Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 270538 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 138 AM EDT Thu Jul 27 2017 .SYNOPSIS... High pressure to the north will continue to wedge south into the area tonight before passing offshore the Delmarva by early Thursday. A cold front will approach from the northwest Thursday, sliding across our area Friday afternoon into Friday night. Cooler and much drier air under high pressure will follow the front over the weekend into early next week. && .NEAR TERM /THROUGH TODAY/... As of 716 PM EDT Wednesday...Little significant change needed to the going forecast. Did adjust sky cover down a bit in the next couple hours and slightly higher later in the overnight but these were mostly noise-level adjustments to the prior forecast. Earlier persistent stratus layer has generally scattered to this point, leaving some scattered cumulus behind. It`s expected that these clouds should dissipate with sunset. Brief period of clearing then advertised before midnight. Recent 18z guidance 925-850 mb RH profiles, including the 20z RAP, continue to show a return of stratus field in easterly flow mainly from the Blue Ridge foothills eastward into the Piedmont. Admittedly, confidence on its development isn`t as high as yesterday as satellite reveals less marine layer moisture to the east in eastern VA than was the case yesterday. It`s possible cloudiness may need to be lowered in subsequent updates if trends continue and/or RH profiles in near-term guidance thin moisture out. Were that to be the case, areas in the Piedmont may be somewhat cooler on forecast lows than presently indicated. 18z suite of guidance shows showers and thunderstorms developing Thursday closer to mid-day along and west of I-77 and later further east, but will leave this period as is for the time being. Previous near-term discussion issued at 230 PM follows... Wedged up low clouds remain slow to exit espcly along the mountains where persistent low level easterly flow continues beneath the inversion. Expect most of the low clouds to gradually mix out to cumulus through the rest of the afternoon so trending with less clouds into early this evening. Any shower threat appears quite limited as well due to lingering clouds so only keeping a mention across the far south at most into early evening. Otherwise high pressure to the north to shift slowly east and offshore overnight allowing the flow to gradually veer more southerly by morning. However potential for the wedge to flop back in after sunset, brings the possibility of seeing some low clouds redevelop mainly Blue Ridge. Therefore keeping things partly cloudy overnight but dry. Given some increase in dewpoints and clouds around supports lows mainly 60s, although perhaps only around 70 in spots out east. Surface high slides farther offshore early Thursday in advance of fast moving lead shortwave energy that will slide east under a passing 5h trough well to the north. Guidance suggests could see a late night MCS type feature with this wave that should push east/southeast and weaken on approach in the region mid/late morning Thursday. However timing and eastward extent remains iffy with some solutions slower which would allow a better convective threat per heating into the afternoon. For now given southward trends of latest models including ensembles, will bring in chance pops for mainly showers mountains by midday Thursday before pushing the lead outflow/trough east during the afternoon. With a quick return of deep moisture/PWATS under decent diffluence aloft, think enough coverage to warrant likely pops west by afternoon, with possible stronger convection east. Severe parameters look rather marginal although given deep moisture could see an isolated threat from strong winds espcly if forcing arrives later and some multicellular organization takes shape. High temps remain tricky given potential early clouds followed by a few breaks which could again hold the mountains in the 75-80 range while allowing the east to bump up into the mid to upper 80s. && .SHORT TERM /TONIGHT THROUGH SATURDAY/... As of 205 PM EDT Wednesday... Models are suggesting convection will move over the mountains Thursday evening from a decaying Mesoscale Complex (MCS) that will track over the Ohio Valley during the afternoon. The GFS has evening convection jumping from the Blue Ridge to the piedmont where a lee trough resides. Convection continues overnight across the area but will primary be along the VA coast. The NAM has the MCS convection stalling over the mountains Thursday evening, with weaker storms remaining in the area overnight. The ECM is similar to the NAM however convection stalls further north, north of hwy 460. To cover all scenarios, we will keep high PoPs across the mountains, lower in the east. We will also keep PoPs through the overnight hours which is a change from yesterday thinking of diurnal showers fading after sunset. The forecast for Friday remains a challenge with an upper level closed low coming out of the Great Lakes generating one or two low pressure systems over the Mid Atlantic region. One low may develop closer to the upper low and track across Pennsylvania Friday into Friday night. Another low will track along the front from the Midwest across the southern Ohio Valley. Which low becomes dominate will determine the severity and areal coverage of storms Friday afternoon and evening. The GFS merges both lows over eastern Virginia with the strongest convection remaining with the northern and upper level low. The NAM has the southern low dominating, coming across the forecast area Friday morning with the front exiting the piedmont by sunset. The NAM has the strongest storms tracking over the area during the morning which will keep the chances for severe storms low. The ECM is similar to yesterday with the strongest convection remaining north and just a few strong to severe storms with the frontal passage east of the Blue Ridge during the afternoon/early evening hours. Confidence is high to increase PoPs Friday however, the level and areal coverage of strong to severe storms remains a question. All models are in agreement with convection moving east by midnight with a sharp reduction in PoPs from west to east. A chance for showers will appear again Saturday afternoon as the closed upper low and cold pool track over the region. Majority of the heavier showers will be along the front which will be well east and south of the area. Therefore, we will only carry isolated to scattered showers Saturday afternoon. Temperatures will also be a challenge Friday, depending on the amount of sun. However, with high chance for showers, temperatures will be a few degrees cooler than normal. Under the cold pool and with northerly winds, Saturday`s temperatures will be cooler than normal with highs ranging from the 70s across the mountains to lower 80s east. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 240 PM EDT Wednesday... An upper level trough will sit over a dry surface high through the middle of next week. No rain is in the forecast and everyone should see fair weather cumulus clouds forming in the afternoon, then fading in the evening. Temperatures will be a few degrees below normal on Sunday, moderating back to normal on Tuesday. Dew points will remain low (mid 50s to lower 60s) through Tuesday, then edge up towards being muggy again on Wednesday. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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As of 131 AM EDT Thursday... Stratus/stratocu sticking around along and either side of the Blue Ridge into the Alleghanys. However, most cigs are VFR and should stay that way except tempo MVFR with fog at BCB/LWB this morning before 13z. Confidence is low on how long cigs last, but think they should be VFR after 12z, with mainly mid and high clouds streaming in ahead convection upstream over the Ohio Valley. Which is the main issue later today. Models are all in agreement that most of the area will see clusters of storms, question is who gets hit and who doesn`t. Confidence moderate enough to have tempo thunder at all sites this afternoon, earlier at BLF/LWB and later across the piedmont. Mainly looking at VFR with tempo MVFR/IFR with heavy rain and thunder. Not going to put gusty winds in the tafs at this point, but some of the storms will be bringing some stronger wind gusts. Low confidence after Thursday afternoon, as model consensus takes heavier convection east, but another batch upstream may arrive by 06z. For now will keep VCTS at most sites and VFR. Aviation Extended Discussion... Should start to see a front approaching with better threat of showers/thunderstorms late Thursday night into Friday. Stronger storms and heavy rain possible in and around the mid- Atlantic/Ohio Valley region into the Carolinas this time frame so be prepared for sub-VFR at times. Outside of storms expect VFR conditions. Saturday looks drier with northwest flow, but could see lingering low end VFR/high end MVFR ceilings in SE WV along with a few added shra/tsra mainly southern/eastern sections Saturday afternoon. Expect widespread VFR to return Sunday/Monday under high pressure following the front.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/WP NEAR TERM...AL/JH SHORT TERM...RCS LONG TERM...RCS AVIATION...AL/JH/WP

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