Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 300610

Area Forecast Discussion
National Weather Service Blacksburg VA
210 AM EDT Wed Sep 30 2020

A cold front that extended from upstate New York to the Florida
panhandle early this afternoon will move slowly east today and
tonight bringing a prolonged period of showers and thunderstorms
to the Mid Atlantic region. Cooler and drier high pressure
builds in for Friday through Sunday.


As of 710 PM EDT Tuesday...

A cold front and associated heavy rain will exit the piedmont
in the next few hours. The upper level portion of this system
remains to the west and will bring another round of showers,
although light, to the mountains overnight.

Low thick clouds likely to hang around across the mountains
overnight with temperatures dropping into the 40s. Increasing
west wind will help scour clouds east of the Blue Ridge.
Temperatures in the east will fall into the low to mid 50s.

As of 247 PM EDT Tuesday...

Our frontal boundary remains stalled along the WV/VA line this
afternoon with low pressure just to our south over the Great Smoky
Mountains. Waves of showers and storms continue to move along the
boundary in a south to north northeasterly direction. Currently as
of 1815z convection is ongoing with what looks like a meso-low
feature across the North Carolina foothills. Out ahead ahead of this
feature and the front there is plenty of storm fuel to work with
Temperatures from Lynchburg to Danville continue to sit around 80
degrees with CAPE values of 1000-2000 j/kg. Bulk effective shear
also remains strong at 40-45 kts good Q-vector forcing and a
strengthening upper jet overhead. With that said, the Storm
Prediction Center, continues to highlight this area in a marginal
risk for severe weather. Locally damaging winds look to be the
primary threat heading into the 4-7pm timeframe especially in areas
east of the US-29 corridor where better instability resides.

Further north into the New River and Roanoke Valley the air is a bit
more stable with limited thunder but some training for ponding of
water.Rainfall amounts of up to 0.75-1.5" have been observed
prompting localized poor drainage flooding concerns. Rain has let up
up somewhat west of these locations as temperatures fall into the
upper 40s and low 50s behind the front.

Life behind the front, shows cold air advection that has already
started in over the western mountains and northern sections of North
Carolina. This will keep temperatures in the 50s and low 60s in
those locations for the rest of the day with lows falling into the
mid to upper 40s. Both hi-res and synoptic guidance depict a bit of
a faster progression of the front to the east as we get beyond 02-
04z Wednesday morning. The GFS is still the fastest here with the
NAM and Euro lagging slightly behind. Best guess at this point looks
to be after 2am from west to east across the area. Some leftover
pockets of light drizzle may be noted especially in areas out toward
the Piedmont. Gusty northwesterly breezes will takeover in the
mountains through the overnight into Wednesday as drier/cooler air
filters in.

Overnight lows look to fall into the mid to upper 40s out west with
low to mid 50s out east. Once again these numbers will be dependent
on the placement and overall passage of the front with the highest
confidence in cooler values west of the I-81 corridor.

Some leftover clouds Wednesday mornings as high pressure takes the
wheel. SKies should clear out quickly by mid to late morning with
sunny skies for the afternoon. Sweater weather returns with cold
air advection draining in as temperatures should recover back into
the upper 60s to low 70s out east and low to mid 60s out west.

Confidence remains moderate to high in the near term with
uncertainty to overall progression of the front as it pushes


As of 200 PM EDT Tuesday...

Trend to drier/less humid and cooler weather Wed night, though
another trough/shortwave digs across the Ohio Valley Thursday
pushing another front through. Models entertain at least isolated to
scattered showers Thu afternoon/evening mainly east of the
mountains, as flow backs ahead of the front and wave of low pressure
develops along the front across NC.

Expect mainly clear skies Wed night with increasing clouds Thursday
into Thu evening followed clearing Thu night.

Lows Wed night will range from the upper 40s to lower 50s west, to
lower 50s east. Thursday should be milder ahead of the front,
especially east, with highs from the lower to mid 70s east, to lower
to mid 60s west. Cooler air advects in behind the front Thu night,
but the coolest air doesn`t arrive til after this period. Lows Thu
night will range from the lower to mid 40s west, to lower 50s east.

Forecast confidence is high for this period.


As of 200 PM EDT Tuesday...

Broad upper trough stays in place east of the Rockies and should
keep our forecast area in a cool pattern. Strong surface high builds
in by Saturday morning, potentially bringing a frost for portions of
the mountains, maybe even a few 32 or lower in valleys like Burkes

Another shortwave digs across the midwest Sunday and models close
off a low over the Ohio Valley by early Monday, with associated
front and surface low pushing across the southern Appalachians. At
the moment models are somewhat different on intensity of surface
features, with the 00z Euro more robust and having a fairly decent
shot of showers by Monday morning with the GFS weaker. Will lean
overall toward a blend of the two until we get closer, so at the
moment look for isolate to scattered shower threat late Sunday night
into Monday, with high pressure again building in Tuesday.

Lows will run about 10 degrees below normal Sat and Sun morning with
30s in the mountains to lower to mid 40s east, then warming into or
through the 40s for Monday and Tuesday morning.

High temps will also run about 10 degrees on average Fri-Sun, then
warm to about 5 degrees below normal early next week. Friday and
Saturday will see highs in the 50s in the mountains, maybe even some
40s in the highest terrain, with 60s east, warming through the 60s


As of 210 AM EDT Wednesday...

Axis of deeper moisture moving east early this morning along with
the western edge of the band of showers. Upper trough will be past
the region by 12Z/8AM ending the upslope lift and light showers
over the mountains. Have mountains becoming VFR by 16Z/noon.
expecting some wind gusts in the 15 to 30 knot range today.

A areas of warm air advection and isentropic lift over the Tennessee
Valley and Mid Atlantic states tonight will bring mid level cloud
cover to the mountains and a low probability of light rain in
southeast West Virginia and the Alleghany Highlands. This layer
of warm air is brought in by a 40 to 50 knot southwest low level
jet so have added LLWS into the KBCB TAF late tonight. Expect
ceilings to remain VFR overnight.

.Extended Aviation Discussion...

High pressure cooler and drier VFR conditions for Thursday.
There could be late night fog at KLWB/KBCB and sheltered
valleys Thursday morning. The next chance of rain arrives
Thursday night and Friday but with limited impact on flight
conditions. Saturday and Sunday are expected to be VFR under dry
high pressure.




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