Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 211436 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1036 AM EDT Thu Jul 21 2016 .SYNOPSIS... High pressure will remain over the mid Atlantic region through Friday. Two weak cold fronts will approach the area early in the weekend and again early next week. The first front will slide down from the north on Saturday but may stall across northern Virginia by Sunday. The second front will approach the Appalachians from the northwest on Tuesday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1020 AM EDT Thursday... Latest visible satellite imagery shows that lingering fog/stratus has burned off and some cloudiness has started to develop east of the Blue Ridge. Water vapor imagery and morning RNK sounding indicate that the atmospheric column is dry and expect this to be a limiting factor in precipitation chances today. Developing return flow around high pressure sliding off the mid Atlantic coast will begin to advect low level moisture into the region on SEly flow and orographic effects along the Blue Ridge combined with some diurnal instability warrant some slight chance/chance POPs. However as the column mixes the dry air aloft should temper CAPE and act to erode convective towers. A lot of meso guidance was too robust with precipitation overnight and into this morning, but going with the qualitative overall trend and throttling back the quantitative numbers yields reasonable grids. Will keep a keen eye on how/if things develop and adjust if necessary. Temperatures will be seasonably warm once again today with highs around 90 east to mid/upper 80s west. Previous discussion... Hard to identify the diffuse surface frontal boundary across the Carolinas this morning, but certainly drier air has moved in behind it at all levels except for the surface as see in areas soundings. Still, lingering weak surface-based instability across the Piedmont overnight had helped to keep a couple of very small isolated showers going near and north of LYH, but which have now dissipated. Despite one or two models suggesting further shower or even thunderstorm activity, do not see that redeveloping at this point given lack of any triggering mechanism. Same scenario today as we had on Wednesday with modest instability by midday but mixing of drier air aloft may limit this as afternoon wears on and any isolated convective updrafts will entrain a lot of dry air aloft and struggle to survive. Still, with some weak convergence along the higher terrain of NW NC and simply better instability in parts of the Piedmont of north- central NC and southside VA, cannot rule out an isolated shower or storm. Have included slight chance pops for these areas along and east of Blue Ridge but basically for southern third of forecast area. It is most likely however that nothing will manage to form during the afternoon and evening other than fairly shallow scattered cumulus clouds. With surface high pressure elongated from off the mid-Atlantic coast to over SW VA...winds will be very light and perhaps a tendency to be from the southeast along and east of Blue Ridge. Skies should clear out tonight and this may once again allow some patchy fog to develop in a few favored valley locations, but with each day of no additional rain the coverage of fog will become less each night.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/... As of 230 AM EDT Thursday... During this portion of the forecast an upper ridge over the central portion of the country will gradually shift west. This will allow for a better opportunity over the weekend for a cold front to move into the area and then stall over the region, or just to our south by Sunday night. Friday, precipitation chances will be limited to primarily diurnal and orographical influences along and near the crest of the Blue Ridge. Coverage of showers and storms will increase Saturday ahead of the cold front, and continue into Sunday with its arrival. Prior to the arrival of the front, 850mb temperatures will gradually rise across the region, reaching 22 to 24 C by Sunday afternoon. Forecast highs by Sunday will be in the low to mid 90s across the Piedmont with a range of 80s across the mountains, elevation dependent. Heat index values will top the century mark during the afternoons of both Saturday and Sunday across the southeastern portion of the forecast area. && .LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 230 PM EDT Wednesday... Upper ridge will extend from coast to coast through Wednesday. In the northern stream the next synoptic scale wave crosses through the northeast United States on Tuesday. Models were similar bringing moisture back into the region on Monday and Tuesday ahead of the front. Therefore this will be the time frame with the highest probability of showers and thunderstorms. Temperatures will remain above normal ahead of the front on Monday. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... As of 740 AM EDT Thursday... With high pressure in control, VFR conditions are expected to prevail through much of the TAF valid period, with the exception of patchy valley fog mainly impacting KLWB TAF with brief IFR conditions. This afternoon expect scattered cumulus to develop but most likely not producing any ceilings. There could be an isolated shower or thunderstorm along the Blue Ridge well south of the TAF sites, and cannot rule out an afternoon shower near KDAN but very limited threat. Winds will remain light and variable through much of the TAF valid period, favoring a S-SE direction at 4-6kts Thursday, with more variable and possibly westerly at times on the west side of the Appalachians, then basically calm tonight. Confidence high for ceilings and vsbys today, then medium overnight. Confidence also high for light winds to continue through valid TAF period. Extended aviation discussion... Strong high pressure aloft centered across the Midwest will attempt to build eastward into the weekend, then retreat back to the west early next week. Our region will remain on the eastern periphery of the upper high and thus subject to weak disturbances in northwest flow aloft tracking around the Great Lakes and into the Mid-Atlantic. Moisture and instability will be sufficient for isolated pop up thunderstorms, but a slightly better coverage will be possible by late Friday into Saturday with a weak front/short wave moving into the Mid-Atlantic with a better threat for convection early next week as another front moves into the area, stalls, and lingers over the region for several days. Hot and humid conditions east of the Blue Ridge this weekend which may impact density altitude. Late night and early morning fog will be possible at the usual sites almost every day. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...SK NEAR TERM...MBS/SK SHORT TERM...DS LONG TERM...AMS AVIATION...MBS/RAB/SK

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