Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240600 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 200 AM EDT Sat Jun 24 2017 .SYNOPSIS... A cold front, energized by the remnants of what was Tropical Storm Cindy, will progress east across the forecast area tonight. Showers and embedded thunderstorms are anticipated 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. && .NEAR TERM /THROUGH TODAY/... As of 855 PM EDT Friday... Leading edge of weakening convection to the west slowly heading east toward the western counties attm in association with the Cindy remnant racing through NE Kentucky and into West Va. Once the low passes should see shra/tsra get a bit better push off to the southeast with perhaps low topped bands impacting the mountains into the early morning hours. However latest regional radar loops show a weaker area of showers within the line heading toward southwest VA likely due to strong lift just north and a passing storm cluster to the west. This may tend to fill in and intensify given the passage of the main core of 850 mb winds across the western sections in the next few hours and low level instability off evening soundings but iffy. Other concern is with the heavy rain potential mainly northwest sections as the persistent band of heavy rain now northwest of RLX drops southeast once the remnant low passes. Some solutions including the HRRR do bring between 1 and 2 inches to portions of Greenbrier county between 03-09Z while others weaken the heavier rainfall once better support exits. Given uncertainty and isolated nature to possible heavy rain, wont hoist a watch and handle with short term products if needed at this point. May also see some of the showers push east of the mountains after midnight but in a much weaker state. Thus main update was with timing of higher pops northwest and to bump up QPF mainly Greenbrier/Summers into Bath county. Otherwise expecting a breezy and very muggy overnight with higher gusts possible in showers despite lack of thunder. Lows 60s west to low/mid 70s east. Appears exodus of showers will occur a bit faster later tonight with decent drying developing by early Sat. Since this supported by most solutions, opted to lower pops by daybreak Sat and remove most chances during the morning for now. Previous discussion 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 /TONIGHT 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 /06Z SATURDAY THROUGH WEDNESDAY/... As of 150 AM EDT Saturday... Following the HRRR as a guideline for events over the next several hours since it matches well with current radar trends, would expect bands of showers and isolated thunderstorms associated with remnants of T.S. Cindy to translate many east of the Blue Ridge through the morning hours, exiting the CWA before daybreak. Generally looking at still VFR cigs with this activity, worst case scenario low end VFR cigs as air mass is very tropical. Visibilities could be briefly reduced into the MVFR category. The exception will be KDAN as activity developing along the Blue Ridge south of U.S. 460 could be somewhat more intense, and may reach KDAN as a TS or VCTS. Will need to watch, otherwise, just looking for VCSH/SHRA at this point. Through the daylight hours, generally looking at improving conditions as the remnants of Cindy track well northeast of the area. Looking at VFR cigs throughout the day with scattered CU and some SCT-BKN CI. Visibility will not be an issue through the daytime or into the evening. The exception will be KBLF where upslope clouds will result in several hours of IFR to LIFR cigs through the mid-morning, then improving to VFR there as well. Will need to watch for thunderstorm development across the NC Piedmont near a lingering frontal boundary and tail of moisture from Cindy. These could translate into the KDAN area before shifting southeast during the evening. Otherwise, not expecting any precipitation at the TAF sites after 12Z. Gusty winds associated with a 850mb core of strong winds traversing the area at the current time and associated with Cindy will move east of the area before daybreak. Looking for SSW-SW winds 15-25kts through the next few hours to generally diminish to 10-20kts overnight and through the daytime hours Saturday, veering more to the northwest with time as well. Medium confidence in ceilings through the TAF valid period. Medium to high confidence in visibilities through the TAF valid period. Medium confidence in wind direction/speed through the TAF valid period. 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. && .HYDROLOGY... As of 911 PM EDT Friday... Rainfall forecast/models/ensemble river forecasts suggests greater threat for flooding will remain northwest of our forecast area into tonight. Average rainfall amounts of .50 to 2 inches expected from the NC mountains to Mountain empire of SW VA to southeast WV with 2 to 4 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 especially across the Greenbrier Valley of West Virginia into the overnight where the heaviest rainfall is expected. Timing of heavy rain will be in a 3 to 6 hour window from late this evening into early Saturday morning with drier air quickly returning Saturday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL/JH NEAR TERM...AL/JH SHORT TERM...PH LONG TERM...PH AVIATION...AL/AMS/JH/RAB HYDROLOGY...AL/AMS

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