Area Forecast Discussion
Issued by NWS Blacksburg, VA

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803 FXUS61 KRNK 261740 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 140 PM EDT Tue Jul 26 2016 .SYNOPSIS... Lingering strong high pressure aloft over the Carolinas will result in hot and humid weather into at least mid week. A weak frontal boundary just north of the area should also continue to oscillate across the Mid-Atlantic region through the end of the week. Weak impulses riding along the front will combine with deep moisture to produce periodic rounds of showers and thunderstorms each day. && .NEAR TERM /THROUGH TONIGHT/... As of 1020 AM EDT Tuesday... Heat advisory remains in effect until 8 pm for South Central Piedmont of Virginia. Made some minor adjustments in temperatures with sfc obs and their trends for this morning, then shaped early afternoon temperatures towards LAMP guidance. Despite the scattered convection, it is going to be another hot day. Heat indices can be as high as 105 degrees in the Piedmont. Adjusted pops towards HiResw arw East and NAM for late morning into this afternoon. Storms will form across the western higher terrain and southern Blue Ridge early this afternoon. Much of our forecast area is under a marginal risk for this afternoon with winds the main threat. more changes later... As of 230 AM EDT Tuesday... Main concerns continue with convective potential today as well as heat over eastern sections as the area remains along the southern periphery of the westerlies and within the northern extent of the strong upper ridge to the southeast. Guidance has again slowed up the exodus/weakening of the 500 mb ridge in turn keeping a weak front stalled in the southern Ohio Valley, with main convective focus so far being from orographics and residual outflow. However appears that sheared shortwave energy passing to the north today will gradually help spill the pre-frontal axis ahead of the boundary a bit farther south into higher pwat air including high surface based instability. Most short term solutions show convergence across the north ahead of this feature and gradually unzip tsra bands/clusters southeast into northern sections this afternoon as the front starts to edge south. Also appears that a second area of storms likely southern Blue Ridge/foothills where aided by differential heating and weak westerly flow. However in between a bit uncertain with models lacking on showers within the weak west/nw flow from the New River/Roa Valleys east until coverage merges somewhat into very unstable air east of the mountains later on. Therefore plan to have a period of low likelys north and across the south/southwest this afternoon but slower initially, then overall decent chance pops across the area by this evening. Given weak steering but some unidirectional westerly component aloft, looks like storms will be slow moving with best severe chances coming from downbursts within a highly unstable water loaded type environment. Given such a warm/muggy start and potential to see a bit more insolation prior to convection, bumped up highs a few degrees in spots with 84-89 west and low/mid 90s east pending clouds. Heat indices may actually be higher at times today out east despite not being quite as hot given the degree of moisture that should hold dewpoints up more. Thus little change to the going headline at this point. Weak mid level wave along the front to the north should exit this evening allowing the boundary to sag south at least east of the Blue Ridge late. May even be back in a region of weak subsidence after midnight per latest ensemble output. Therefore trimmed pops back later this evening/overnight with only isolated coverage far northeast, and over the west for continued weak westerly convergence south of the front. Otherwise another very warm/muggy overnight with moisture and debris clouds keeping most outside of the valleys in the low/mid 70s. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... As of 330 AM EDT Tuesday... The upper level pattern will be undergoing some changes through the latter portion of the workweek as it transitions from the flat, quasi zonal regime of the past several days to a shallow trof in the east. This will keep an ample supply of deep moisture over the region, and as the upper jet sags a bit enhanced divergence aloft in the right rear quadrant will create an overall synoptic environment favorable for enhanced convective development. The washed out frontal boundary will remain over the region on Wednesday but shear is still not very impressive so the probability for widespread severe remains low with the main threat being pulse storms and any clusters that can organize due to outflow boundary interactions. The latest guidance for Thursday is now starting to look rather interesting as most guidance converging on a solution that may bring severe weather to our neck of the woods.. An energetic short wave will move up from the Gulf coast and interact with the developing upper trof moving through the Ohio valley. This will create a wave along the frontal boundary draped across the region, developing into a well defined surface low over the middle Atlantic coast. As this system undergoes cyclogenesis the 850mb wind flow will become enhanced and help create significant shear with looping hodographs depicted in Bufkit by late on Thursday. This shear will be entering the hot and humid airmass over the region with ample instability to drive convection. There is still uncertainty in the location and timing with these ingredients for Thursday and Thursday night, but the situation is looking favorable for development of an organized MCS that may impact the forecast area. PWATs will also be running close to 2 inches so even with relatively dry antecedent conditions, an organized convective system will put down a good amount of rainfall and may create hydro issues. At this time, will start trending POPs and QPF upward for Thursday/Thursday night, but will not go all in until later runs confirm or deny this scenario. Additional short wave energy riding up the lingering frontal boundary will bring a good chance of showers/storms to the region again on Friday, especially west of the Blue Ridge, but a less dynamic environment and lower instability will help limit severe potential. Temperatures will remain above normal through the period. Highs will generally be in the low/mid 90s east of the Ridge, with low/mid 80s to the west. Lows will be in the lower 70s east to the 60s west. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
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As of 130 PM EDT Tuesday... Overall pattern situation favors storminess every day but especially in the afternoon/evening. A frontal boundary situated from the Ohio Valley over to the Central Plains will slowly shift southeast into the Virginias and North Carolina over the weekend. Airmass will remain unstable and moist. Some issue with how far this front makes it as 5h ridge may be hard to break, but still looks like belt of westerlies will shift at least into our northern forecast area over the weekend. Highs will be seasonal with lows slightly above normal through most of the period. Highs will range from the lower to mid 80s mountains, to upper 80s to around 90 east. Lows will be in the lower to mid 60s mountains, to upper 60s to around 70 east.
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&& .AVIATION /18Z TUESDAY THROUGH SUNDAY/... As of 138 PM EDT Tuesday... SCT to BKN VFR ceilings with pockets of MVFR/IFR convection expected this afternoon into tonight. Given strong instability would expect greater coverage of storms this afternoon compared to the widely scattered activity Monday. However, models continue to vary on expanse and locations of coverage as ridging aloft appears stronger than earlier forecast, while focus on residual outflow...southeast lee trough and the upstream pre-frontal zone could lend to multiple clusters in spots later this afternoon. Mention VCTS in most taf locations for this afternoon into this evening. Believe the taf site with the best chance of a thunderstorm is KROA. Widely scattered clusters of shra/tsra may linger into tonight before again fading to patchy fog/stratus overnight. KLWB is the taf site with the best chance for low clouds and fog late tonight into Wednesday morning. Scattered MVFR showers and thunderstorms are expected Wednesday especially during the afternoon and evening hours. Medium confidence on ceilings,visibilities and winds during the taf period. Extended aviation discussion... The upper ridge will remain just south of the area and a residual front nearby through Saturday. This should allow for periodic daily convective coverage through much of the period until the boundary either fades or sinks farther south. Appears more widespread sub- VFR possible in convection later Thursday per a stronger wave along the front with perhaps some decrease in coverage by the weekend as this feature passes. However given uncertainty appears at least diurnal MVFR/IFR possible each afternoon/evening through Saturday for now. Late night and early morning fog will likely occur at the usual valley locations, and those chances increase across the entire region if any rain occurs during the afternoon or evening at any site. && .CLIMATE... Record high temperatures for Tuesday (7/26) Roanoke.....101 in 1930 Lynchburg...104 in 1934 Danville.....99 in 2012 Blacksburg...94 in 1987 Bluefield....91 in 1993 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Heat Advisory until 8 PM EDT this evening for VAZ046-047-058- 059. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH/KK SHORT TERM...MBS LONG TERM...WP AVIATION...JH/KK CLIMATE...PM is the U.S. government's official web portal to all federal, state and local government web resources and services.