Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 171134

Area Forecast Discussion
National Weather Service Blacksburg VA
634 AM EST Tue Jan 17 2017

Warm front lifts north across our area today. A cold front
will track into the mountains this evening then southeast to the
coast by Wednesday morning. Weak high pressure builds over by
late Wednesday, then another storm system moves over the
southern Appalachians by Friday.


AS OF 620 AM EST Tuesday...

Increased threat of showers over the New River Valley northwest
to SE WV and the Alleghanys this morning. Getting less fog now
that showers moving in. No other changes to the overall forecast
as appears best threat of showers will stay across the north and
west this morning then slide east by late morning.

Previous early morning discussion...

Surface high situated over Eastern North Carolina shifts east today
with warm front lifting north across us as approach of cold front
with showers erodes the shallow cold air east of the WV/VA border.

Expect the fog this morning to erode after 12z/7am, maybe later near
Fancy Gap, VA and the higher ridges of NC before showers move in
scour it out. Coverage of dense fog is not enough for an advisory
but will keep the Special Wx Statement going.

Cold front early this morning from Northwest Indiana, south to the
lower Mississippi Valley should be close to the mountains by late
afternoon. Best lift should stay over WV into the Alleghanys today
with less threat of showers further south. Nonetheless, should still
see high enough coverage with the warm front/cold front over most of
the area to warrant high pops, from categorical in the Greenbrier
County area toward Bath County, VA to high chance in southside VA
and NW NC mountains. The best threat will be late this morning into
early afternoon.

Some hint of instability showing up in the high-res models later
this afternoon, so thunderstorms not out of the question over the
Mountain Empire, but do not see a good enough chance to have mention
in the forecast, given extent of clouds/showers, and main energy
heading north.

Temperatures should stay mild as warm front moves north of us today,
despite the showers and cloud cover. Most locations should reach the
mid to upper 50s, with some lower 60s not out of the question.

Front moves southeast through the forecast area tonight. Some
difference in model solutions, and could see area of deeper
convection form east or southeast of our forecast area. Appears
another area of lift with shortwave energy moves toward Southeast WV
during this time so could see a lull in precip in between. Went with
this trend but slower. Running likely pops over WV to NW NC
mountains to chance pops east. Temperatures staying mild with lows
in the mid 40s west to mid 50 southeast.


As of 415 AM EST Tuesday....

After an initial northern-stream trough exits Wednesday,
relatively quiet weather conditions are anticipated for
Wednesday night into the first part of Thursday. Late in the
forecast period, a complicated mid-level pattern begins to
evolve with southern stream becomes rather active/unsettled. The
first of what may prove to be several southern-stream
disturbances (continuing into the medium- range period) begins
to affect the Appalachians and Piedmont Thursday night into

For Wednesday: Relatively sharp positively-tilted trough treks
across northern West Virginia into the mid-Atlantic region through
the day, while an associated 850 mb front sags southward into North
Carolina. Despite otherwise favorable broader lift associated with
the trough, moisture becomes rather shallow through the column of
atmosphere. Maintained chance-level PoPs across southeast West
Virginia and into the Virginia Mountain Empire, give I`ve included
mention of patchy drizzle along with chance showers. Thus, I really
don`t think upslope QPF Wednesday will be all that substantial,
maybe amounting to a couple hundreths at most. Despite variable
cloud cover (greatest west), it will turn rather breezy Wednesday
due to sharp cold advection occuring during the day. Per NAM/GFS
BUFKIT soundings, northwest winds at the top of the mixed layer
during the aftn are around 35 kts at Roanoke and Bluefield. Feel
that northwesterly gusts in the 30-35 mph would be possible in
western areas into the NC foothills, more in the 20-25 mph range
east of the Blue Ridge into the Virginia and NC piedmont areas.
Highs in the lower 60s furthest east aided by downsloping and
greater sun, but may only top out in the 40s to mid 50s west of the
Blue Ridge.

For Wednesday night: High pressure builds in and essentially anchors
itself over the CWA overnight. Result should be slackening winds and
generally clear skies allowing for good radiational cooling. I
wasn`t confident enough to place in the grids given the time of
year, but I did think of the prospect of radiational fog across
parts of the area given a few days of damp conditions, light winds
and crossover temps in the upper 30s/low 40s being exceeded.
Something for later shifts to evaluate. Temps should cool into the
30s, coolest in the Greenbrier Valley/Alleghany Highlands.

For Thursday: Ridging sfc and aloft begins to shift east. It is
replaced late in the day by isentropic lift in advance of a potent
closed low over the Central Plains and surface low near northeast
Arkansas. 00z suite of guidance shows significant differences in
timing eastward increase in PoPs. The 00z NAM and the past couple
runs of the SREF mean are the most aggressive in introducing PoPs
into our western zones later Thursday, while the GFS is later
Thursday evening and the ECMWF is more into Friday. Given the
amplified nature of the pattern in mid-levels which can lend itself
to slower eastward progression, I`ve sided toward the slower
solutions, keeping Thursday dry. I would however expect an increase
in clouds, capping high temperatures only in the lower to middle

For Thursday Night and Friday: Upper closed low deamplifies as it
advances into the Ohio Valley Thurs night and Friday. Surface warm
front looks to advance into our western zones early Thursday night
and continuing to progress northeast overnight and Friday. Result
should be PoPs increasing into the Likely/low Categorical range.
Given PWAT values that increase to near 1" and strong mid-level
diffluent flow, appears that a window of steady widespread rain may
be possible during the overnight hours and into Friday associated
with the warm front`s advance. PoPs then decrease into Friday
evening as system moves off the coast. Highs on Friday may be
tempered significantly by abundant clouds and leftover CAD and I`ve
leaned on the colder end of guidance, in the low 50s.


As of 330 PM EST Monday...

The upper pattern will be in a state of flux for the end of the week
and through the weekend, as one closed low which weakened/moved east
Friday is replaced by a very large closed low which dominates the
eastern US by Sunday night. This will keep a progression of weather
system s affecting our region through Monday.

In response to the changing upper pattern, another low will develop
over the mid Mississippi valley and push what appears to be a
dynamic occluded front into the area on Sunday. By this time deep
moist southerly flow will be well established above the boundary
layer, and when coupled with the dynamics of the front has the
potential to produce a significant amount of rainfall. Guidance also
indicates the presence of some instability which would support a
chance for some thunder in an environment of robust low level winds.
This is still quite a ways off but well worth keeping an eye on in
the coming days. We eventually get into the dry slot, but moisture
and colder air wrapping well around the vertically stacked system
will keep a good chance of rain around through Monday.

Temperatures will remain well above normal through the weekend, but
colder air wrapping around the vigorous low on Monday will bring
temperatures down a bit west of the Blue Ridge.


As of 625 AM EST Tuesday...

Should see IFR or worse cigs/vsbys start to rise to MVFR to VFR
at times this morning as showers move in from the west. At times
the showers may be heavy enough to keep vsbys/cigs down under
1kft especially BLF. A brief period of low level wind shear with
a 35-40kt 8h jet occurs in the mountains this morning.

Front expected to get into the mountains this evening, and main
focus for showers will be from the ROA/BCB areas west to BLF/LWB
and at times LYH with DAN with lower threat. Some instability
may sneak into the mountains of Southwest VA southwest of BLF
this afternoon bring some thunder. Showers should start to taper
off this afternoon/evening in the ROA/LYH/BCB area and south,
while keeping it showery into tonight at BLF/LWB per upper level
support with front.

Question overnight to be mixing of winds and cigs/vsbys. At the
moment expect enough wind to keep most places no worse than
MVFR, except the BLF airport which is more elevated and cigs
will be around 1kft.

Extended Aviation Discussion...

Clearing may take place behind the front on Wednesday with
potential period of VFR Wednesday into early Thursday. However,
this is expected to be short-lived per return of unsettled
weather by weeks end as moisture returns from the southwest. No
precipitation type issues are expected through the end of this
week...just plain ole rain.




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