Area Forecast Discussion
Issued by NWS Blacksburg, VA

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424 FXUS61 KRNK 072350 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 650 PM EST Thu Dec 7 2017 .SYNOPSIS... Low pressure will develop along a frontal boundary over the Southeast United States Friday and move northeast and off the Mid-Atlantic Coast Friday night into Saturday. This will produce a wetting rain across much of the Southeast, with potential for snow, or rain mixed with snow, where the cold air interacts with the system over parts of the Mid-Atlantic States. A stronger front with a shot of colder air and better chances for mountain snow showers will arrive Saturday night into Sunday. && .NEAR TERM /THROUGH FRIDAY/...
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As of 640 PM EST Thursday... No changes needed at this time. As of 330 PM EST Thursday... Full latitude longwave upper level trough extends from Hudson Bay Canada to northern Mexico with a high amplitude ridge along the West Coast. A pipeline of subtropical moisture extends from the Gulf of Mexico across the Southeast CONUS along the eastern periphery of the upper level trough. This moisture coincides with a strong baroclinic zone, basically dividing a colder winter airmass which resides over much of the country from the remnant warmth from weeks past which has now been confined to the Gulf Coast States. Over the next 24 hours there will be several changes within the configuration of the upper trof that may require closer scrutiny to our forecast as any subtle change may introduce higher amounts of rain/snow to the forecast for parts of the forecast area. There is a strong short wave trough embedded withing the longer wave over northern Mexico. This short wave energy is forecast to eject northeast across the Gulf Coast, dynamic lift causing an area of surface low pressure to develop along the existing baroclinic zone over the Southeast. This low is forecast to track across Georgia Friday and to the North Carolina Coast Friday night. Models are currently at odds as to how much backing flow (negative tilt) will take place over the Mid Atlantic as the shortwave ejects from the long wave trough, which makes a profound impact as to how much qpf, or liquid equivalent precipitation, gets imposed over the Carolinas and Virginias... models ranging from less than a tenth liquid to as much as 1 inch, and most of this occurring Friday night (after this forecast period) and over the VA/NC piedmont. Current radar and satellite trends indicate flow to be very confluent over the Southeast with parallel flow aloft and no sign of backing. As such our forecast reflects the lower end qpf with respect to rain/snow. Overall, the main concern through tonight will be more tied to a cloud/temperature forecast with less cloud cover leading to colder temperatures, most areas slipping into the 20s west of the Blue Ridge and 30s to the East. For Friday, do expect mid/high clouds to re-emerge across the forecast area as low pressure develops over Georgia. Have maintained at least chance threat for precipitation along and south of the VA/NC border (mainly in the afternoon), profiles supporting a mix of rain/snow if any occurs, boundary layer temperatures or surface temperatures above freezing for all but the NC Mountains. Model ensemble qpf is close to a tenth of an inch between 1PM-7PM (18-00Z) so even if it were to all fall as snow the only place during the day Friday that would have a chance to accumulate would be the NC Mountains south of Boone where upwards of an inch may occur later in the afternoon. Elsewhere, ground temperatures and boundary layer temperatures are just too warm to promote accumulation... at least before 7PM Friday. Aside from the forecast challenge which mainly impacts our southern and eastern CWA, the areas along and west of the I-81 corridor look to escape any major consequence to any subtle change in the models near term.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 230 PM EST Thursday... Residual deep moisture axis will continue to pivot slowly northwest Friday night as the final and stronger upper wave rides up from the southwest allowing for sharpening of the upper trough. Guidance now suggesting much stronger dynamics heading across at least southern sections late Friday night/early Saturday as southeast sections end up in the right rear of the upper jet. Good diffluence associated with the approach of a slowly phasing northern stream trough may set the stage for a deformation band of heavier precip espcly across southeast sections from just before daybreak Saturday into midday before ending. Profiles suggest this would take the form of wet snow given rates/lift and sufficient low level cold air espcly when it is still dark. Models have also trended much farther northwest with QPF later Friday night into early Saturday with quite a range in ensemble members from little accumulation to headline criteria. Thus have bumped up pops with some light snow possible into the Blue Ridge with likelys southeast. However warm ground and snowfall intensity farther northwest into the drier air makes accumulations iffy across the region. For now bumped up amounts to an inch or so from then northwest NC mountains along the NC/VA border into far eastern Piedmont sections although uncertainty remains large. Otherwise will have a tight gradient in pops with little over northwest sections and higher coverage south. Lows mostly 20s to lower 30s southeast. Coastal wave should exit Saturday afternoon ahead of an upstream clipper that will bring a surge of much colder air from the northwest Saturday evening into Sunday. This combined with deepening northwest flow should lead to more western upslope snow showers Saturday night into Sunday morning before drier air cuts things off Sunday afternoon and overnight. This could again result in enough snowfall over western Greenbrier county for an advisory by early Sunday. Therefore planning to keep mention in the HWO for now. Otherwise will become windy and much colder from late Saturday into Sunday night with local guidance suggesting at least wind advisory criteria along the Blue Ridge Saturday night as stronger 850 mb winds coincide with subsidence and arrival of colder air aloft. Temps mainly 30s to lower 40s for highs Saturday, then much colder Saturday night with winds driving mountain wind chills down to near zero as lows topple into the teens/low 20s. Only slow recovery expected Sunday under the cold pool aloft with highs in the 30s despite increasing sunshine over the region. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 200 PM EST Thursday... Highly amplified pattern will continue next week with ridging over the west and lingering troffiness in the east. This scenario to be reinforced by added rounds of digging shortwave energy that for the most part look to keep well below normal temperatures in place. However will start with a brief reprieve on Monday as the region slides in between clipper systems within a flatter flow aloft that should bring a quick shot of warm advection ahead of the next cold front with highs 40s to around 50. This strong cold front aided by a very deep 500 mb trough will slide through late Monday night/early Tuesday with the next shot of perhaps even colder air on strong northwest winds and upslope snow showers Tuesday into Wednesday morning. May even see some snow showers jump east of the mountains Wednesday night as possible interaction to energy offshore occurs. Pattern should again relax Wednesday afternoon into Thursday under flatter upper flow and some subsequent moderation in temperatures but still below normal through Day 7. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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As of 650 PM EST Thursday... Generally VFR tonight. Some MVFR ceilings possible on the west slopes of the Appalachians in eastern West Virginia. Otherwise widespread mid and high level clouds across Virginia and North Carolina. A low pressure system over the Gulf of Mexico will track to the Carolina coast by Saturday morning. This will spread light precipitation into the Virginia and North Carolina piedmont Friday afternoon into Friday night. High confidence that as the air mass saturates from the top down, ceilings will lower to MVFR, especially south and east of KDAN. Temperatures will be warm enough for rain Friday in all but the mountains of northwest North Carolina. Extended Discussion... Conditions may trend close to MVFR late Friday into early Saturday as potential for light snow to develop across southside VA and into the piedmont of NC, associated with the coastal low. An arctic cold front this weekend could also bring snow showers to the mountains with potential sub-VFR ceilings and intervals of sub-VFR vsby mainly at Bluefield and Lewisburg, with post- frontal strong northwest winds Saturday night into early Sunday. VFR conditions then return Sunday evening through Monday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...AMS/PM SHORT TERM...JH LONG TERM...JH AVIATION...AMS/PM

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