Area Forecast Discussion
Issued by NWS Blacksburg, VA

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206 FXUS61 KRNK 170531 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 131 AM EDT Thu Aug 17 2017 .SYNOPSIS... 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. && .NEAR TERM /THROUGH TODAY/... As of 1140 PM EDT Wednesday... Just a minor update to bring T/Td readings in line with current trends and adjust pops according to latest trends. Since HRRR has a fairly good handle on the trend of showers moving into southwest VA at this hour, will follow its trend through the night. Activity in Smyth and Grayson counties should work its way through the CWA near the I-81 corridor over the next few hours, then gradually dissipate across the Piedmont by 10Z. New activity will likely begin developing/moving into western areas toward daybreak. Thursday looks like an active day in terms of convection as a short wave interacts with a very warm and humid air mass. A good theta-e ridge evident through the heart of the CWA Thursday afternoon. Would expect fairly widespread convection Thu. Instability and shear values are not overly impressive, but an isolated strong storm certainly could not be ruled out. Heavy rainfall will likely be the more significant problem. As of 825 PM EDT Wednesday... Isolated showers still feeding off lingering surfaced based instability early this evening across the Mountain Empire of VA through the piedmont of NC. Expect generally dry conditions overnight but some isolated activity may be wafting around mainly east of the Blue Ridge. Previous discussion... HRRR and 3-km NAM solutions display good consistency in showing ongoing shower activity trudging slowly to the east. This general area roughly bounded along and southwest of a line from Bluefield to Hillsville VA to Danbury NC. 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. 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 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. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... 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. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... 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. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... As of 123 AM EDT Thursday... Upper short wave is pulling off to the east but surface frontal boundary continues to linger over the region with showers scattered south of BCB into the HLX area. High-res models showing this area moving east this morning with more development possible in the LYH/DAN area after 11z. Will have VCSH at these sites. Otherwise, mainly VFR. Exceptions should be fog at the usual spots LWB/BCB with LIFR possible. MVFR also likely at other sites except ROA. However, confidence in fog/stratus formation is medium at best as it is tied to extent of higher cloud cover. More upper clouds will result in less fog. Confidence higher at LWB where less clouds are expected. Better forcing and increasing moisture along the frontal boundary tomorrow will yield greater coverage of showers/thunderstorms by Thursday afternoon, so have added VCTS at all sites in the 16z-00z time frame, but keeping cigs VFR. Showers/storms may linger into late evening in the piedmont, but will take VCTS after 00z. Winds will generally be light through the period mainly out of the south to southwest. Extended Aviation Discussion... Models vary on late Thursday night, with GFS showing storms/showers lingering overnight into LYH/DAN with NAM similar but further west. Coverage is expected to stay scattered, so overall VFR with some MVFR or lower in showers. Cloud cover will dictate fog development but would not be surprised to see fog at most sites Friday morning, at last MVFR. 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/MBS/RAB SHORT TERM...JH LONG TERM...JH AVIATION...MBS/WP EQUIPMENT...RAB/WERT is the U.S. government's official web portal to all federal, state and local government web resources and services.