Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 251927

Area Forecast Discussion
National Weather Service Blacksburg VA
327 PM EDT Sun Sep 25 2016

High pressure following the backdoor front will wedge south along
the eastern slopes of the mountains tonight into early Monday.
Another cold front will shift in from the west Monday night into
Tuesday bringing better chances of showers and isolated
thunderstorms for early next week.

As of 330 PM EDT Sunday...

A frontal boundary will drop south this afternoon into tonight in
the South Carolina and then lift northward as a warm front on
Monday. High pressure located over eastern Ontario will move east
tonight into Monday and push off the New England coast Monday.
Low level moisture is trapped under subsidence inversion in
combination with light upslope flow into the higher terrain.
This is resulting in plenty of clouds especially across
eastern portions of the forecast area. The shallow nature of the
moisture this afternoon is allowing cloud cover to erode and thin.
convective cu fields are forming in the west and parts of the south
this afternoon. Tonight, the flow turns more southeast across the
Southern Appalachians. The increased lift along with upslope flow
will produce areas of light rain,drizzle and fog overnight
into Monday morning. Low temperatures tonight will range from the mid
50s across the north to the mid 60s along the southern Blue ridge
mountains. The warm frontal boundary will return north fairly
quickly Monday as a strong cold front approaches from the west.
Considerable cloudiness, fairly warm and humid conditions with
scattered showers and thunderstorms are expected in advance of the
front. Swody2 has placed a marginal risk of severe thunderstorms to
our west across western Pennsylvania and West Virginia. The main
concern to our west is for organized line  segments which could
produce strong winds. The cold front will move across our area
Monday night. High temperatures on Monday will the mid 60s along
the northern  Blue ridge to the upper 70s in the far west mountains.

As of 300 PM EDT Sunday...

Cold front will slowly cross the region from west to east Monday night
and early Tuesday in advance of a closing off upper system rotating
through the Great Lakes into midweek. Models have again trended
slightly slower with deeper pre-frontal convection over the mountains
during the evening, with this swath of lift gradually shifting east,
reaching the Piedmont during the overnight hours. Best support will
also reside west of the Blue Ridge during the evening with somewhat of
a residual low level wedge out east until it mixes out overnight. Thus
still appears best thunder chances western third early Monday night,
with a possible decrease in coverage upon crossing the Blue Ridge late
evening, before some redevelopment possibly occurs out east as the axis
of deeper moisture slows into midday Tuesday. This supports keeping
likely to categorical pops west through midnight with highest qpf
across the northwest and perhaps southwest. Then high chances to low
likelys east overnight where could still see some isolated thunder far
southeast per latest weak progged instability. Still appears a half to
locally one inch of rain possible pending just how fast showers move
east before loss of heating takes place.

Boundary likely to stall just southeast of the area Tuesday
afternoon/evening before finally getting a bit more of a push as
another faint wave rides along and northeast of the region overnight.
This should finally let a strong push of 850 mb cool advection take
place with a nose of surface high pressure sneaking in between the
departing front and moisture with the developing Ohio Valley cutoff
low. Will still be rather mild with lagging cool advection Tuesday
allowing highs to reach well into the 70s Piedmont and 69-75 elsewhere
given some sun. 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. Otherwise kept in some low pops southeast third Tuesday
afternoon/evening, then dry with clearing from northwest to southeast
Tuesday night.

Any drying however to 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 per latest ECMWF. This to
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 200 PM EDT Sunday...

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 Wednesday night 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 154 PM EDT Sunday...

Scattered to broken MVFR clouds this afternoon into tonight with
brief period of VFR west of the Blue ridge. The shallow nature of
the wedge from the high center to our north will allow for some
improving ceilings this afternoon. Then this evening into tonight,
the flow turns more southeast and lower cigs will again be
encountered at all terminals, with most experiencing MVFR with
some IFR possible. Light rain is possible but best chance resides
south of ROA/BCB/DAN.

Overrunning will begin in earnest by early Monday in advance of an
approaching slow moving cold front. Morning low clouds and fog
with widespread sub- VFR cigs/vsbys will continue Monday morning
ahead of the cold front. A few prefrontal showers are possible in
the west late morning.

Medium confidence in ceiling, visibilities and winds
during the taf period.

Extended aviation discussion...

A strong cold front that will approach the area from the west
late Monday. A healthy line of showers can be expected in advance
of the front as it moves through the region during this time
frame. Conditions will improve by midweek, although as an upper
low sinks southward over the Appalachians, conditions could become
unsettled enough to continue periods of sub-VFR cigs and scattered




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