Scientific Forecaster Discussion
fxus61 krnk 130553
Area forecast discussion
National Weather Service Blacksburg Virginia
1253 am EST Wed Dec 13 2017
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
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
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 quantitative precipitation forecast 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
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/...
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 west-northwest-northwest 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 west-west-southwest 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.
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.
Virginia...Wind Chill Advisory until 9 am EST this morning for vaz007-
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-
Wind Advisory until 7 am EST this morning for ncz001-002-018.
Winter Weather Advisory until 7 am EST this morning for ncz001-
WV...Wind Chill Advisory until 9 am EST this morning for wvz042>044-
Winter Weather Advisory until 7 am EST this morning for wvz042-