may produce storms across the mountains Tuesday afternoon. Increase mixing ahead of an approaching cold front may be a deterrent to limit convection activity to scattered coverage. Any afternoon storms will fade during the evening. Prefrontal showers enter southeastern West Virginia after midnight as a cold front begins to move into the Ohio Valley. Muggy conditions are expected through the period with highs Tuesday in the 80s west of the Blue Ridge and low 90s east. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 255 PM EDT Sunday... Models remain consistent with having a cold front tracking across the region on Wednesday. One inconsistency is timing. Models have the front stalling west of the central Appalachian mountains Wednesday morning. It appears the front loses jet support and the short wave axis runs parallel to the front. The surface front is expected to slowly move across the mountains, while precipitation hangs back on western slopes during the morning. The front will jump to a lee trough in the piedmont during peaking heating in the afternoon. Models have the best jet dynamics over the northeastern US, but enough instability and lift will generate some strong to severe storms across NW NC and Southside VA during the late afternoon and evening. Again with the lack of jet support, the front may stall across the coastal piedmont of southern Virgina and North Carolina Wednesday night. This will delay the cooler and drier air from enter the region until later Thursday when another low pressure system pivots over New England. Temperatures Wednesday will vary from the mid to upper 70s across the mountains to the upper 80s towards the piedmont. Following the front passage, temperatures will run below normal going into next weekend. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... As of 1230 PM EDT Sunday... Overall VFR will continue under light east to southeast winds this afternoon. However should see cumulus fields start to increase by mid to late afternoon with bases between 4-6k feet. Models hinting at isolated to widely scattered, short-lived showers/isolated storms developing along the Blue Ridge from mid afternoon through early this evening, but still not enough coverage to mention in the TAFS. Appears best chances along the highest ridges between KROA and KTNB at this point. Expect any convection to fade after sunset, although some clouds may linger along the mountains and northern sections until around midnight or so. With winds shifting southerly this evening, expecting a continued increase in moisture overnight. This combined with clearing skies across the region late, expect another round of fog espcly across the western valleys. This most likely around KLWB where another episode of IFR/LIFR cigs/vsbys likely, while a period of IFR possible at KBCB, although models suggest a bit more uncertainty. Also may see low clouds advance northward toward the eastern terminals late as tropical moisture returns. Thus including a brief MVFR mention at both KLYH/KDAN late per latest HREF progged cigs/vsbys. Otherwise VFR outside of any patchy fog or stratus elsewhere late tonight. Flow turns more south/southwest on Monday as high pressure slides east and weakens. This should bring about a more widespread cumulus field by afternoon once any low clouds/fog burns off. However still appears it should remain mainly VFR Monday afternoon with only localized MVFR under isolated to widely scattered mountain convection. Extended Aviation Discussion... Moisture will continue to increase on Tuesday as high pressure moves farther offshore ahead of another cold front that will arrive from the northwest around midweek. This will again lead to more afternoon clouds and isolated convection mountains Tuesday afternoon but overall VFR at this point. Better potential for sub- VFR will come Wednesday when more widespread showers and storms arrive with the next cold front. Flying conditions should slowly improve behind the passing cold front on Thursday although sub-VFR cigs may linger across southern/western sections a while longer. && .EQUIPMENT... As of 400 AM EDT Sunday Aug 20th... 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 this 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/NF NEAR TERM...JH SHORT TERM...RCS LONG TERM...RCS AVIATION...JH/NF EQUIPMENT...RAB/WERT/WP

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