Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 161727 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 127 PM EDT Wed Aug 16 2017 .SYNOPSIS... Weak high pressure over our area will be sandwiched between a front over the mid-Atlantic and a trough off the southeast coast. This pattern stays the same into Thursday, before a cold front shifts from the western Ohio Valley into our area Friday. This cold front will slide into the Carolinas during the weekend. && .NEAR TERM /THROUGH TONIGHT/...
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As of 125 PM EDT Wednesday...Earlier fog has since fully burned off, and in its place has been cumulus clouds with insolational heating. Deepest cumulus per visible imagery in far southwest VA in Smyth, Wythe and Tazewell counties, and in the mountains of NC. Regional radar shows scattered showers starting to develop north of Abingdon and Bristol that should brush across those aforementioned counties this afternoon associated with weak surface low and associated weak disturbance aloft. A few thunderstorms possible, with brief downpours in these as well, but not expecting storms to become overly strong at this point with SBCAPE values presently at/below 1000 J/kg per SPC mesoanalysis. Current forecast reflects this pretty well with no significant changes needed. Other than accounting for current T/Td observational data and blending in with current forecast, no other alterations were made attm. Previous near-term discussion issued at 236 AM follows... Overall airmass does not change much this period. Will see deeper moisture/higher pwats to our south while a frontal boundary stays draped across the mid-Atlantic into the midwest, with Hurricane Gert veering out to sea. This morning will be concentrating on fog, as some areas have dropped to under 1sm, but overall still not seeing enough coverage for a dense fog advisory as rainfall yesterday was less numerous than expected. Will monitor conditions and issue a Special Weather Statement at least to cover the patchy dense fog, and possibly upgrade to a Dense Fog Advisory if it becomes more widespread. Once fog lifts there should be a period of mainly sunshine though some higher clouds will stream by in the westerly flow aloft. Will see the fog turn into a cu field then some towering cu to CBs after midday in the southwest mountains of VA into NC, as weak upper disturbance and moisture convergence along the spine of the Appalachians acts to form showers and storms. Coverage today will be low, with most not seeing any rain. With more sunshine will see warmer temperatures with highs in the 80s, around 80 NC mountains, higher ridges of WV/VA to upper 80s east of the Blue Ridge and including Roanoke, Lexington, VA. Tonight, the upper heights rise with weak ridging. Still models showing weak convergence into the overnight across the southeast, then another area of lift reaches the central Appalachians by dawn. Very low confidence on much shower coverage overnight, after evening showers/isolated storms fade over the NC foothills and piedmont. Added a slight chance of showers around dawn in the WV mountains as well. Fog will form in the river valleys as well late tonight but increased cloud cover appears to limit coverage. Lows staying above normal with mid to upper 60s mountains, to lower 70s out east.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 300 AM EDT Wednesday... Weak ridging aloft will continue Thursday as a weak warm front aloft slides back through the area from the southwest. A low level southwest flow will advect deeper moisture north Thursday afternoon. The combination of a boundary, low level convergence and stronger instability will result in scattered showers and thunderstorms. The Day Two Convective outlook keeps our area in general thunderstorms with marginal risk to our west across the Ohio Valley. High temperatures Thursday will range from the mid 70s in the mountains to around 90 degrees in the piedmont. A few showers and thunderstorms may linger into Thursday evening before loss of heating and exodus of the weak boundary takes place Thursday night. Low temperatures Thursday night will vary from the mid 60s in the mountains to the lower 70s in the piedmont. A cold front should slowly pivot toward the region Friday afternoon preceded by a lead band of showers that could jump into the eastern lee trough during the afternoon where deeper convection possible. Weak lift is expected ahead of the boundary, given that most upper support shearing out to the northeast with the passing weak shortwave. Behind the pre-frontal axis, a west winds may hinder convection in the mountains. However progged instability still quite strong east of the mountains and if timing is slower then could see better thunderstorm coverage in the east. There is enough shear, better instabilities and healthy lift to support isolated strong storms. High temperatures Friday will generally be from around 80 degrees in the west to near 90 degrees in the east. Temperatures will depend on cloud cover and timing of convection. Any convection Friday evening will taper off with the loss of solar heating Friday night. Low temperatures Friday night will feature readings from the upper 50s in the mountains to the lower 70s in the piedmont. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 300 AM EDT Wednesday... Transition to more of an upper troughing regime will again occur over the weekend allowing another surface cold front to push into and perhaps south of the region by early next week. Latest models again have differences on the southward push of drier air behind the front espcly given another upper disturbance that looks to pass just north by later Sunday. However appears passage of this feature may be enough to briefly shunt deeper moisture to the south/east before the trough flattens into broad ridging early next week. Therefore will leave in some low pops mainly southeast third later Saturday where the front may slow up during heating, and slight to low end chance pops espcly southern/southwest sections Sunday as the wave/trough passes. Monday look drier but rather hot with surface high pressure building in from the north behind the front resulting in better subsidence under a weak wedge. Thus limiting any pops to start the week to mainly the southern Blue Ridge, with overall more sunshine elsewhere for now. Added some light pops to Tuesday and Wednesday as weak shortwaves rotate through. Confidence is low on shortwave energy in the extended that may rotate further north around the subtropical ridge as hinted at by ECMWF. Despite the frontal passage, will actually see warmer high temps given heating of somewhat drier air and warming aloft to start next week. This supports overall above normal highs with low/mid 80s mountains to near 90 east through much of the period. && .AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
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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.
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&& .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...JH/WP NEAR TERM...AL/WP SHORT TERM...KK LONG TERM...JH/KK AVIATION...AL/WP EQUIPMENT...RAB/WERT

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