Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 161819

Area Forecast Discussion
National Weather Service Blacksburg VA
119 PM EST Mon Jan 16 2017

A warm front across the Carolinas slowly lifts northeast today
stalling across southwest Virginia. A cold front enters the area
by Tuesday afternoon shifting off to our south Wednesday. This
front edges northward again Thursday into Friday.


AS OF 1100 AM EST Monday...

Fog finally lifting across the New River Valley. Satellite
imagery still shows quite a bit of stratus, so not much hope for
sun unless you are west of the Appalachian Divide.

Confluent flow over the northeast at 500 mb will keep high
pressure situated over the area, with a wedge of cool air down
the east side of the Appalachians. Temperatures will struggle to
get out of the 40s on the east side of the mountains this
afternoon, but on the west side (west of a Boone NC to Bluefield
WV line), it will be a much nicer afternoon with temperatures
climbing into the U50s to near 60 along with some sunshine.

Weather boils down to a tale of two weather regimes today with
our far southwest CWA much warmer than the eastern three
quarters. Wedge front will be aligned near I-77 and this will
likely remain static for most of the day. Models try to erode
this boundary from the west per increasing southwest winds
aloft, but cold air is dense and heavy and does not like to
move, so unless we can get some peaks of sun through this
stratus to promote deeper mixing, then the thermometer is just
not going to budge much this afternoon.

Tonight, a decent shortwave pushes across the midwest. At the
same time, surface high over the mid-Atlantic shifts offshore
and well east of the NC coast. Will still see some cold air
damming in the east tonight. Expect temperatures to drop a few
degrees in the east with upper 30s to mid 40s, while the west
drops a little more, though still mild for mid-January with mid
40s to lower 50s from Southern WV to the NC mountains. Upstream
cold front stays far enough west that pops are expected to be
less than previous forecast with limited forcing. With some low
level convergence near the Blue Ridge, will see some low chance
pops here overnight, with slight over little chance in the
piedmont. Rain showers should get pretty close to southeast WV
by dawn so have pops close to 50.


As of 415 AM EST Monday...

While not overly significant, it continues to project to be a
relatively active short-term forecast period with at least two and
potentially three upper-level disturbances focusing rain chances
with above-normal temperatures continuing through most of the period
as we remain on the northern periphery of dominant, if rather flat
southeast CONUS mid-level ridging.

For Tuesday: 500 mb closed low roughly positioned over the mid-
Mississippi Valley deamplifies as it moves northeast. It streams an
weak warm front northeastward across the mid-Atlantic and
Appalachian region during the day, the warm front associated with
warming 850 mb temps and chances for rain. Given the best upper-
level support for large-scale lift is confined to our northern
counties (and frankly is better into northern WV and PA), I`ve
showed something of a gradient in PoPs/QPF associated with the warm
front with the highest respective values furthest north and lowest
in the southside Virginia and the foothills and Piedmont region in
North Carolina. PoPs then begin to decrease slightly into the
Chance/low Likely range into the afternoon, in a narrow warm sector
characterized by overcast conditions and weak to nil surface
convergence in deep southwest flow, ahead of a cold front likely to
be positioned over the Ohio/West Virginia border area. Rainfall
amounts range from a few hundreths south to up to one-quarter inch
furthest north. Tough forecast as it pertains to highs given the
likelihood of overcast and early-day rain offsetting southwest flow
and 850 mb temps warming to near +8 to +10C. I`ve sided toward some
of the more pessimistic/cooler raw guidance into the mid and upper
50s. Should greater clearing be realized, these may be 2-4 degrees
too cool.

For Tuesday Night: Quasi-zonal confluent mid-level flow is replaced
by 500 mb height falls as a positively tilted shortwave trough
interacts with a southeastward-sagging cold front Tuesday night.
Expect any break in afternoon showers to blossom back up along the
boundary, with the highest PoPs across far southwestern Virginia,
southeast West Virginia and building into northwestern North
Carolina. Westerly boundary-parallel flow would argue for a slower
southeastward advance of the front and potentially higher QPF values
across the NC mountains into the Grayson Highlands/Mountain Empire
region (forecast near 0.25") to a tenth of an inch or less elsewhere
of additional rain. Cold advection should allow for low to dip into
the mid/upper 40s, but areas east of the Blue Ridge into the
Piedmont stand to only fall into the low 50s, a couple degrees
cooler than daytime highs.

For Wednesday: Baroclinic zone gradually builds southward into
central North Carolina with any early-morning showers dissipating in
southern sections. More variable cloud cover is anticipated with
greatest coverage along and west of the Continental Divide into
southeast West Virginia in weak ridging. 850 mb temperatures fall in
cold advection to values a couple degrees either side of 0C, and
Wednesday may be the only time our low-level thermal structure is
this cool for the foreseeable future. While post-frontal downslope
flow should push temperatures east of the Blue Ridge into the mid
60s, highs in the lower/mid 50s are more likely west of the Blue
Ridge into the Alleghany Highlands.

For Wednesday Night: Mid-level heights then begin to rise Wednesday
night in response to digging upstream trough over the central
Plains. Surface high pressure the rule in an otherwise uneventful
forecast, with some high clouds advancing in toward early Thursday
morning. Temps look to fall back into the upper 30s under better
radiational cooling conditions than the past couple nights.

For Thursday/Thursday Night: Ridging should generally prevail
through most of Thursday, but is gradually replaced by neutral to
negative height falls in mid-levels by Thursday night as highly-
amplified mid-level closed low over the central Plains moves into
the Mississippi Valley. Deep southerly flow across most of the South
yields a large shield of precipitation, some potentially convective
well to our southwest over the Tennessee Valley. It`s not likely we
see rain into our western zones until overnight Thursday/early
Friday morning. Think in reality the QPF shield may look similar to
that depicted in the 00z ECMWF, so I`ve weighted PoPs more heavily
in that direction. Highs Thursday in the 50s to near 60 in northwest
NC, with lows in the upper 30s/low 40s.


As of 330 PM EST Sunday...

The upper pattern will become highly amplified as we head into the
weekend with a sharp ridge along the east coast and a southern
stream closed low moving across the deep south. This pattern will
eventually bring abundant moisture to the region as we head into the
start of next week.

Focus for steadiest widespread rain remains on Friday. High pressure
over New England will then form a wedge down the east side of the
Appalachians for the weekend. Isentropic lift will be steadily
increasing as low pressure develops over the lower Mississippi
valley and low level southerly winds ride up over the wedge. This
will steadily increase our chances for precipitation, especially for
Sunday into Sunday night.


As of 100 PM EST Monday...

Wedge of cool stable air on the east side of the Appalachians
will promote low cigs, and in some areas low vsbys, through the
24 hour valid Taf pd. Cigs of 1-2kft were common just east of
the Blue Ridge with cigs of 300 to 900 feet withing the vicinity
of the Blue ridge in addition to IFR VSBYS.

Have low confidence that the majority of the area will rise
above MVFR today. Wedge in place will keep cigs down against the
mountains and foothills and this will likely persist until we
can get better mixing ahead a an approaching cold front later
Tuesday. Until then, the expectation is for flight conditions to
improve to no more that MVFR.

Extended Aviation Discussion...

A strong front will arrive from the west Tuesday associated with
showers and widespread sub-VFR. Some actual bonafide 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 ol rain.




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