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-- Discussion containing changed information from previous version are highlighted. --
459
FXUS61 KRNK 240130
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
830 PM EST Thu Jan 23 2020

.SYNOPSIS...
The cold front associated with a storm system over the mid
section of the country will cross the area tonight providing a
focus for moderate to heavy rain. Some of the rain may begin as
a period of freezing rain early Friday before transitioning to
all rain.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 800 PM EST Thursday...

After analyzing several models and BUFKIT soundings, having a
difficult time supporting winter precipitation for any length of
time or significant accumulation Friday morning. However, given
that the advisory is out and all have planned accordingly, not
wise to make wholesale changes at this point. Went carefully
through the hourly temperatures to make adjustments to better
match the non-diurnal trend in place overnight. Namely,
temperatures will likely reach minimums around mignight, then
begin to rise slowly under the thick blanket of clouds and
general southeast wind flow. Then, as the precipitation reaches
the western part of the CWA near or just before daybreak,
temperatures will bottom out at their coldest levels as the
initially dry air mass saturates and results in temperatures
lowering to the dewpoint, which should result in a brief period
of temperatures near or just below freezing in the 09Z-14Z
period. Once the temperatures fall, they will be slow to rise,
but likely will rise just enough or remain close enough to 32F
to limit ice accumulations on anything other than elevated
objects such as trees and power lines. Really do not anticipate
major road problems at this point, especially any treated or
previously treated roadways. Ice accumulations have been lowered
across the board and confined mainly to the southern Shenandoah
Valley where temperatures have the potential to remain at or
below freezing the longest into the day. In addition, the Winter
Weather Advisory was pulled back to end at 18Z as opposed to
23Z. Even the coldest models do not support holding onto the WSW
that late into the day. All temperatures really should be above
freezing by noon at the latest, although northern areas and
deeper valleys could hover around 33-34F for several hours into
the early afternoon.

The bigger concern by evening may be the potential for a line of
heavy rain showers resulting in pockets of minor flooding and
also the potential for gusty winds as yet another negatively
tilted trough brings the chance for a QLCS rotating up from the
southwest. However, in comparison to the severe wind event of
two weekends ago, CAPE with this event, given the much colder
temperatures than were in place on the 11th, is negligible.

Otherwise, timing of pops generally good, perhaps a bit fast in
the early part of the forecast cycle, but otherwise seem to
catch well the heaviest rain and best chances for rain. Finally,
references to moderate freezing rain were reduced to just light
freezing rain.

Previous Near Term Forecast Discussion...
.. As of 300 PM Thursday...

Increasing clouds and dry conditions persist through the remainder
of Thursday. Meanwhile, a low pressure system continues to push
eastward across the U.S. As it enters our area tonight, it will
still be fighting dry air at the surface and will have to fight to
saturate this layer, so any precipitation starting off will be
light.

The expectation is still for precipitation to enter in from the SW
after midnight. As this precip enters the area, it will mainly start
as rain. Shortly after starting though, evaporative cooling looks to
cool air at the surface, bringing temps below freezing. The Blue
Ridge is still the prime area of possibility for this given
increased cooling from upslope flow, but we`ve gone ahead with
highlighting parts of the foothills, as well as parts west of the
Blue Ridge, mainly north of 460, where for a very short period of
freezing rain will be possible. By 10-11 AM, temps look to be warm
enough for it to be all rain.

The plan is to split a winter weather advisory to emphasize the
greater risk/amounts of ~0.1" of ice accretion along the Blue Ridge,
and another for areas east and north of Roanoke with one to allow
the better description of just trace amounts of ice. Areas like the
foothills will be such a brief window, that we currently plan cover
it with an SPS as it warrants.

Changing the subject, with the system will also be a stout low level
jet that will be humming along at 50-60kts. This favors
strengthening winds across the higher elevations (ridge crests) west
of the Blue with the potential for damaging winds on the west side
of the Appalachian Divide. This sort of set-up drives some pretty
healthy wind gusts above 4000 feet in elevation, or lower if it can
mix down. In addition, downsloping into the TN valley typically
takes place. Tazewell County typically has wind damage issues in
this sort of set-up. A wind advisory will be issued to cover
this wind risk from Watauga Co, north to Tazewell and into
western Greenbrier.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 240 PM Thursday...

A cold front will move east Friday night and push off the coast. The
main batch of rain will move out of eastern Virginia. An isolated
thunderstorm may be possible in the southern portion of the forecast
area. In the west, temperatures will fall enough in the mountains to
see the rain showers change to snow showers. Low temperatures Friday
night will range from mid 20s in the mountains to the upper 30s in
the piedmont.

The closed upper low over the Great Lakes Region will slowly travel
east Saturday into Saturday night. Winds will turn toward the west
and increase Saturday morning. Boundary layer mixing should tap into
the 850 mb winds around 35 knots and bring decent gusts down to the
surface Saturday afternoon. Upslope rain and snow showers will
continue through Saturday along the western mountains from Boone to
Lewisburg, while the Piedmont should remain dry. High temperature
Saturday will vary from the lower 30s in the northwest mountains of
Greenbrier county in southeast West Virginia to the lower 50s in the
piedmont with some sunshine. Snow showers and snow flurries will
linger in the western mountains Saturday night. Low temperatures
will generally be from the mid 20s in the west to the lower 30s in
the east.

Clouds will linger along and west of the Blue Ridge Sunday and
Sunday night. Upslope precipitation should taper off to snow showers
mainly for southeast West Virginia. High temperatures Sunday will
range from the upper 20s in northwest mountains to around 50 degrees
in the piedmont. Snow flurries will continue in northwest Greenbrier
county Sunday night. Look for low temperatures Sunday night into
Monday morning to feature readings from the mid 20s in the mountains
to the lower 30s in the piedmont.

Moderate confidence in the short term period.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 240 PM Thursday...

The base of the upper trough will pivot east Monday. A reinforcing
shot of cold air from the northwest will continue isolated snow
showers in western Greenbrier Monday. Richer cloud cover will
continue in the western mountains Monday into Monday night. While a
downsloping winds will provide more sunshine in the east. High
pressure will build overhead Tuesday and slide east Tuesday night.
Cloud cover will gradually increase Wednesday ahead of the next
system that takes aim to move in late next week. By Thursday into
Friday, the GFS and ECMWF start to diverge. The ECMWF develop a
coastal low off the North Carolina Thursday which lifts northeast
into Friday. The GFS becomes drier with a more zonal flow, then
upper ridging builds in. Used a model blend for Wednesday into
Thursday and kept highest pops in the west.

Moderate confidence in the long term period.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
As of 800 PM EST Thursday...

VFR conditions are expected to continue to persist through
midnight at most if not all TAF sites with just a mid-high level
overcast in the 100-150 range. The lower levels of the
atmosphere are dry from the recent Canadian air mass and will
require precipitation late tonight/early Friday to saturate and
result in lowering ceilings/visibilities. Ceilings will
transition quickly through MVFR in the 06Z-10Z time frame, then
to IFR or worse at most TAF sites by 12Z-14Z.

Precipitation is expected to be mostly rain and for that matter
all rain at DAN, LYH, and ROA. For BCB, BLF, and LWB, included
some brief winter precipitation, mainly in the form of -FZRA,
but a brief period of PL or even SN is possible at BLF and
especially LWB where the cold air is a bit deeper than further
south and east. Areas of fog will also be prevalent through the
day, especially by afternoon.

By late afternoon or early evening Friday, expect a narrow
squall line with brief heavy rain and possibly gusty winds to
move through the region from southwest to northeast and this has
been reflected in the BCB, BLF, and LWB TAFs. Confidence in this
reaching areas further east is low, so it has not been included
there at this time.

Winds will remain northeast to southeast overnight at speeds of
6-10kts with low end gusts possible in most areas as the night
progresses, but especially at BLF and also LYH. Strong and gusty
southeast winds will become more prevalent early morning at BLF
as the low-level jet moves into that region from the southwest.

/Confidence in Forecast Parameters/
Ceilings - Moderate to High,
Visibilities - Moderate to High, Winds - Moderate to High
Freezing Rain Threat - Low to Moderate.

Extended Aviation Discussion...

Once the front passes, conditions will improve east of the
mountains Friday night. However, anticipate lingering MVFR
cloud bases across the mountains into Sunday with snow showers.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Winter Weather Advisory from midnight tonight to 1 PM EST
     Friday for VAZ011-013>020-022>024.
     Wind Advisory from midnight tonight to 6 PM EST Friday for
     VAZ007-009-010-012-015.
NC...Winter Weather Advisory from midnight tonight to 1 PM EST
     Friday for NCZ001-002-018.
     Wind Advisory from midnight tonight to 6 PM EST Friday for
     NCZ001-018.
WV...Winter Weather Advisory from midnight tonight to 1 PM EST
     Friday for WVZ044-507-508.
     Wind Advisory from midnight tonight to 6 PM EST Friday for
     WVZ042-043-508.

&&

$$
SYNOPSIS...PM/RR
NEAR TERM...PM/RAB/RR
SHORT TERM...KK
LONG TERM...KK
AVIATION...PM/RAB/RR