Area Forecast Discussion
Issued by NWS Blacksburg, VA

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365 FXUS61 KRNK 061041 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 541 AM EST Tue Dec 6 2016 .SYNOPSIS... An area of low pressure over the lower Tennessee Valley will move northeast today, providing a widespread wetting rain across the central and southern Appalachians. Mixed wintry precipitation will occur farther north from northern Virginia and West Virginia into Pennsylvania. The low is forecast to exit the region tonight, with a period of higher pressure Wednesday. A strong cold front will cross the region on Thursday. && .NEAR TERM /THROUGH TONIGHT/... As of 515 AM EST Tuesday... A wet day on tap for the mid atlantic region as low pressure moves overhead from the Tennessee valley. Widespread rain is expected with amounts of a half inch to an inch. Some very light wintry precip, freezing rain may occur at the highest elevations in the VA highlands and from the wilderness area in northern Amherst northward toward Montebello, but coverage limited to the ridge tops with little or no impact to the remaining 99.9 percent of our CWA. Temperatures today will struggle to rise east of the Appalachian divide per wedge of cooler air on the east side of the mountains, temperatures remaining in the 30s to lower 40s for most of the day. Warmer air will be confined to the west side of the divide into the TN valley were readings will climb into the lower 50s Bristol and vicinity. Winds on the west side of the divide will also be gusty, with wind gusts of 30-40 mph across Tazewell and Smyth counties. Primary surge in dynamic lift will occur between now and 18Z/1PM ahead of a negatively tilted short wave trough lifting northeast through the southern Appalachians. Once this surge of PVA passes then precip will wane with rain tapering off from southwest to northeast between 18z/1pm and 21z/4pm. Gradual clearing will follow for most of the CWA tonight as winds become northwesterlybehind the departing storm system. Lingering clouds and light precip will be found across the mountains, with potential for drizzle and or freezing drizzle for the WV Highlands. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... As of 500 AM EST Tuesday... A fairly active period on the whole, particularly for the Thursday and Friday time period owing to a modified Arctic cold frontal passage Thursday, ushering in some of the coldest air yet experienced to this point in this winter season. Daily details below... Wednesday: Lingering precipitation across the western hillsides in Greenbrier County tapers down during the morning hours. Pretty stout dryslot noted in BUFKIT soundings supports idea that any rain/snow showers may end as a brief period of freezing drizzle. Not likely that this will cause much of any impact in this area, though, other than some riming on tree tops. Skies then begin to clear out through the rest of the day, but will see increasing high clouds across the northwestern mountains of North Carolina and southwest Virginia late in the day. Highs should range from the mid 40s in Greenbrier, Bath and Alleghany counties to the mid/upper 50s in the southside of Virginia. Wednesday Night: Clouds should continue to increase from SW to NE through the evening ahead of the approaching Arctic front. Forecast is generally dry until at least midnight as there should be some antecedent dry air that should need to be overcome first. PoPs are limited to the slight to low Chance range for areas west of the Blue Ridge for rain and/or higher elevation snow showers with no accumulations anticipated. I think areas east of the Blue Ridge remain dry; though one thing to watch in later model runs is the potential for some light drizzle in the piedmont areas in North Carolina and into the southside of Virginia. Lows range from the low 30s for far western VA into southeast WV, mid 30s for the NRV and northwest NC mountains and upper 30s/near 40 east of the Blue Ridge into the VA/NC piedmont. Thursday: Arctic front marches east across our forecast area during the morning to early afternoon hours Thursday. As indicated earlier and continuing with the 00z guidance suite, the trend continues to be less enthused about precipitation chances with the frontal passage. I really couldn`t justify going higher than Chance associated with the frontal passage itself. Given the timing of the front itself, timing of highs will likely vary to some extent across the forecast area (i.e. earliest in western areas and not until early afternoon across the central Virginia piedmont/southside and the North Carolina piedmont). Once the front clears, expect a marked increase in northwest wind speeds across the entire area, strong cold advection and PoPs taking on a more NW-flow orographic character with any rain transitioning to light snow showers. Temps range from the low 30s west to the upper 40s/low 50s in far southeastern sections. Temperatures should be falling pretty steadily by Thursday afternoon, however. Thursday Night: In a word, cold. Strong cold advection commences during this period with 850 mb temps falling to values around -10 to -15C across the area according to the GFS. For areas in southeast WV and far western VA, northwest flow should be enough to wring out some upslope snow showers, but an unsaturated -12 to -18C dendritic snow growth layer should mitigate any significant accumulations. It will be quite breezy Thursday night which will make it feel even colder. Looking at apparent temps, I have wind chill values ranging from the single digits pretty much west of the Blue Ridge and the NC mountains to the teens in eastern and southern areas. Given lows in the teens to low 20s and wind chill values this cold early in the season, definitely want to dress warmly for any outdoor plans Thursday night. Friday: Continued cold and breezy with any leftover clouds and snow showers across western sections eroding through the day. Despite abundant sun, highs should only top out in the 20s to the mid 30s. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... As of 430 AM EST Tuesday... Trough lifts out later Saturday with fast zonal flow aloft developing Sunday into Monday as the surface high shifts east with warming/moistening return flow developing. This will allow pieces of energy ejecting out of the Rockies to perhaps bring some chance of precip by late Sunday into Monday at this point. However guidance shows considerable spread with consensus having a stronger northern stream wave push a weak surface front with faint southern connection toward the region by Monday. This could bring some chance of precip to mainly the western counties by Sunday evening per faster GFS despite residual low level dry air. Appears this would be mostly rain Sunday night into Monday given column warming with possibly some mix late Sunday pending timing. However quite iffy espcly given much slower timing off the 12z ECMWF. Should see temps rebound into the upper 30s/40s Sunday and 40s to mid 50s Monday as warm advection aloft reaches the surface. && .AVIATION /10Z TUESDAY THROUGH SATURDAY/... As of 1200 AM EST Tuesday... Poor aviation conditions expected through the TAF valid period. Negatively tilted short wave will move northeast from the southeast U.S. Tuesday bringing widespread rain, fog, and low clouds to the area through much of the TAF valid period. Local radars and surface observations indicate that light rain has reached the U.S. 460 corridor. Saturation of the lower air mass continues to occur, but will occur fairly quickly once the rain overspreads the region. KBLF already seeing MVFR cigs. All areas should be IFR by 08Z with areas of LIFR thereafter. Visibilities will quickly fall into the MVFR range, then settle into the IFR range in the 08Z-10Z time frame and hold there through much of the day Tuesday. Strong short wave will lift to the northeast of the area in the 20Z-22Z time frame, at which time the steady rain will end, but expect widespread low clouds, fog, and drizzle to remain in place into Tuesday evening, smilier to what we saw yesterday, until better drying can develop after daybreak Wednesday. Expect northeast winds east of the Blue ridge and southeast to the west of the Blue Ridge, with the possible exception of KLWB, where a northeast wind is also possible from cold air drainage. An 850mb southeast jet will traverse the region overnight, which will equate to a period of gusty winds in the usual western locations, namely KBLF. Some low end gusty winds are also possible across the Piedmont later tonight into Tuesday morning. Otherwise, generally look for speeds of 7-10ts through the TAF valid period. Winds will become light and variable Tuesday evening after the short wave passes, but then shift to the northwest toward 06Z west of the Blue ridge. Medium confidence in ceilings and visibilities through the TAF valid period. Medium confidence in wind direction/speed through the TAF valid period. Extended aviation discussion... Drizzle, low clouds, and fog will likely remain in place Tuesday night before drier air and westerly flow evolve across the area Wednesday. Wednesday will be the transition day ahead of an Arctic cold front that is expected to move through the region Thursday. Limited moisture may result in a few showers in advance of the front Thursday, but upslope conditions should support snow showers across the mountains Thursday into Friday along with a significant surge of cold air. This will lead to VFR conditions east of the Blue Ridge through much of the period beyond Thursday, but MVFR conditions may linger into Friday across the Alleghanys. West to northwest winds behind the front will be strong and gusty into Friday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...PM SHORT TERM...AL LONG TERM...AL/JH AVIATION...KK/PM/RAB is the U.S. government's official web portal to all federal, state and local government web resources and services.