Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 260911

Area Forecast Discussion
National Weather Service Blacksburg VA
511 AM EDT Mon Sep 26 2016

High pressure will remain wedged against the east side of the
Appalachians today. Meanwhile a cold front will be moving east
across the Ohio Valley. This front will pass through the area
tonight bringing showers and thunderstorms.


As of 430 AM EDT Monday...

High pressure will remain wedged against the east side of the
Appalachians today...east-southeast wind flow resulting in
upslope cloudiness drizzle and fog. West of the Appalachian
divide, downslope wind flow will result in a lot less cloud cover
with little or no precip until the arrival of a cold front later
in the day.

A surface cold front extends south from the Great Lakes into the
western OH/TN valleys. This front is forecast to move
east...approaching the appalachians this afternoon. An axis of
instability just ahead of the front will fuel scattered to
numerous showers and thunderstorms, some of which will be rather
strong as they move into West Virginia during the peak heating
hours of the afternoon. There is a Marginal Risk for severe storms
across West Virginia this afternoon and evening, the main threat
from strong damaging wind gusts.

These storms are expected to weaken as they cross the mountains
this evening, encountering the stable airmass, cool wedge, on the
east side of the mountains. Never the less, still anticipate a
period of moderate to heavy showers this evening and early tonight
with potential for a half inch of rain as the front moves over and
east of the Blue Ridge. Forward progress of the front is expected
to slow per the upper level support lagging, models trending
toward a slower solution with respect to the eastward push of the
front with time.

Temperatures today will be relatively mild in spite of the
cloudiness with highs in the 70s. Any sun, and we could easily test
80. Slower frontal progress also means lingering warmth tonight,
especially east of the Blue Ridge where we are looking at another
night with minimums above 60.


As of 400 AM EDT Monday...

The Cold front will continue to slowly cross the region from west to
east early Tuesday in advance of a closing upper low rotating
through the Great Lakes into midweek. Synoptic scale models continue
to trend slower with the frontal passage, and differ widely with the
ultimate stalling location of the boundary. For now, have slowed the
progression of said boundary by a few hours out of respect to the
ECMWF and GFS solutions. All in all, expect to see impacts in the
form of broken to overcast skies and scattered showers mainly east
of the Blue Ridge Mountains through the day Tuesday, with a rumble
of thunder or two possible in the far east if enough daytime heating
is allowed, which for now seems doubtful.

Boundary likely to stall just southeast of the area Tuesday night.
Models begin trending differently at this point, with smaller scale
solutions allowing the boundary a bit more progression toward the
coast and introducing cold air advection more rapidly, while the
GFS/ECMWF have it sliding back NW as the low begins to slip down
into the mid-Atlantic Wed/Thr. With this uncertainty, have re-
introduced POP/QPF values a bit earlier in the east, but continue to
keep temperatures trending cooler into midweek with cloudcover and
cold air advection in play, especially to the west. Appears enough
progged 850 mb cooling out west under a period of clearing and light
winds to result in some of the coolest lows of the season with
low/mid 50s east, and 40s west including perhaps a few 30s deepest
valleys outside of any fog by early Wednesday.

Any drying between the frontal passage and the closed low will be
short lived as the flow quickly backs aloft Wednesday in response to
the upper low dropping south and deeper moisture to the east
shifting back inland. This will bring a combination of increasing
shower chances Blue Ridge east by afternoon with weak surface low
pressure to the east, and showers along the western slopes along the
leading edge of the deeper moisture axis on the front of the upper
low. Kept pops mostly low end chance most sections with less
southern Blue Ridge for now where expecting a gap between the two.
Otherwise becoming partly/mostly cloudy with highs mostly 60s
mountains to low/mid 70s east.


As of 400 AM EDT Monday...

Trend toward a cloudier/wetter and much cooler scenario continues at
least to start the period with models swinging a strong upper low
from the Ohio Valley early Thursday morning southeast into/and
around the region into early next weekend. This would allow for some
degree of deeper moisture with the residual front along the coast to
get swung back inland around this feature, espcly by weeks end per
latest GFS. However, model spread remains rather large with earlier
ECMWF farther to the southwest with this feature, while the 12z GFS
much stronger now with the cold pool about directly overhead
Thursday into Friday night before the upper low lifts out. Latest
ensembles were closer to the earlier weaker 00z runs excluding the
Euro with a much weaker upper system keeping most showers to the
north and east. Since great uncertainty plan to just continue trend
of including more chance pops Wednesday night into Friday night, at
least for diurnally driven showers under the cold air aloft. Should
finally see this slow moving system weaken and lift out during the
weekend with drier air aloft spilling in.
This will allow for a return to dry weather for later Saturday into
Sunday although clouds may linger given possible redevelopment of
another low level wedge under leftover weakness aloft at this point.

Temps likely to be below normal under very cool air aloft with highs
60s to low 70s through Friday with potential 50s at elevation
pending clouds. Some rebound likely during the weekend as highs
slide back into the 70s overall. Lows also chilly, mostly 40s to
low/mid 50s, with possibly even a few 30s again deeper valleys early
Thursday if a period of more clearing develops early on.


As of 200 AM EDT Monday...

Poor flight conditions will persist today per cool moist airmass
banked up against the east side of the Appalachians. Low clouds
cover much of VA/NC with ifr/lifr conditions right along the Blue
Ridge per clouds intersecting the terrain. In addition to the low
cigs, areas of drizzle and fog will restrict visibility, esp along
the Blue Ridge where MVFR-IFR vsbys are likely.

The cool wedge will begin to erode from the southwest late in the
day per approach of a cold front from the Ohio Valley. Cigs are
forecast to improve into MVFR category after 18Z. Strong cold
front will push into WV/KY by late afternoon with a line of
showers and a few thunderstorms expected to accompany the front,
diminishing as it reaches the KBLF and KLWB TAF sites. Have
included showers for all of the taf sites this evening and
overnight as the front slowly works its way across the mountains.

Extended aviation discussion...

The front will slow in its forward progression across the Piedmont
Tuesday, so showers and a few thunderstorms may linger across that
region as a result. Initially to the west conditions will improve
with mostly VFR to MVFR conditions within this time frame. During
midweek, a rather unsettled pattern is expected to evolve as a
deep upper low sags southward from the Great Lakes into the
Central Appalachians. This will keep variable clouds, MVFR cigs,
and periods of -SHRA across the region through much of the period.




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