Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 161942 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 242 PM EST Thu Nov 16 2017 .SYNOPSIS...
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A building ridge of high pressure will become established across the region tonight, allowing for breezy northwest winds to abate tonight. Sunny skies, dry and seasonal temperatures for Friday. Increasing clouds and breezy southwest winds Saturday precede a strong cold frontal passage for the Saturday overnight period. Front will bring gusty northwest winds and rain changing over to brief snow flurries/snow showers at higher elevations in southeast West Virginia. Temperatures then trend below normal late in the weekend into early next week.
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&& .NEAR TERM /THROUGH FRIDAY/...
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As of 230 PM EST Thursday... Post-frontal northwesterly gradient flow in place across the Appalachians, Blue Ridge and VA/northern NC Piedmont behind a weakening initial "cool" front. Front`s only real noticeable affect on sensible weather has been breezy conditions with gusts at times up to 30 mph, with closer to 35 mph wind gusts along the ridges. Broad cyclonic flow exists aloft. Expansive shield of stratus/stratocu associated with a 925-700 mb thermal trough axis and shallow trapped moisture underneath it lies to our west from northern KY northeastward into western PA. Regional Doppler VWP trends show a general decrease in northwesterly low-level winds. Though we may see a secondary, brief surge in winds and as best pressure rises move into our western mountains between 21-00z, it`s expected that wind gusts should begin to abate quickly after dark as boundary layer stabilizes. Main forecast challenge for the evening is on temperatures - specifically how quickly do winds decouple to promote sharp radiational cooling. NAM/GFS guidance remain steadfast in progressing the thermal trough across our forecast area, with shallow moisture layer manifesting itself as stratus along the western slopes of the Appalachians into southeastern WV. In those locations, overnight cooling should be slower. While the 3-km NAM and WRF-based solutions show some very low QPF amounts, I`m less optimistic this will transpire as modeled given the ice nuclei layer (around -8 to -10C) is not saturated. May see some rimed tree tops along the Greenbrier hillsides but unlikely to see anything more than that. Kept lows mostly in the upper 20s to around 30 in the western mountains (if clouds are thinner or don`t advance this far south, temps may be 3-5 degrees lower), and closer to freezing in the foothills, Piedmont and Roanoke Valley. Broad area of high pressure establishes itself over the region on Friday, with warming 850 mb temperatures and rising heights aloft. Given that moisture associated with stratus layer is thin, it should easily/quickly mix out. By mid/late morning, most areas will see abundant sunshine, becoming a little more filtered late in the day with an increase in high clouds from northwest to southeast. Given that several locations today reached the upper 50s to middle 60s and our 850 mb temperatures tomorrow afternoon are warmer (about +4 to +6C), opted to side closer to the warmer 12z MAV for highs. This leads to highs again well into the 50s, with upper 50s/lower 60s in the foothills and Piedmont areas.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
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As of 230 PM EST Thursday... Ridging will begin to break down Friday night in advance of the strong upstream shortwave trough that will arrive over the weekend. Warm front also lifting northeast toward the area should allow some increase in cloud cover overnight as deeper warm advection arrives aloft but still dry given better lift west of the mountains through early Saturday. However will see the southwest jet aloft really ramp up as the gradient tightens although appears much of the stronger speeds will remain aloft until overall mixing increases during Saturday. Lows likely to occur early then rise overnight espcly ridges as mixing/warming picks up. Models now generally agree with timing of lead shortwave energy along with its associated cold front arriving Saturday evening followed by trailing waves of upper energy through late Sunday. May initially see a few showers with the warm front lifting across Saturday otherwise appears lobe of deep moisture will remain just west of the western mountains until after dark. This should allow for a mainly breezy but mild Saturday ahead of the front with low chances of showers during the day staying mainly west of the Blue Ridge. Pending the degree of cloud cover enough warm advection/mixing ahead of the boundary to support upper 50s to mid 60s across the region. Guidance now still a bit slower in bringing the front across Saturday night although stronger with the initial surge of winds aloft per 850 mb forecast of speeds of 40-50 kts across the west. Latest local scheme wind guidance showing enough for wind advisory headlines for spots across the mountains with warning criteria possible mainly over the northwest NC ridges into the Grayson Highlands. Limiting factors appear with cloud cover and rather high inversion levels while subsidence and cold advection impressive. Thus will keep mention in the HWO as still 5th period for any watches at this point. Otherwise models suggest a decent swath of showers along/behind the front progressing from west to east Saturday night before ending early Sunday. Still not seeing much instability at night but wind fields and speed of movement of perhaps a low topped embedded convective band could result in stronger winds as forecast soundings continue to indicate. Thus will keep categorical/likely pops going from west to east but without any thunder mention again at this point. Somewhat slower exodus of the moisture could allow more time for much colder air to catch up resulting in a quicker change to snow showers out west before things taper to more upslope nature by Sunday morning. Most solutions giving a half to an inch of snow in spots skipping across the higher elevations from western Watauga across far southwest Virginia to western Greenbrier where the highest amounts look possible despite warm ground temps. Rounds of deeper cold advection will continue Sunday as one spoke of energy crosses Sunday morning with the main core of the upper system arriving Sunday afternoon during better mixing. This could be enough to warrant continued wind headlines although latest forecast soundings suggest less transport to the surface per a weaker jet despite lower inversion levels. Otherwise will be a breezy/windy day under decreasing moisture Sunday with chance pops western ridges and increasing sunshine elsewhere. Latest progged 850 mb temps not quite as cold as previous and with eastern downslope would expect some 50s out east with 40s Blue Ridge and 35-40 west Sunday afternoon.
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&& .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
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As of 130 PM EST Thursday... Upper trough will lift out Sunday night allowing surface high pressure to build in overnight into early Monday. However likely enough of a gradient to continue gusty northwest winds Sunday evening before the gradient relaxes overnight. May also be a lingering snow shower or flurry far northwest ridges otherwise clearing, but cold overnight, with lows mostly in the 20s. Rather quiet period for most of next week with flat upper flow early on giving way to passing troffiness aloft by midweek and an overall split flow regime into Thanksgiving. This should keep things mainly dry but cool through the period with perhaps more clouds with the impulse/front Tuesday night per latest model consensus. Temps to stay below normal, mainly 40s/low 50s for highs, except for a little bump ahead of the front on Tuesday when most should reach the milder 50s.
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/... As of 100 PM EST Thursday... VFR conditions through the remainder of the afternoon. Main impact to aviation is the west-northwest wind gusts which approach 30 kts, though 20-26 kts will be more commonly observed. These gusts unlikely to subside until boundary layer decouples by around mid-evening. Aircraft can also expect areas of localized turbulence in/around the mountain ridges as satellite trends had been showing stable wave clouds earlier this morning. For tonight, continued VFR conditions should be prevalent at most TAFs, along with wind speeds abating to light and variable values. Winds at elevation should also begin to subside to around 15-20 kts by midnight, with less turbulent conditions more probable. Shallow moisture gets shunted down the western slopes of the Appalachians, leading to a period of MVFR stratus ceilings being at Bluefield after 03z. Confidence was lower on MVFR ceilings further east into Lewisburg and Blacksburg and only shown SCT020 here. Moisture depth is shallow and convective mixing should thin out the stratus layer by early morning. VFR high clouds should overspread the TAFs Friday with light and variable winds as surface ridge builds overhead. Extended Discussion... VFR conditions anticipated through Saturday. May see brief low-level wind shear during the Friday overnight period as southwest low- level flow increases to around 40 kts atop calm/decoupled winds. Breezy southwest winds for Saturday with gusts to around 20-25 kts west of the Blue Ridge. A potent cold front moving across the forecast area during the Saturday overnight period into early Sunday. Main impacts are from sub-VFR ceilings/visbys due to low clouds and rain (changing to higher elevation snow showers Sunday morning) and a wind shift to northwest leading to gusty surface and ridgetop winds. May have lingering MVFR stratus western Appalachians Sunday night but should be VFR everywhere else with wind gusts abating. Conditions then transition to VFR Monday into Tuesday. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/WP

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