Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 130553 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1253 AM EST Wed Dec 13 2017 .SYNOPSIS... Mountain snow showers will continue tonight, as a strong shot of modified Arctic air encompasses the region tonight into early Wednesday. Strong northwest winds persist into early Wednesday before weak high pressure builds in. Another clipper moves in Wednesday night, followed by high pressure to finish out the week into the weekend. && .NEAR TERM /THROUGH TODAY/... As of 630 PM EST Tuesday... Adjusted pops and weather to line up better with WSR-88D images and their trends, then shaped towards a blend of HiResW-ARW-East and NAM tonight. Modified temperatures with latest obs, blended to GLAMP and may need to lower Minimum temperatures later tonight. More changes later... As of 258 PM EST Tuesday...It`s beginning to look at lot like winter, and it will certainly feel that way over the next 24 hours. Winter Weather Advisories for snow and blowing snow, Wind Advisories and Wind Chill Advisories for parts of the western Appalachians/Blue Ridge still remain in effect. Radar mosaic reveals several bands of light to moderate-intensity snowfall across the western side of the Appalachians and Blue Ridge. These features in association with strong mid-level shortwave and Arctic surface front, infused by a connection to Lake Michigan moisture. Snow showers tonight, at times with visibilities as low as one-half mile, should be the most persistent across southeast West Virginia, and starting later this afternoon working on southward into Watauga and Ashe County. Snow showers should prove to be more intermittent from the New River Valley eastward to I-81/Blue Ridge, but may still at times lead to difficult travel during the Tuesday evening commute. As temperatures fall tonight, any wet pavement or accumulated snow is likely to freeze and lead to slick travel overnight especially on untreated roads. See the recently issued Special Weather Statement to address potential travel difficulties with periods of snow showers. Looking for snow showers to initially begin on the wet side, but as the -12 to -18C dendrite growth layer collapses amid strong cold advection, looking at more powdery, high-SLR type snow (approximate 16:1 snow-liquid ratio or better at elevation). Most significant change in terms of the snowfall forecast is that by mid-evening, we`ll start to lose the Lake Michigan moisture connection as 700 mb winds become more west- southwest. This should in turn result in snow likely ending sooner from Bluefield to Boone, focusing the main area of snow into western Greenbrier County where accumulating snows should continue well into the evening. Did opt to lower accumulations to 1-2" in Ashe and Watauga County, but given that snow will blow around a good deal there`s probably enough impact to leave the Advisory going for those counties. Otherwise, forecast snow amounts are little changed ranging from 3-6" in western Greenbrier County (such as Rainelle and Duo), 2-4" further south into Summers and Mercer Counties, 1-2" Greenbrier Valley to a coating to an inch to the Blue Ridge. To this point, gusty winds associated with the Wind Advisory have been only marginally close to Advisory level. Boone had recorded a northwesterly wind gust to 43 kts around 15z. A secondary surge of wind gusts is anticipated early to mid evening owing to pressure rises and passage of shortwave aloft. In addition, local research wind guidance board still supports solid Advisory winds along the southern Blue Ridge, so for now will leave Wind Advisory in effect given those highlighted expectations. Northwesterly winds should ease into early Wednesday morning. Looking into the overnight, focus begins to shift to very cold temperatures and low wind chill values. 850 mb thermal trough settles over the forecast area in that time period, with 850 mb temperatures dropping to -10 to -16C across the entire forecast area. Cold advection should have no trouble helping to plunge low temperatures in the teens areawide, even with dampening effect of cloud cover in the far west. With ongoing breezy to locally gusty northwest winds, this will force lowest wind chill values in the pre- dawn hours in the single digits below zero along and west of the Blue Ridge, with values in the single digits above zero to the low teens in the Piedmont and the foothills of North Carolina. If you have to be outside, especially in the Wind Chill Advisory area, make sure to dress in layers. Leftover upslope clouds/snow showers in western Greenbrier should be ending by early Wednesday morning. Though it will still be quite cold into the mid-morning hours, the good news is that the cold spell is short-lived and pattern turns into one of a warm advection regime. Warm advection occurs on southwesterly 850 mb winds of 40-45 kts boosting 850 temps to values -3 to -6C by afternoon. Though much of the day should see sunshine, will start to increase mid-level clouds late in the day ahead of the next Clipper system. Maintained a dry forecast though. High temperatures between the mid 20s to upper 30s, with the coldest readings in the western mountains with a light snowpack. && .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/... As of 258 PM EST Tuesday... Will basically be in between features to start Wednesday evening with the current system well to the northeast and the next fast moving clipper quickly approaching and passing to the north Wednesday night. Guidance again showing less digging with this feature with mainly a passing cold front followed by a brief period of upslope snow showers later Wednesday night into midday Thursday. With the best pocket of deeper moisture progged to pass just north with the main system, should only see light QPF behind the front resulting in perhaps an inch or two of snow northwest mountains including flurries farther south. Elsewhere some clouds/flurries possible along the northern Blue Ridge with mainly clear/partly cloudy skies elsewhere overnight. Winds could again become an issue espcly late as an impressive 40-50 kt jet accompanies this vigorous wave into early Thursday. However latest forecast soundings indicate much of the higher speeds to stay just off the surface until just before daybreak when cooling aloft arrives allowing some mix down of higher speeds at elevation as the inversion lowers. Otherwise expect mixing and clouds to keep temps from bottoming with lows likely early on before bumping up some late. Flow flattens Thursday with moisture and winds decreasing during the day allowing a bit more sunshine by afternoon. However still chilly with highs 30s west to 40s east. Extension of weak high pressure to remain in place Thursday night ahead of yet another weak clipper that looks to pass even farther north Friday into Friday night. This feature to likely bring clouds back overnight and espcly Friday ahead of added split flow energy heading in from the southwest. Models now keep these features separate with dry air limiting any precip to mainly upslope snow showers again far northwest late Friday into Friday night where will again carry some chance pops. Otherwise will keep it dry with more clouds across the region Friday and mainly west Friday night. Temps again to remain below normal. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 258 PM EST Tuesday... High amplitude pattern seen during the past week will finally begin to flatten this weekend and gradually work into weak upper ridging across the southeast states early next week. This should basically act to cut off any added cold air intrusions with more of a Pacific nature airmass from Sunday through Tuesday. However a leftover wave across the Gulf states may push far enough north within increasing warm advection aloft to bring some light precipitation Sunday into Sunday night before drying returns Monday. However timing of this system remains iffy with some solutions much slower with rain lingering into Monday. This would support mainly a rain event with perhaps some mix at the onset if earlier arrival occurs Sunday morning. Otherwise running with mainly chance pops Sunday into Monday morning, then dry Monday afternoon/Tuesday under slightly higher heights and south of most northern stream energy at this point. Good moderation in temperatures to occur through the period as the cold pool aloft lifts out Saturday followed by decent west/southwest flow through Tuesday. This supports highs rebounding to near normal to start and then above normal into Day 7 including some 50s ahead of the next weak front && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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As of 1230 AM EST Wednesday... Aviation conditions will improve through the TAF valid period as the Alberta Clipper lifts northeast of the area, surface winds back more to the west, and upslope clouds and snow showers come to an end by morning. Overnight, we will still see bands/fingers of snow showers with brief moderate snow bursts, accompanied by brief MVFR vsbys and MVFR-IFR ceilings, tracking from the Great Lakes southeast toward the Alleghanys and Blue Ridge. As the upper flow becomes more westerly, the trajectories will become far less favorable for this to continue. Snow showers should end by or before 12Z in the west. VFR conditions are expected throughout most if not all of the TAF valid period in the east. Gusty WNW-NW winds will continue for the next several hours with gusts of 25-30kts common west of the Blue Ridge and 20-25kts east of the Blue Ridge, decreasing slowly after 08Z. Winds will become more W-WSW after daybreak with speeds mostly in the 8-12kt range through the day. It should be noted that some gusts to 50kts will be possible across Watauga, Ashe, and Grayson counties at the higher elevations for the next few hours before that threat diminishes. VFR conditions expected from late Wednesday morning through the remainder of the TAF valid period. Medium to high confidence in ceilings, visibilities and winds during the TAF period. Extended Discussion... Weak high pressure builds in Wednesday evening followed by another weaker clipper system Thursday that passes by mostly to our north. Conditions then trend VFR for the weekend, with the next chance for sub-VFR being Sunday night into Monday with a system emanating out of the mid-Mississippi Valley.
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&& .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Wind Chill Advisory until 9 AM EST this morning for VAZ007- 009>020-022>024. Wind Advisory until 7 AM EST this morning for VAZ015>017-022. NC...Wind Chill Advisory until 9 AM EST this morning for NCZ001-002- 018. Wind Advisory until 7 AM EST this morning for NCZ001-002-018. Winter Weather Advisory until 7 AM EST this morning for NCZ001- 018. WV...Wind Chill Advisory until 9 AM EST this morning for WVZ042>044- 507-508. Winter Weather Advisory until 7 AM EST this morning for WVZ042- 043-508.
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&& $$ SYNOPSIS...AL NEAR TERM...AL/KK SHORT TERM...JH LONG TERM...JH AVIATION...AL/KK/RAB

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