Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220016

Area Forecast Discussion
National Weather Service Blacksburg VA
716 PM EST Tue Feb 21 2017

An upper level low to our south along with a weakening surface
cold front will serve as a focus for clouds and showers tonight
into Wednesday. A warm front returns northward from the
Carolinas across the region on Thursday, focusing additional
opportunities for rain showers. Weak high pressure then builds
in for Friday, with a strong cold front expected to move across
the forecast area early in the weekend. Temperatures through the
rest of the workweek remain above normal.


As of 321 PM EST Tuesday...Much of the central Appalachians and
VA/NC Piedmont region remain under the influence of ridging
from a broad surface anticyclone centered over coastal New
England. Residual cold-air damming/easterly flow is also holding
tough in northern and central VA Piedmont areas. A light
southeast moisture return is in place across the southern Blue
Ridge/NC mountains and foothills areas. Sunshine has been
filtered from high clouds spiraling north- northeastward in
advance of an upper low centered over western TN; moisture from
which streams northeastward into the Ohio Valley.

For Tonight: Expect clouds to lower and thicken through midnight
from west to east as deeper plume of moisture shifts east-
northeastward. Chances for precipitation also increase in kind as
well, though it will be fighting lingering lower-tropospheric
dryness as well. That being said, recent indication from most of the
global and higher-resolution output suggests that as the mid-level
low builds southeastward into the Gulf later tonight, precipitation
becomes focused along a mid-level deformation axis that slowly
pivots southeastward from southeast WV/southern Shenandoah Vly into
our central CWA after midnight and into the early morning hrs. With
a shallow inversion developing later tonight and an increase in low-
level moisture along with light to calm winds, periods of patchy fog
would be possible especially in the eastern Piedmont. Rainfall
amounts tonight range from a couple hundreths east to up to two-
tenths north and west. Best chance of rainfall looks to be after
midnight. I sided toward the warmer guidance tonight for
temperatures, which are generally in the mid to upper 40s areawide.

For Wednesday: Cold front more or less washes out overnight, with
aforementioned rain band building southeastward into NC during
the day on Wednesday. However, as in-situ CAD signature is
maintained along with the shallow inversion that appears to be
very tough to mix out per forecast BUFKIT soundings, I`m not
hopeful for much improvement. Since mid-level moisture profiles
dry out due to increasing/zonal flow aloft, periods of drizzle
or mist/fog and low clouds appear likely to hang tough into the
afternoon. Thus, with a wetter/cloudier outlook for Wednesday,
I`ve biased temperatures toward a colder end of guidance
(actually, a few degrees colder than 12z MAV/MET MOS). These
highs are actually pretty uniform, mainly in the upper 50s to
low 60s, a couple degrees cooler than today despite milder 850
mb temperatures. Any locations that were to mix out would stand
to be several degrees warmer than indicated here; but I`m not
optimistic that outcome will happen. Winds Wednesday are light
and variable.


As of 321 PM EST Tuesday...

During the Wednesday night into Thursday time period, the axis of
surface high pressure will shift east to off the Atlantic coast. The
general prevailing flow across much of the region will be south to
southwest on the west side of this ridge axis. Low level moisture
will progress into the area, and help maintain clouds and patchy
light rain in areas primarily along the crest of the Blue Ridge.
Also, a weak upper level disturbance will progress eastward along
the Ohio Valley, and allow for precipitation more in the form of
isolated to scattered showers across the far northwestern portion of
the region. The coverage of these showers will shift eastward to
near the crest of the Blue Ridge by thursday afternoon, all while
the extent of the stratiform precipitation decreases and becomes
confined to the far eastern part of the forecast area. Also on
Thursday afternoon, guidance is suggesting surface based instability
may be large enough to support some isolated thunderstorms across
parts of southeast West Virginia, south into the Northern Mountains
of North Carolina

Late Thursday afternoon through Thursday night, a warm front is
expected to develop across the area and head north. Expect some
patchy precipitation associated with the front early in the evening.
By early Friday morning, we expect only some limited light rain near
the crest of the Blue Ridge as southerly flow yield some light
upslope precipitation in this area.

Friday night, a strong cold front will approach the area from the
west. Very strong southerly winds are progged to increase in advance
of this cold front, helping to maintain very mild conditions across
the area. Showers will be on the increase through the night,
especially across western sections of the region. Instability may
increase enough for some isolated thunderstorms across the far
southwest section of the forecast area.

Temperatures during this portion of the forecast will average around
twenty degrees above normal.


As of 321 PM EST Tuesday...

A strong cold front will cross the area on Saturday. Anticipate a
distinct line or area of showers to cross the region during the
course of the day, with coverage greatest across the northern third
of the region. Surface based instability may be great enough in the
afternoon for some isolated thunderstorms generally along and east
of a line from Danbury, NC to Bedford, VA.

Colder air will move into the area Saturday night. Gusty northwest
flow behind the cold front will help maintain isolated showers
across parts of southeast West Virginia that will transition to
isolated snow showers by late Saturday night.

For Sunday, our pattern transition to zonal flow aloft with surface
high pressure providing dry weather to the region.

Sunday night into Monday, a weak disturbance within this flow is
expected to bring isolated to scattered showers to the far western
parts of the area.

Monday night into Tuesday, southwest flow increases across the area
in advance of a developing upper level trough. Look for a warm front
to develop over the region and move northward. Anticipate cloud
cover and patchy light rain to develop across the region, and be
concentrated the greatest across the northern sections of the area,
nearest the warm front.

Temperatures will still be some ten to twenty degrees above normal
on Saturday, but trend to near normal by Sunday behind the cold
front. Temperatures will slowly moderate Monday into Tuesday to
reading about ten degrees above normal.


As of 715 PM EST Tuesday...

High confidence in TAFs trending to IFR through the first part
of the valid period and remaining as such through the remainder
of the period.

A wedge of high pressure will linger over the region through
tomorrow with weak isentropic lift and some short wave energy
running over the top of the wedge. This will lead to lowering of
clouds with showers gradually spreading across TAF sites from
west to east after Midnight. Lowest IFR cigs will set in by
daybreak but may trend upward by the end of the period. Best
chances for precipitation will be through mid morning but low
visibilities in drizzle and fog will linger into the afternoon.
Winds will be light through the period.

Extended Aviation Discussion...

Significant improvement does not appear likely until Wednesday
evening. As a warm front returns northward on Thursday, an
eventual redevelopment of light showers is possible with likely
MVFR conditions Thursday evening through Friday.

Area will slide in between weak low pressure off the southeast
coast Friday and the next stronger upstream cold front that
looks to arrive by midday Saturday. This should allow for some
improvement in cigs/vsbys within the warm sector Friday as the
low level jet increases. Expect a mix of MVFR and low end VFR
ceilings and mainly VFR visibilities during the day with surface
wind gusts of 20 to 30 kts possible across the higher terrain.
Band of showers including a period of sub-VFR to arrive late
Friday night into early Saturday with the front, followed by
drier weather and increasing northwest winds by afternoon behind
the boundary.




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