Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 232306 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 706 PM EDT Fri Jun 23 2017 .SYNOPSIS...
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A cold front, energized by the remnants of what was Tropical Storm Cindy, will progress across the forecast area tonight. Showers and embedded thunderstorms are anticipated late this evening into the overnight, a few of which may produce gusty winds and heavy downpours. The cold front will slowly push southeastward into eastern Virginia and North Carolina on Saturday followed by a cooler and much drier air mass for the rest of the weekend.
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&& .NEAR TERM /THROUGH SATURDAY/... As of 339 PM EDT Friday... While currently on the quiet side, heading into a more active evening with several potential issues to contend with, owing to the approaching cold front which has merged with Cindy`s remnants. Through the rest of the afternoon.... we continue to carefully monitor the potential for isolated thunderstorms. Visible satellite reveals some cloud breaks taking place from the central/southern Blue Ridge eastward. LAPS-based CAPE values are on the order of 1000-1500 J/kg, and we do have strong low-level winds already in place (40-50 kts are common per area VWPs). The strong low-level winds contributing to deep layer shear values near 40 kts per SPC mesoanalysis. If any isolated storms do develop, as reflected in recent runs of the HRRR and the 3-km NAM, they may produce localized gusty winds as they move northeastward off the Blue Ridge. Otherwise, looking at generally quiet conditions into the early evening. For tonight... weather turns increasingly more active, as we will watch ongoing storms across the Ohio and Tennessee Valleys progress eastward across the forecast area. Present indication from a consensus of high-resolution guidance is to bring an evolving line of heavy showers and embedded thunderstorms into our western WV/VA/NC doorstep by around 01z, progressing eastward to the Blue Ridge foothills toward midnight. While it is a diurnally unfavorable period of time for strong convection, wind shear values both through the lowest 0-3km and deep layer owing to Cindy`s wind field only increase. Though it`s difficult to fully trust higher-resolution guidance later in the model cycle, through midnight several members do depict some concerning radar structures as storms move into far western counties. I suspect that we`ll have a high-shear/low-CAPE environment in place tonight, even well after dark. In that environment, even heavy showers could offer the potential to produce gusty winds which may topple dead or shallow-rooted trees. Based on 0-1 km shear values progged to be in the 40 to 50 kt neighborhood, this also poses a concern for isolated embedded spin-ups in stronger convective elements in the line. Given those reasons, to raise awareness of the potential threat I`ve added enhanced wording for gusty winds in the zone forecast through midnight at least, roughly from the Blue Ridge foothills westward into southeast WV. After midnight, heavier showers and storms seem to lose structure as they drift into the Piedmont areas, so I`ve kept the gusty wind wording to central and western counties away from the VA/NC Piedmont areas. Another concern in this humid, tropical air mass will be heavy rainfall potential. Even showers should be efficient rainfall producers. Potential for hourly rain rates to be enhanced by convective elements as well. I`ve maintained heavy rainfall wording in the zones for the tonight period. Fortunately, rainfall we did receive last night was not substantial. And the fast-moving nature to tonight`s rain/thunderstorms will probably keep flooding more localized. Guidance QPF values continue to generally paint some consensus in depicting the highest forecast values northwest of our forecast area, and official forecast ranged from a half to one inch west of I-81, a quarter to third of an inch to the Blue Ridge escarpment, tapering to a couple tenths or less east. For those reasons, confidence was too low on an issuance of a flash flood watch with this forecast package. For more on hydro, see the Hydrology section below. Cold front will have advanced east of the Blue Ridge toward Saturday morning. However, the 12z NAM, ECMWF and GFS suggest that the front may make more slow southeastward progress into the NC Piedmont and foothills areas. While much drier, more refreshing air filters into the northwestern two-thirds of the forecast area on Saturday, I`ve kept lower Chance level PoPs through the day for showers and thunderstorms for the southeastern third. With the front roughly bisecting the forecast area, lows tonight range from the lower to middle 60s west of the Blue Ridge but will be a muggy lower to mid 70s from the Roanoke Valley eastward. Highs tomorrow should top out in the mid 70s to lower to middle 80s. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 339 PM EDT Friday... By 00z Sunday (8 pm Saturday Night), most of the medium range models forecast the position of the cold front across the far southeast portion of our forecast area. This boundary interacting with lingering instability will result in keeping a slight chance for storms from South Boston to Yanceyville early Saturday evening. Dry conditions are expected for the remainder of the short term period as a deep northwest-west flow prevails across our region through Monday. In the upper levels, a broad upper trof will gradually deepen across the Great Lakes into the northern U.S., which will allow dew points to drop through the period resulting in cooler temperatures Monday morning compared to Sunday morning. Leaned toward the cooler guidance values or lows Monday morning. Highs will also be cooler Monday as 1000-850mb thicknesses continue to fall. For late June, humidity levels will be quite comfortable. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 339 PM EDT Friday... The long range models are in good agreement with the overall upper air pattern during this period with an anomalously deep upper trof over the eastern U.S. early in this period, with 500mb heights 2-3 standard deviations below normal,transitioning to a zonal flow by mid week, with ridging building by the end of the week. The 12Z GFS is still fastest with the eastward progression of the upper trof Tuesday night into Wednesday, but this has little impact on the tranquil weather expected during this time. A weak frontal boundary moving through early Tuesday morning may generate some spotty showers in the mountains, but forecast RAOBS indicate there will be very little moisture available. Otherwise, PWAT values will remain below average until Thursday night/Friday so expect a prolonged period of dry weather. By late in the week...a deep southwest flow developing on the west side of an upper ridge and ahead of an approaching upper trof in the upper mid west will increase moisture and provide the opportunity for weak upper level disturbances to pass over our area. This will result in increase chances for scattered thunderstorms during this period. As for temperatures, forecast 850mb temps from the GEFS are 2-3 standard deviations below average, which will lead to some cool mornings Tuesday and Wednesday. Adjusted low temperatures down a couple of degrees below guidance during this period. Temperatures will moderate late in the week. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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As of 650 PM EDT Friday... Overall active aviation forecast period in the first 12 hrs. VFR conditions should prevail into early this evening across the mountains and out east through about midnight. Areas of mechanical turbulence along the spine of the Blue Ridge are likely to continue into this evening as the low level jet ramps up to around 50 kts ahead of convection to the west. Surface winds should also remain gusty at times with KBLF perhaps gusting to 30 kts at times. Deterioration to sub-VFR flight conditions will occur from west to east after sunset with most of the showers either passing east or dissipating during the early morning hours of Saturday. This ahead of a cold front along with the remnants of Cindy that are likely to produce at least a broken band of moderate to heavy showers and embedded thunderstorms. Models have continued to slow down the eastward progress of this line while showing a weaker scenario with best energy passing by to the northwest. Thus have only included a prevailing period for showers at KBLF/KLWB with continued VCSH/VCTS and TEMPO elsewhere although thunder chances look rather small at this point. Appears some low clouds may develop across the mountains as the showers exit with potential IFR/LIFR mainly around KBLF late. Strong wind gusts to 35 kts may also accompany any of the heavier showers. For Saturday, the front should make slow but steady progress southward, with steadily improvement to VFR conditions along with a westerly wind at 8-15 kts with potential gusts to around 25 kts during Saturday afternoon. Extended Aviation Discussion... VFR conditions to then prevail Saturday night through Sunday night. Potential for radiation fog and related visibility restrictions Sunday night. A cold front passing on Monday may produce VFR/possible MVFR conditions and breezy northwest winds behind it. VFR then lasts through Wednesday.
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&& .HYDROLOGY... As of 735 AM EDT Friday... Rainfall forecast/models/ensemble river forecasts suggests greater threat for flooding will be west and north of our forecast area. Average rainfall amounts of 1 to 2 inches expected from the NC mountains to Mountain empire of SW VA to southeast WV with 2 to 3 inches further west into the KY/TN/OH corridor. Not seeing enough signals to warrant a watch, but given efficient rainfall rates, any convective elements or training will increase the threat. Timing of heavy rain will be this evening into Saturday morning. In summary, based on models looking wet, but still not high enough confidence in a flood/flash flood watch. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL/JH NEAR TERM...AL SHORT TERM...PH LONG TERM...PH AVIATION...AL/AMS/JH HYDROLOGY...AL/AMS

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