Area Forecast Discussion
Issued by NWS Blacksburg, VA

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891 FXUS61 KRNK 150607 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 107 AM EST Fri Dec 15 2017 .SYNOPSIS... Variably cloudy skies will continue through tonight. A weak disturbance passing through the region Friday may produce light wintry precipitation during the morning hours. Though amounts are not significant, it may impact the Friday morning commute. High pressure then returns Friday afternoon through Sunday. After Friday, the next chance for precipitation is Sunday night into early next week. && .NEAR TERM /THROUGH TODAY/... As of 306 PM EST Thursday...Variably cloudy afternoon across the central Appalachians, Piedmont and mid-Atlantic region this afternoon. Still some shallow moisture being drawn southeastward on lower-tropospheric northwesterly flow, despite surface ridging building into the region. Broad cyclonic mid- level flow exists across the eastern two-thirds of CONUS. In that flow, a shortwave trough/vort max now over the central Plains will serve as a focal point for sensible weather into the morning hours Friday. Though sky conditions are variable currently, greatest further west one goes, a west-east increase in cloud cover is expected per bufkit soundings during the late-evening/around midnight, especially for the northern/central half of the forecast area. Should see good radiational cooling in most areas through that period of time with most locations in the 20s to low 30s around midnight. Beyond midnight, forecast becomes a bit less certain given some discrepancies in 12z NWP solutions on extent of QPF. Though it`s really a difference between no QPF and limited QPF, the potential for significant impact is possible for locales in/around the Blue Ridge, as much of what falls may be in the form of freezing drizzle. Aforementioned mid-level trough acts on low-level frontal zone, and generates limited to no QPF as it moves eastward, inducing weak cyclogenesis off the NC coast later Friday. The NAM and the higher- resolution CAM solutions generate QPF values less than 0.05", but closer inspection of the sounding indicates the moisture layer is shallow and confined between -2 and -8C, suggesting supercooled droplets with a dry ice crystal layer. Using the top- down methodology, this leads to either freezing drizzle or flurries within about 30 miles either side of the Blue Ridge for the pre- dawn hours into mid-morning Friday. On the other hand, the GFS/ECMWF depict hardly any QPF as the wave aloft makes its way into the mid- Atlantic region. From an impact perspective, freezing drizzle produces the same level of impact as light freezing rain: potential for accidents and glazed trees. Given that confidence in the NAM solution is low, will not issue any winter advisories at this time but will allude to the potential for difficult travel during the morning commute from freezing drizzle in the HWO. Temperatures should slowly warm into the mid/upper 30s with decreased clouds in the afternoon. The upper Piedmont of NC should see highs in the lower 40s. As clouds clear, westerly winds increase with some occasional breezes to 25 mph. && .SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/... As of 301 PM EST Thursday... Will see increasing subsidence as well as drier air punch into the region Friday night into Saturday behind the passing complex of shortwave energy from later Friday. This along with high pressure should make for quiet weather to start the weekend with little more than a flurry far northwest mountains Friday night. However will again see northwest winds ramp up behind the departing system with the next shot of modest cold advection. Expect strongest speeds Friday night as the 850 mb jet increases to around 40 kts and the inversion lowers but for now soundings indicate speeds below advisory levels. Should see winds diminish on Saturday as surface high pressure builds across allowing for a switch to warm advection in the afternoon. Otherwise partly cloudy Friday night with lows in the 20s to mainly sunny on Saturday with highs in the 40s. Heights slowly build Saturday night with southeast ridging taking shape Sunday into Sunday night. However residual upper low heading northeast out of Texas looks to ride up along the west side of ridging across the Tennessee Valley Saturday night/Sunday with some of this lift spilling east toward the region by Sunday night. Models remain inconsistent in just how much deeper moisture will make it into the area given westerly flow and ridging aloft as some solutions dry most precip up crossing the ridges at this point. Thus will continue the trend of bringing low shower chances into the west later Sunday and chance pops most sections Sunday night. Should be warm enough for all rain given late day arrival with temps in the 40s Sunday, and only falling into the 30s to around 40 Sunday night. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 301 PM EST Thursday... Appears mild period to maintain itself for much of next week as flatter upper flow around ridging across the southeast states remains in place for most of the period. Will see some digging northern stream shortwave energy make for a brief return to passing eastern troffiness around midweek but overall weaker now with each run. Other than for much warmer temperatures, main concern will be with shower chances that could linger into Monday, and then perhaps expand again Tuesday with a residual shearing upper system from Mexico that will attempt to undercut the ridge. Guidance spread remains quite large in northward extent of deeper moisture into westerly flow aloft as well as timing through Tuesday night. Appears initial system much weaker so only running with low pops into Monday night with somewhat higher chances Tuesday pending added support coming out of the southwest. Should finally see a cold front cross from the northwest Wednesday allowing for drier but only slightly cooler weather before temperatures rebound again Thursday as high pressure slides offshore. Highs to rebound to well above normal levels for early next week with mostly 50s to near 60 east before cooling some to 40s/near 50 mountains to 50-55 east Thursday. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... As of 100 AM EST Friday... MVFR ceilings developing over the past few hours across eastern WV as an upper trough approaches from the west. VFR ceilings continue elsewhere, but expect MVFR ceilings to spread across a larger area from the west associated with the upper trough and across the Piedmont toward the Blue Ridge as east-northeast surface flow develops across these regions. Several models suggest that low clouds/stratus and potentially toward daybreak that -DZ/-FZDZ could develop along especially the northern and central portions of the Blue Ridge. Moisture is very shallow, but deep enough to support drizzle given the upslope easterly flow along the Blue Ridge. Thus, some light icing will be possible as a significant hazard to aviation during the early morning to mid-morning hours in these areas. QPF, however, will be miniscule trace amounts and at this point there are no winter weather headlines. Winds over night with a pseudo wedge in place will be northeast to east on the order of 3-5kts west to 4-8kts east. By late morning/midday today, another clipper system will be passing by to the north of the area, shifting winds back to the west as well as with increasing speed. Wind speed should ramp back up to the 10-12kt range with gusts into the 20-25kt range later in the day/afternoon. Upslope clouds will linger west of the Blue Ridge with MVFR ceilings likely persisting well into the afternoon. East of the Blue Ridge, clouds will disperse leaving just some high cirrus clouds and VFR ceilings. In general visibility will not be an issue through the TAF valid period, although a brief period of MVFR in -FZDZ BR will be possible near the Blue Ridge early Friday. Medium confidence in ceilings through the TAF valid period. Medium to high confidence in visibilities through the TAF valid period. Medium to high confidence in wind speed/direction through the TAF valid period. Extended Discussion... Generally VFR conditions are expected Friday night into Saturday. . A better chance for sub-VFR conditions from a weather system forecast to emanate from the mid- Mississippi Valley Monday into Tuesday. Sub-VFR conditions will likely return by Sunday afternoon as another system rapidly approaches from the southwest. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/RAB

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