Area Forecast Discussion
Issued by NWS Blacksburg, VA

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031 FXUS61 KRNK 161920 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 320 PM EDT Wed Aug 16 2017 .SYNOPSIS...
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Though the majority of the forecast area is dry tonight, a weak upper-level disturbance should spark scattered showers and embedded thunderstorms mainly south and west of Interstate 77. An even more humid air mass settles in for Thursday, with a better chance for showers and thunderstorms during the afternoon. A cold front moving into the region on Friday sparks another round of showers and storms followed by clearing along with drier conditions for the weekend.
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&& .NEAR TERM /THROUGH THURSDAY NIGHT/...
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As of 305 PM EDT Wednesday... Weak surface ridge of high pressure presently is in control across much of the Alleghanies, the northern and central Blue Ridge into the Piedmont. Though a few instability showers noted near the Triad north to Patrick and Pittsylvania counties, mostly sunny skies with cumulus clouds along with noticeable humidity levels were common for many locales this afternoon. However to the west, over eastern KY into the Clinch River basin, monitoring a weak mesolow and associated weakness in the 500 mb height field (amid a background regime of mid-level height rises). In conjunction with modest convective instability (SBCAPEs near 1300 J/kg per LAPS analysis), this feature is contributing to a deepening cumulus field and a greater coverage of scattered showers from Lexington KY through just north of Abingdon VA on southeast into Avery and Caldwell counties in NC. For the rest of today/tonight: Recent HRRR and 3-km NAM solutions display good consistency in showing ongoing shower activity trudging slowly to the east and southeast through mid-evening tonight. This general area roughly bounded along and southwest of a line from Bluefield to Hillsville VA to Danbury NC. There still are a few more hours of insolation left, so there could be a few thunderstorms through mid-evening as well in this area. With storm cells more or less drifting east-southeast, could see localized higher rainfall footprints but overall scenario is not vastly different than experienced recently. Otherwise, the rest of the forecast area stays dry through the evening. Some question if showers can linger after midnight in weak convergence field in the NC Piedmont and Southside as modeled by the GFS and the 3-km NAM. I am dubious as mid- level feature should have advanced far enough east by that point to allow for weak subsidence. Will need to monitor, but for now have left this area dry. After midnight, it appears that we see a considerable increase in moisture levels and clouds amid light southwesterly low to mid-level flow. GFS LAMP guidance dewpoints at MOS forecast points all trend upward overnight, so if you thought today was on the humid side, more is on the way. I`ve indicated some patchy fog in for the overnight hours in the Greenbrier and New River Valley, but less cooling due to clouds cast doubt much will develop. It`s currently at least highly doubtful we see as much fog as experienced this morning. A mild and rather humid evening with lows from the mid upper 60s to the lower to mid 70s, mildest further southeast. For Thursday/Thursday Night: Looking at quite a muggy air mass areawide. Outside of the higher elevations along the Blue Ridge, upper 60s to mid 70s dewpoints should be rather common Thursday. This moisture extends at depth as well, reflected in PWAT values progged from 1.9-2.2". Though we should likely have a good amount of cloudiness, BUFKIT soundings show an air mass that is weakly capped. So it won`t take much heating to generate enough instability to spark at least scattered showers and thunderstorms. While 500 mb height tendencies are neutral early in the day, they start to fall by the afternoon and should be enough lift, augmented by a pre- frontal or lee trough. Indicated high chance to lower Likely PoPs for showers and thunderstorms. I wouldn`t necessarily discount a couple of SPSable type storms tomorrow, but the deeper layer shear and tall, skinny CAPE profiles summing to values 1000-1500 J/kg should keep severe threat mitigated. With Corfidi vector speeds ranging from 5 to 15 kts indicating slow cell motion and potential for nearby cell growth on old outflow, along with the aforementioned high PWAT air could have some potentially heavy downpours. Showed rainfall amounts from a quarter inch up to localized 0.50-0.70" amounts along the lee of the Blue Ridge south of Roanoke. Confidence wasn`t high enough to address in the HWO or to add heavy rain wording, though WPC`s overnight Day-2 outlook does have our Blue Ridge counties in a Marginal Risk for excessive rain. A general decrease in coverage and intensity of showers and storms should be expected with sunset and generally ending near midnight, though may have a good deal of cloudiness ahead of the primary cold front. Kept highs in the mid to upper 80s with at least partly/mostly cloudy skies, with lows still on the muggy side in the upper 60s to mid 70s.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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As of 230 PM EDT Wednesday... Weak shortwave trough should lift by to the north on Friday allowing a surface cold front to slowly pivot east toward the region by late in the day. Expect some degree of residual showers preceding the boundary across the west early in the day that could jump into the eastern lee trough during the afternoon where deeper convection still appears possible. Most guidance still not overly impressive with lift ahead of the front, given most upper support shearing out to the north with the passing weak 500 mb trough. This while seeing the boundary layer flow turn more westerly espcly mountains behind the pre-frontal axis early on. However progged instability still quite strong east of the mountains where models suggest a narrow band of shortwave energy crossing into the piedmont late. Late day timing could allow for better convergence from the Blue Ridge east espcly if the low level flow backs southwest, and progged very high 850 mb theta ridging materializes. Therefore keeping chance pops with highest out east. Highs again mostly 80-85 mountains to lower 90s east given downslope but also convection dependent. Surface cold front should cross the region Friday night into early Saturday, booted along by a stronger upstream shortwave trough that will pass just north Saturday night. Model consensus shows some lingering convection along the front mainly southeast early Friday night before deeper moisture gets shunted to the east early Saturday. However just how far southeast dry advection can get Saturday remains iffy espcly ahead of the strong upstream shortwave. Latest trends suggest perhaps just enough low level moisture with heating to still spark an isolated shower/storm mainly southeast sections later Saturday so left in a mention. Otherwise should be a bit drier Saturday afternoon with dewpoints dropping off espcly west, and only slight evening pops far south Saturday night for now. Still quite warm Saturday with highs low/mid 80s to near 90 southeast with somewhat cooler lows in the 60s overnight behind the front.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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As of 130 PM EDT Wednesday... Exiting upper trough by late in the weekend will evolve into strong ridging across the region for early next week as Bermuda high pressure links with building heights over the southeast states. Appears this in combination with weak surface high pressure ridging in from the north and lack of upper support, will act to keep a lid on most organized convection for Sunday into Monday. However given close proximity of at least low level moisture near the residual front over far southern sections, will need to leave in an isolated diurnal convective mention espcly southern Blue Ridge for now. Otherwise will be leaving things mostly dry with more clouds far south and less northern half into Monday. Ridging will begin to flatten Tuesday in advance of the next digging 500 mb trough that looks to push another cold front toward the region by midweek. Return moisture ahead of the boundary and less cap should support a scattering of showers/storms Tuesday afternoon especially mountains. Expect a bit more organized convection ahead of the front and under cooling aloft Wednesday when will have medium to high chance pops. Better ridging and warming 850 mb temps to around +22C suggests a rather warm/hot period with highs mostly 80s, except around 90 or warmer piedmont, before cooling slightly on Day7 per more showers around.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/... As of 125 PM EDT Wednesday... VFR conditions through much of tonight across the terminals tonight. Some potential for an isolated to widely scattered shower or thunderstorm affecting far southern VA into the southern Blue Ridge in NC. Indicated VCSH at Bluefield, which would be the northeastern extent of this threat this afternoon, but more of an awareness for routes through the southern Blue Ridge toward TN. Any showers or storms should dissipate and/or pivot into the central NC Piedmont by 04z. However, through mid-evening there should be a considerable degree of VFR low to mid clouds associated with southwesterly moisture advection. This lends uncertainty on the extent of radiation fog after 06z, even in the more customarily fog-prone river valleys. Indicated BR in at Lewisburg, but only at 5 SM as confidence is on the low side. Should fog develop, it would in all likelihood not be as dense or extensive as observed earlier this morning. Winds light west to southwest becoming light and variable by morning. Multi-layered VFR cloudiness expected Thursday. With humid dewpoints, air mass tomorrow is only weakly capped to convection, so any breaks in the cloudiness should produce at least scattered showers as early as mid-morning. Thunderstorms becoming more likely after 16z Friday. Couldn`t rule out this threat anywhere, but a somewhat more favorable area would be within a few miles either side of the Blue Ridge. While strong storms are not anticipated before 18z, a few will produce lightning and short- duration MVFR to LIFR visibility restrictions in heavy rain. Indicated VCSH prior to 16z, with VCTS indicated after 17z to reflect potential thunderstorms in vicinity of TAFs. Winds southeast to south 4-6 kts, though may tend southwest at Bluefield. Extended Aviation Discussion... Expect a continued increase in shower/thunderstorm coverage and strength after 18z Thursday, lasting until 03z Friday. Outside of thunderstorms look for VFR conditions. Overnight fog is a possibility in the river valleys and in areas that do see rain, though cloud cover/convective debris may limit its spatial extent. Another round of scattered afternoon showers and thunderstorms Friday associated with the primary cold front, though VFR prevails outside of showers or storms. Dry advection and light to moderate west to northwest winds behind the front Friday night could keep overnight fog coverage limited, but a potential sub-VFR ceiling is possible at Lewisburg and Bluefield. Mainly VFR conditions for the weekend under weak high pressure. && .EQUIPMENT... As of 755 PM EDT Saturday Aug 12th... KFCX doppler radar will continue to be down for the rest of the month due to a failing bull gear. It will only be operational during this time frame for brief periods if an imminent significant widespread severe weather and/or hydrologic event occurs. Operating the system for any amount of time until the bull gear is replaced risks a catastrophic failure that would further extend system downtime. From the 17th through the 27th the radar will most likely be down completely as the repairs are expected to be made during this period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/WP EQUIPMENT...RAB/WERT

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