Area Forecast Discussion
Issued by NWS Blacksburg, VA

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328 FXUS61 KRNK 132336 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 636 PM EST Wed Dec 13 2017 .SYNOPSIS...
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Increasing clouds expected tonight ahead of an Alberta Clipper system. This storm system moves across the region overnight, producing a period of light accumulating snow and strong gusty northwest winds overnight into early Thursday. High pressure then returns Thursday afternoon through the weekend with dry conditions and temperatures trending above normal.
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&& .NEAR TERM /THROUGH THURSDAY/...
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As of 630 PM EST Wednesday... Raised pops in the west for this evening into tonight closer to a blend of the HRRR, ECMWF and NAM to capture the light chances in the northwest portion of forecast area and to add scattered snow flurries further south highlighted on WSR-88d images. Adjusted temperatures with last surface obs, their trends and shaped towards GLAMP. More changes later this evening... As of 324 PM EST Wednesday...Potent 500 mb shortwave trough and associated 992 mb Alberta Clipper low centered near the Chicago area is trailing a surface warm front southeastward from it into the western Appalachians. Currently the central Appalachians to the mid-Atlantic region is in a southwesterly warm advection pattern, in the warm sector of the cyclone. This clipper will be the primary weather maker for the region`s weather over the next 24 hours, producing another round of light accumulating upslope snow showers in southeast WV and into the mountains of NC late tonight, and another burst of strong northwesterly winds overnight into Thursday. Due to the potential for gusty to strong northwest winds, I`ve issued a Wind Advisory with this forecast, taking effect at 7 PM tonight and lasting through noon on Thursday for areas along and adjacent to the southern Blue Ridge and the Interstate 81 corridor near/south of Roanoke. Will be looking at a general deteriorating trend through tonight, with clouds on the increase and pre-frontal southwesterly winds/gusts increasing in lockstep. Southwest wind gusts project to be as much as 40 mph at elevations above 3000`, but it will be relatively breezy in most areas through the first part of the night. Would expect temperatures to fall slowly if much at all through midnight given warm advection, blanket of clouds and breezy conditions. Moisture into the clipper system will be very slow to recover, as dewpoints in the warm sector air mass in the Deep South are only in the low 20s. Thus expect any precip with the cold frontal passage to probably evaporate. Despite being well-developed, the clipper system is really fast-moving, and the entire forecast area should be post-cold frontal by the pre-dawn hours. Look for light snows to once again develop into the favored northwesterly upslope areas in southeast West Virginia into Tazewell County after midnight tonight. There may also be periods of light snow showers or flurries into the Blue Ridge and NW North Carolina mountains into the early morning hours, as indicated by the past several cycles of the HRRR and 3-km NAM. But bear in mind this is not the same situation that we saw unfold yesterday. Though low level instability is present given sharp cold advection, we simply don`t have the Great Lakes moisture connection, the progressive nature of the system limits storm total QPF/snow amounts, and poor dendritic snow growth microphysical conditions to produce the near-whiteout conditions we saw at times Tuesday. Snow should come to an end in the early-morning Thursday hours. So from an accumulations perspective, showed an coating to a inch from Lewisburg to Boone northwest, 1-2" in Summers and Mercer Counties to 2-3" in the favored western Greenbrier mountains. Northwesterly wind gusts are the more notable aspect of the clipper. 850 mb northwest winds increase to around 55 kts behind the cold front during the after-midnight/pre-dawn hours, while the strongest 6-hourly pressure rises occur essentially during the Thursday morning commute. Local wind guidance research indicated marginal Advisory criteria conditions for the southern Blue Ridge given those conditions. Given the timing of the strongest winds being around the morning commute with some potential travel impact, and in collaboration with surrounding WFOs who already have wind headlines in effect, opted to issue for areas along/adjacent to the Blue Ridge and along I-81. I-77 in the Carroll County/Fancy Gap area and along the Parkway may encounter some difficult travel as well. Peak gusts should top out around 55 mph, with a better likelihood of achieving gusts this high being at elevations above 3000`. As low-level jet eases, should see northwesterly winds begin to ease up into the afternoon hours. Lows tonight in the upper 20s to near 30, while forecast highs Thursday are in the low 30s to mid/upper 40s.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/... As of 324 PM EST Wednesday... More in the way of zonal flow will develop Thursday night in advance of the next complex of both northern/southern stream energy that will approach and pass across the region on Friday. These continue to remain out of phase with deeper moisture remaining to the east Friday with a surface wave offshore, while just ahead of the northern shortwave trough. Guidance showing lots of mid/high clouds from later Thursday night through much of Friday but with little precip. Thus leaving it dry for now Thursday night and Friday followed by spotty upslope snow showers far northwest sections Friday night, but only 20ish pop for now given lack of moisture with this system. Otherwise expecting another cold Thursday night under lighter winds with lows mostly upper teens to mid 20s pending how fast clouds arrive. Went below Mos on Friday given progged clouds for much of the day which should hold the mountains in the 30s and low 40s east. Fast west/northwest flow to continue behind this system into the weekend with slowly building heights by Saturday as the 850 mb cold pool starts to lift out. This warm advection wont deepen enough to be realized at the surface until Saturday with sunshine helping to push highs into the 40s to perhaps around 50 southeast. Some increase in high clouds possible Saturday night but likely thin enough to still allow lows to reach the 20s to around 30. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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As of 324 PM EST Wednesday... Warming trend will continue late in the weekend into early next week as weak southeast ridging keeps a more zonal trajectory in place until Wednesday when a passing trough aloft digs back into the region. However with the warm advection will see moisture return perhaps by late Sunday but moreso Sunday night into early Monday with a residual upper low ejecting northeast across the Gulf states. Appears much of this precip would be light and mostly liquid given westerly flow and well above freezing 850 mb temps. Models then diverge on whether or not this feature helps develop a weak low offshore, aided by yet another shortwave headed out of the southwest states, or things remain more progressive and drier into Tuesday. Given somewhat wetter trends will continue chance pops Sunday afternoon into Monday while leaving in low pops northwest Monday night/Tuesday for showers ahead of the next front and over the south per lingering shortwave energy. Drier and colder air should follow the front for Wednesday as deeper northwest flow kicks in. Otherwise looking at highs rebounding to above normal Sunday-Tuesday and well into the 50s east for early next week.
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&& .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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As of 636 PM EST Wednesday... VFR conditions will prevail this evening into tonight. After 04z, aviation conditions begin to deteriorate particularly for the western mountain terminals, due to northwesterly upslope snow showers and snow flurries, low ceilings and strong post- frontal northwest winds. Though VFR conditions should predominate Lynchburg, Danville and Roanoke through the TAF period, should see ceilings drop to MVFR at Bluefield and potentially at Lewisburg and Blacksburg. Lower confidence in overnight ceiling forecast for Lewisburg and Blacksburg. Will see most persistent snow showers with highest confidence mainly on the West Virginia side and in the southern Blue Ridge in North Carolina, but there may be periods tonight especially between 09-14z with potential snow showers/flurries as far east at Lewisburg, Blacksburg and Roanoke. Snow showers and snow flurries look to abate by noon Thursday, with ceilings also trending VFR. Northwesterly winds become gusty post-frontal overnight lasting into midday Thursday, though the core of strongest winds should be between around 07-13z offering potential for low- level wind shear and turbulence along the spine of the Blue Ridge. Medium confidence in ceilings, visibilities and winds through 04z. Extended Discussion... Looking at VFR conditions with abating wind speeds/gusts Thursday afternoon and evening. Other than an outside shot of MVFR/VFR ceilings and possible upslope snow showers at Bluefield Friday night, VFR conditions should prevail until Sunday night. A better shot exists for sub-VFR conditions from a system system forecast to emanate from the mid-Mississippi Valley Sunday night into Monday.
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&& .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Wind Advisory until noon EST Thursday for VAZ009-012>017-022. NC...Wind Advisory until noon EST Thursday for NCZ001-002-018. WV...None.
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&&to high $$ SYNOPSIS...AL/KK NEAR TERM...AL/KK SHORT TERM...JH LONG TERM...JH AVIATION...AL/KK

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