Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 231006
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
506 AM EST Tue Jan 23 2018

.SYNOPSIS...
A strong cold front will continue to move east across the region. It
will usher in colder temperatures, gusty northwest winds, and
mountain snow showers this afternoon and continue into Wednesday.
Temperatures will moderate to above normal again later in the week
before the next frontal system arrives during the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 450 AM EST Tuesday...

997 mb surface low over northern Michigan trails a frontal
occlusion/Pacific front across central WV into roughly the I-77
corridor early this morning. This boundary best shows itself as a
wind shift from south to west-southwest/west. Parent upper level low
currently per water vapor imagery is spiraling east-northeast across
IL/IN.

This frontal system will lead to a relatively active 24 hour period
with ongoing heavy showers early this morning in the Piedmont,
falling temperatures today/tonight, a surge of northwest winds along
the Blue Ridge this afternoon and tonight and potential minor
accumulations of snow tonight continuing into the Wednesday
timeframe.

Primary cold front and dry slot aloft moves eastward through the
morning hours per the GFS. This will lead to a west to east rather
rapid decrease in PoPs along with clearing skies except in the
western mountains. By early afternoon, most of the eastern two-
thirds of the forecast area should see at least partly sunny skies.
Falling 850 mb temperatures should also result through today, with
most areas west of the foothills having already seen their high
temperature occur overnight. Would expect any additional rains
amounting up to 0.15" in the far eastern Piedmont counties. By
afternoon, temperatures should range from the mid 30s in the far
west to the lower 50s in southeastern counties.

In terms of wind headlines, have opted to cancel the Wind Advisory
for the south/southwest winds that was in effect until 6 AM for our
far southwestern mountains. However, have opted to issue a impact
based wind advisory in effect from 1 PM today until 10 AM Wednesday
from the Blue Ridge from Roanoke to Watauga County. Will see winds
also flip to west and eventually northwest by mid to late this
morning. These should become at least breezy by early afternoon as
cold advection aloft steepens lapse rates. Based on GFS/MAV
guidance, local research wind guidance indicates only marginal Wind
Advisory criteria for Roanoke and Boone, with the highest pressure
rises anticipated to occur around 4 pm to around midnight, while the
core of the northwesterly low level jet of around 50 kts looks to
occur after 7pm into early Wednesday morning timeframe. Generally
think peak wind gusts of around 50-55 mph should occur between 6 pm
and 3 am, and continuing to diminish into the Wednesday morning
period. Appears too marginal north of Roanoke and have opted to hold
on further extension per collaboration with NWS Sterling.

And then there`s upslope snow to talk about. Cold advection and
enhanced northwest flow will again favor a changeover from rain to
mountain upslope snow showers from western Greenbrier County
southward into Watauga County. Changeover should occur by late this
afternoon in southeast West Virginia and toward early evening from
Grayson through Watauga County. Though flurries or light snow
showers may at times advance to the New River Valley, simulated
reflectivity from most SPC HREF members suggests any accumulating
snows would be in southeast West Virginia, Tazewell County VA and
Ashe/Watauga Counties in NC. Snow accumulations through 12z
Wednesday should add up to between 1-2 inches, greatest in western
Greenbrier County. Additional accumulations are expected into
Wednesday as well.

Low temperatures tonight range from around the mid 30s in the
Piedmont/foothills, to the upper 20s/around 30 west of the Blue
Ridge.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
As of 430 AM EST Tuesday...

The deep low pressure area moving through the Great Lakes today
will be moving rapidly northeast away from the area early
Wednesday. At 12Z Wednesday, it is expected to already be in the
eastern Canadian provinces. This will allow winds to begin to
diminish and thus the Wind Advisory is slated to end at 15Z or
10 AM EST. Unlike the first two weeks of January, the upper flow
will remain progressive and essentially zonal as this low
pressure area departs. We will see a quick shot of colder air
with 850mb temperatures dropping into the -8C to -10C range
which will equate to lows in the 20s. Temperatures are not low
enough at this point for wind chills to be a concern. Upslope
snow showers across the Alleghanys will linger into Wednesday
morning, with generally 1-2 inch accumulations western
Greenbrier, less elsewhere in the west.

Given that the flow is fairly progressive and zonal, the next
upper short wave, a fairly weak one, will move into the region
let Wed or Wed night, a psuedo Alberta Clipper type system. The
main dynamics are across the Great Lakes and moisture is
limited. However, it will provide just enough dynamics to
enhance upslope snow showers across the Alleghanys where another
1-2 inches of snow is possible by Thursday morning. This short
wave will quickly exit the region later Thursday as a short wave
ridge in the central U.S. moves into the eastern U.S. for
Friday.

Wednesday and Thursday will be on the cool side, but actually
really close to normals for this time of year. Look for lows
mainly from near 20 west to near 30 east. Maximum temperatures
will range from the 40s in the west to the 50s in the east.
These are within a few degrees of normal for mid-January, but
about 20-30 degrees warmer than we were much of the first half
of January!

Friday as the upper ridge approaches and 850mb temperatures
surge back above zero, to perhaps as warm as +8C, look for
surface temperatures to warm into the 50s west and 60s east,
about 10 degrees above normal.

Other than the upslope snow showers, little meaningful
precipitation is expected during this period leaving monthly
precipitation totals a bit below normal for this time of year.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 450 AM EST Tuesday...

As we move into the weekend, the next in the series of short
waves within a continued progressive upper flow will move from
the Midwest into the eastern U.S. The ECMWF suggest that this
could be a decent liquid precipitation maker for the forecast
area as the system becomes more amplified as it moves into the
eastern U.S., and low pressure develops off the northern Mid-
Atlantic coast, while the GFS is much weaker, faster and less
excited bringing about just brief period of showers. WPC seems
to be favoring the slower ECMWF, which brings a general 1-2
inches of rainfall to the area in the Sun-Mon time frame.

Little concern about winter weather with the weekend/early next
week system as 850mb temperatures remain above 0C for several
days, topping out in the +8C to +10C range as the front
approaches Sun-Mon.

Given the differences in the intensity and timing between the
ECMWF and the GFS, the degree of cold air that follows int the
wake of this frontal system early next week and the amount of
post frontal upslope snow showers will largely depend on which
solution verifies. For now, will carry snow showers across just
the Alleghanys following frontal passage Sunday night into
Monday. 850mb temperatures fall back toward -8C per the ECMWF,
but barely reach 0C per GFS with a flatter, more zonal flow.

At any rate, temperatures for the remainder of January will
hover much closer to normal than the first two weeks did, with
essentially equal periods of moderately above and moderately
below normal. It could be said that the temperature variability
will likely be greater than the degree above/below normal.

&&

.AVIATION /10Z TUESDAY THROUGH SATURDAY/...
As of 1222 AM EST Tuesday...

Several potential weather-related issues to TAFs and aviation
operations in the next 6-8 hrs of the 06z TAF period. This spans
mostly low-level wind shear, gusty frontal showers and MVFR
ceilings. Initial flight categories should stay MVFR due to low
ceilings, but frontal rains expected to begin between 08-12z
per recent guidance and will add 4-6 SM SHRA/-SHRA in this
period for most TAFs. May see a rumble or two of thunder with
the best chance being in the Piedmont/southern Blue Ridge
foothills. Low- level jet of up to 60 kts out of the
south/southeast will lead to low- level wind shears of 40-45 kts
producing mechanical turbulence. Given the strong wind fields,
even heavier showers/embedded thunder expected between 08-12z
may lead to gusty winds, with best chance again mainly for
Lynchburg and Danville but can`t entirely be ruled out
elsewhere. Will need to TEMPO if TSRA or stronger convective
gusts more imminent at the TAF as confidence in timing and
placement is too low to place in TAFs attm.

Greater improvement in ceilings/visbys expected post-frontal,
with showers/possible thunder moving east into eastern Virginia.
Should see rapid clearing of ceilings to VFR along and east of
the Blue Ridge after 14z. Post-frontal VFR stratus at Bluefield
and Lewisburg will be slower to thin out into the afternoon.
Late aftn- evening, ceilings lower for these two TAFs to
VFR/MVFR levels as post- frontal northwest upslope becomes
enhanced. May see 3-6 SM -SHSN in this period, becoming more
likely after 03z. Should see winds steadily veer to
west/northwest 8-14 kts. May need to add/increase afternoon wind
gusts in later TAF issuances once degree of clearing becomes
more apparent.

Medium confidence in ceilings, visibilities and winds through
the TAF valid period.

Extended Discussion...

MVFR ceilings with low clouds and potentially -SHSN can be
expected across the westernmost sites, and possibly as far east
at times as KBCB. High pressure and VFR conditions along with
diminishing winds should return to the entire area by Wednesday.
Dry weather including good flying conditions can be expected
Thursday and Friday.

&&

.EQUIPMENT...
As of 355 PM EST Monday...

NOAA Weather Radio, WXL60 (Roanoke Transmitter) which
broadcasts at 162.475 MHz remains off the air. The phone company
reported that the circuit line had been cut and is being worked
on. We are told it is a fiber optic line, so it is taking some
time to repair.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Wind Advisory from 1 PM this afternoon to 10 AM EST Wednesday
     for VAZ015>017-022.
NC...Wind Advisory from 1 PM this afternoon to 10 AM EST Wednesday
     for NCZ001-002-018.
WV...None.

&&

$$
SYNOPSIS...AL
NEAR TERM...AL
SHORT TERM...RAB
LONG TERM...RAB
AVIATION...AL/DS
EQUIPMENT...PM/RAB


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