Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 212033
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
433 PM EDT Sat Oct 21 2017

.SYNOPSIS...
High pressure will control our weather tonight into Sunday. A strong
cold front will approach Sunday night and then cross the region
Monday through Tuesday, bringing good coverage of showers and some
gusty winds. High pressure returns to the area by the latter half of
the upcoming week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 200 PM EDT Saturday...

High pressure at the surface and aloft will slowly slide east
tonight into Sunday, Light winds and low level moisture will create
patchy mountain and river valley fog late tonight into Sunday
morning. Low temperatures tonight will range from the lower 40s in
the mountains to around 50 degrees in the piedmont.

The upper ridge will push out into the Atlantic ocean Sunday. The
upper flow will become increasingly southerly as a deep trough
develops across the Tennessee Valley in response to the combination
of a southern stream upper low moving northeast from the Mid-South
and a northern stream upper trough moving southward. Clouds will
increase Sunday as Gulf Moisture is advected northward with
increasing PVA/WAA aloft. The 850 mb temperatures will be around
plus 12 to plus 16 on Sunday. Afternoon high temperatures will vary
from the mid 60s in the mountains to the upper 70s in the Piedmont.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 400 PM EDT Saturday...

This period of the forecast will feature unsettled weather with
a pronounced change from dry to wet, yielding upwards of an
inch of rain across the entire CWA with isolated higher amounts
of close to 2 inches near the Blue Ridge Parkway. Latest model
runs suggest the bulk of the QPF will fall withing a 12 hour
time frame from 3PM Monday through 3AM Tuesday.

The upper flow will become increasingly southerly/meridional as
a deep trough develops across the TN Valley in response to the
combination of a southern stream upper low moving northeast from
the Mid-South and a northern stream upper trough moving
southward from the north central United States. This will bring
a marked deterioration in the weather Sunday night into Monday
with clouds and rain spreading across the forecast area from
southwest to northeast. Rain will initially be light Sunday
night and early Monday, then increase in intensity Monday
afternoon as the southern stream low lifts northeast toward the
area, and culminating as a period of moderate to heavy rain as
the trough passes overhead Monday night.

Occasional heavy rain showers are expected with PWATS around
1.6, well above normal for late October, and strong PVA/dynamics
as the southern stream upper trough takes on a negative tilt
and begins to phase with the northern stream upper trough. QPF
should average in the 1.0 to 2.0 inch range for much of the
CWA. Only brief, minor flooding (poor drainage from leaves
clogging ditches and drain pipes)problems are expected given
the recent long dry spell, and relatively fast movement of the
system.

With respect to the thunderstorm threat, there is a small window
of opportunity for thunderstorms as the upper trough passes
overhead Monday night, mainly southeast of a line from UKF-
ROA-LYH.  During this time frame the combination of a strong
50+kt 850mb LLJ, upper diffluence, weak instability on the order
of 100-400 J/kg CAPE, and notable shear will spread across the
area in association with the LLJ and trough phasing. Still
cannot completely rule out the potential for a QLCS and
potential brief spinups within the line across mainly
Piedmont/Southside areas. We are not currently outlooked, but
this remains something that will need to be watched closely.
Otherwise, the bulk of the heavier precipitation will shift
east-northeast by Tuesday morning leaving scattered showers in
its wake as the northern stream upper low digs southward into
the area. A dry slot (drying aloft) may limit overall coverage
of showers Tuesday and will make the temperature forecast a bit
tricky (potentially warmer than forecast) as the colder air from
the northern stream trough will not arrive in earnest until
Tuesday night.

Until the arrival of the northern stream trough Tuesday night,
temperatures will remain mild, supported by rising dewpoints
associated with the southern stream system. Passage of the of
the southern stream trough will leave the forecast area within
an area of neutral advection early Tuesday with Tuesday`s
temperatures more of a reflection of whether we develop any
breaks in the cloud cover from the dry slot. A more bonafide
push of colder air will take place late Tuesday and Tuesday
night with the passage of the northern stream trough.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 415 PM EDT Saturday...

On Wednesday, the upper trough will begin to shift east of the
region, but keep cold advection and clouds across the region.
This will result in some continued threat for showers across
mainly the Alleghanys southward into the western mountains of
NC. Possibly early in the morning, low-level temperatures and
thickness values will support a mix of rain/snow in the higher
terrain, but nothing supports any accumulation or much QPF at
this time.

With regard to the remainder of the period, all of the extended
models have trended much more progressive with the upper trough,
lifting it out Thursday, west to southwest flow aloft returning
to the area and allow temperatures to moderate quickly.
Wednesday looks like it will be the coolest day of the week,
then moderating for Thursday and Friday.

Models are now hinting at another upper level long wave trough
for the weekend with the potential for showers and bonafide
cold frontal passage. The GFS brings clouds/showers into the
area as early as Saturday whereas the ECMWF is 24 hours slower
holding off on any appreciable QPF until Sunday. Attm will
introduce low chance threat for showers Saturday, but nothing
more until models can resolve the timing.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
As of 132 PM EDT Saturday...

High pressure will remain across the area through the TAF valid
period yielding primarily VFR conditions. The exception will be
a few hours Sunday morning of some mountain and river valley
IFR/MVFR fog, especially west of the Blue Ridge. LIFR fog is
possible at KLWB Sunday morning. Any sub-VFR conditions will
quickly improve to VFR a couple of hours after sunrise. Outside
of an isolated gusts at LYH, winds will be light and variable
through the TAF valid period.

High confidence in ceilings, visibilities and winds during the
TAF valid period.


Extended Aviation Discussion...

A deep upper trough and associated strong cold front will move
into the region from the west by Monday with sub-VFR conditions
in rain/showers along with increasing wind. Colder, blustery,
unsettled weather with perhaps largely MVFR conditions across
the mountains will linger through the later half of next week.
MVFR showers may return by early next week.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...KK
NEAR TERM...KK
SHORT TERM...PM/RAB
LONG TERM...PM/RAB
AVIATION...KK/RAB


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