Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 210007

Area Forecast Discussion
National Weather Service Blacksburg VA
807 PM EDT Thu Oct 20 2016

A cold front will slowly move into the region from the west tonight,
then exit the area to the east late Friday morning.  This will spread
showers and possibly a few thunderstorms across the Appalachians and
central mid Atlantic region later tonight into Friday. Strong gusty
winds behind the front will usher in much colder air for the weekend,
and some wet snowflakes are possible at the higher elevations west
of the Blue Ridge.


As of 330 PM EDT Thursday...

Expect quiet weather into this evening before the cold front
approaching from the west can start to push some showers into the
region. Guidance continues to trend slower with this system and the
bulk of the precipitation looks to remain west of the Blue Ridge until
well after Midnight. However, there will be a tight gradient to the
precipitation so a few miles either way will make a big difference as
to whether locations far west of the Ridge making it into some decent
rainfall earlier tonight. Steep lapse rates aloft will also make for
the possibility of some embedded thunder especially far west, with the
thunder chances decreasing overnight.

The front looks to be right along the Ridge by daybreak, but the best
forcing will be shearing off to our north. This is expected to weaken
the precipitation considerably as it progresses east of the Ridge but
some redevelopment of convective elements will be possible in the
piedmont early in the afternoon. Conditions will improve as the front
departs, but upslope winds will keep scattered going west of the Ridge
through Friday afternoon.

Winds behind the front will shift around to the northwest and favorable
momentum transfer indicated in model soundings will make for quite
gusty conditions, especially at the higher elevations from the Blue
Ridge westward. Expect the winds to be on the increase after daybreak
and continue blowing throughout Friday.


As of 330 PM EDT Thursday...

This portion of the forecast to be dominated by sharp upper trough
rotating across the mid-Atlantic region with attendant cold front
pushing well off the east coast east Friday night into Saturday.
Lingering rain and few high elevation snow showers will persist into
early Saturday with a diminishing trend as the day progresses. 850mb
temps fall below 0C Friday night and should allow for some snow or
mixed precipitation in the elevations above 3000 ft. as noted.
Strong winds may well be the most notable feature of this system as
deepening low over the northeast increases pressure gradient through
Saturday. Wind gusts over 30 mph will be common through this period.
In addition some of the coolest temperatures of the season to date
are expected although that is not saying much given the
extraordinary persistence of above normal temperatures over the past
six to eight weeks. Lows Saturday morning upper 30s to low 40s and
similar on Sunday morning. No widepspread frost or freeze is
anticipated as clouds and wind Friday night and wind Saturday night
persist enough to maintain mixing and poor radiational conditions
both mornings. The lows in fact will be right around the long-term
normals for the 3rd week in October. Saturday will feel quite chilly
perhaps owing to clouds/wind and highs from mid-40s west to near 60
east. Those are 5-10F below normal.


As of 330 PM EDT Thursday...

Surface low pressure and associated upper system will be located
over southeast Canada on Sunday with the flow across the lower 48
beginning to flatten out into a zonal pattern. This will promote a
rather dry week ahead with few temperature extremes. A mostly dry
front will push south across the area on Monday with only the
slightest chances for some light precip in the far northwest. Not
until about late Thursday does another front organize over the
midwest and begin advancing eastward. This front looks to arrive
early Friday with timing not too dissimilar to tonights frontal
passage and again with best dynamics and deeper moisture to our


As of 750 PM EDT Thursday...

Clearly the main focus through this TAF valid period will be on a
strong frontal system and negatively tilted upper trough slated
to cross the region overnight and early Friday. Timing has slowed
down on all models and this is in good agreement with current
radar analysis. Thus, while some pop up showers are expected
across southwest VA/southeast WV over the next few hours per HRRR
and NSSL WRF, the main line of convection is not expected to reach
the western part of the CWA until just before daybreak, into the
KBCB/KROA area in the 12Z-13Z time frame, and into the Piedmont
between 14Z and 16Z. With this in mind, there should be little no
potential for any thunder as all instability of significance is
gone within the next few hours. Having said this, it is still
possible to get a rumble of thunder from elevated instability, but
not worth putting in the TAFs at this point.

Thus, cigs/vsbys are largely expected to remain VFR over the next
few hours, with BKN-OVC cigs AOA 040 mainly west of a KBCB-KLWB
line, with cigs AOA 050 further east across the Piedmont. Starting
around 09Z, -SHRA will arrive in the western parts of the CWA at
which points cigs will lower into the MVFR and then IFR range by
12Z with vsbys dropping into mainly the MVFR rain in -SHRA. Cigs
will drop into the MVFR range further east after 12Z accompanied
by a period of -SHRA and MVFR vsbys as well. The initial line of
showers/convection will lift out of the eastern areas by early
afternoon, but upslope showers will continue across the western
mountains. While there may be a brief break in the showers western
areas during the early afternoon, there is clear evidence of a
deformation zone and moisture wrapping back into these areas as
the upper trough takes on a negative tilt and rotates through the
region during the afternoon. At times, given the site setting at
KBLF, expect the cigs to drop to observation height and the cigs
to drop below 005 and vsbys to drop to LIFR.

Further east across the Piedmont, cigs should improve to high end
MVFR or low end VFR after 18Z with no further restrictions to
vsbys at that point.

Until the front passes daybreak or later, winds will be mostly
SSE-SSW 4-8kts, highest in the western areas. After the frontal
passage, winds will become WSW-WNW 10-15kts with gusts 22-30kts.
Winds will likely be stronger on Saturday with better mixing and
insolation than what we will see on Friday.

Medium confidence in cigs and vsbys throughout the TAF valid
Medium to high confidence in wind direction/speed throughout the
TAF valid period.

Extended aviation discussion...

A return to VFR conditions is expected for most areas Friday
night into Saturday. The exception will be western parts of the
region where a healthy northwest upslope flow, and lingering low
level moisture, will prolong an IFR/MVFR ceiling during this time
period. Even the mountain areas will become VFR again for Sunday
and Monday.




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