Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170544 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1244 AM EST Fri Nov 17 2017 .SYNOPSIS... A building ridge of high pressure will become established across the region tonight and 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. && .NEAR TERM /THROUGH TODAY/...
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As of 1240 AM EST Friday... Winds continue to diminish across the area with the bulk of the region experiencing clear skies. The exception to this rule is across parts of southeast West Virginia and the neighboring tier counties of southwest Virginia. Here, strato-cu from the earlier stronger upslope winds persist. With winds now waning, look for these clouds to start to dissipate as well, especially once we get past sunrise Friday and much drier aloft mixes downward. Have tweaked hourly temperatures and dew points to reflect the latest observations and expected trends through early this morning. As of 920 PM EST Thursday... Winds have diminished at all but the highest elevations this evening. Surface high pressure will continue to builds south out of the Ohio Valley into the Mid Atlantic region, resulting in winds speeds becoming lighter by morning. Experimental GOES16 difference images showed expansion of upslope clouds on the western side of the Appalachians. NAMNest and Gridded LAMP guidance had a good handle on the evolution of the clouds overnight so used that for minor adjustments to the sky cover forecast. Also lowered minimum temperatures a couple of degrees outside of the cloud cover in the some of the western valleys and across parts of the foothills and piedmont. While the 3km 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. 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 /TONIGHT THROUGH SUNDAY/... 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. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... 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. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... As of 1130 PM EST Thursday... Wind speeds will continue to lighten and will become variable to calm as surface high pressure moves from the Ohio Valley into the Mid Atlantic region through 12Z/7AM. Satellite pictures showed MVFR clouds over southeast West Virginia. As the upslope flow diminishes, the clouds will erode. High confidence ceilings will be VFR after 11Z/6AM. Conditions will be VFR throughout the day Friday. Once daytime mixing begins winds will generally be from the south to southeast. Mid and upper level clouds spread into the area from the west on Friday night. Ceilings will remain VFR through Saturday morning. A low level jet develops from the southwest late Friday night but models show any resulting low level wind shear will be after the 06Z end of the TAF forecast period. Extended Discussion... VFR conditions anticipated through Saturday. May see low-level wind shear Friday night as southwest low-level flow increases to around 40 kts. Breezy southwest winds for Saturday with gusts to around 20-25 kts. Even stronger gusts are possible at higher elevations. A potent cold front moving across the forecast area Saturday night 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 very 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/AMS/DS SHORT TERM...JH LONG TERM...JH AVIATION...AL/AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.