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NWS Area Weather Forecast Discussion
County Warning Area [CWA]: RNK
Regional NWS Weather Office: Blacksburg, VA

FXUS61 KRNK 200850

Area Forecast Discussion
National Weather Service Blacksburg VA
450 AM EDT Fri Oct 20 2017

High pressure will continue to keep our weather pattern dry heading
into and through the weekend. A potent cold front will approach and
cross the region Monday into Tuesday, bringing a return of showers
and gusty winds to the region. Drier weather returns to the area by
the middle to end of next week.


As of 200 AM EDT Friday...

Surface high pressure was centered over eastern KY/TN. A broad upper
level ridge was positioned from the Florida Panhandle northwest into
the Upper Mississippi River Valley. Today into tonight, the center
of the surface high will shift east and be situated over the
Delmarva Peninsula by Saturday morning. The upper ridge also will
have shifted east a couple hundred miles so that its axis will be
overhead, just slightly east of the region.

The result will be another day and night of very limited cloud cover
and no precipitation. The one change will be a trend towards
slightly milder temperatures as both geopotential heights increase
across the area and 850mb temperatures inch up about 0.5 to 1.0 deg
C more as compared to those of Thursday.

We are expecting another morning with some short-lived patchy fog
and stratus mainly in the mountain and river valley regions. Any fog
and low clouds that do manifest themselves this morning are
expected to have mixed out by the mid to late morning.

High temperatures today will range from the low to mid 70s across
the mountains with mid to upper 70s across the Piedmont. Low
temperatures tonight will range from around 40 degrees to the mid
40s across the mountains to the mid to upper 40s across the Piedmont.


As of 410 AM EDT Friday...

A strong 590dm+ upper ridge will be anchored over the region
Saturday with 850mb temperatures hovering in the +14C to +16C
range. Expect a continuation of abundant sunshine and well
above normal temperatures with afternoon max temperatures on the
order of 10+ degrees above normal. Although Sunday will still be
warm, Saturday will be the last really warm day with near full
sunshine. Lows will be in the 40s and 50s with high temperatures
in the 70s, close to 80 across the Piedmont.

For Sunday, the axis of the upper ridge will slowly shift off
the coast as the combination of a southern stream closed low
moving out of the Arklatex region and a northern stream digging
trough continue to amplify the flow across the eastern U.S.
Given the increasing meridional and digging/developing nature
to the upper flow, the approaching weather systems will be slow
as the upper ridge trudges eastward. The net result will be a
continuation of an unseasonably warm day Sunday. Clouds will be
on the increase, but do not expect any precipitation to arrive
yet during the day Sunday. Afternoon temperatures will continue
to average about 10 degrees above normal, despite the increased
cloud cover. Morning low temperatures will be well above normal,
mostly in the 50s.

During the day Monday, the southern stream system will move into
the southeast U.S., while the northern stream system digs into
the Midwest. This morning`s model runs show these remaining as
two distinct separate systems, with the southern system
containing the bulk of the deeper moisture, instability, and
shear. Overnight Monday into Tuesday morning, the increasingly
negatively titled upper trough or perhaps closed low (per GFS)
will begin to lift northward into the Mid-Atlantic. Expect a
marked increase in showers and rainfall as this occurs. Healthy
rainfall amounts of 1-2 inches will be possible by Tuesday
morning daybreak, especially along the southern Blue Ridge.

The threat for severe weather with this system late Monday
night/early Tuesday is definitely non-zero as a 50kt 850mb LLJ
traverses the region with a strong upper jet and upper
diffluence. While instability is not impressive, enough CAPE
exists given strong low-level shear to have some concern for
severe thunderstorms during the late night/early morning hours.
The favored mode would appear to be a embedded QLCS, with the
greatest concern across the NC Foothills, Piedmont, and into
Southside Virginia.

Clouds and precipitation will keep temperatures down Monday, but
given no change yet in the air mass, temperatures will still run
5-10 degrees above normal, especially minimum temps with cloud
cover and much higher than normal PWATS/dewpoints.


As of 435 AM EDT Friday...

A deep trough will evolve across the eastern U.S. Wednesday as
the aforementioned southern stream and northern stream energy
phase together, most likely along or near the Mid-Atlantic
coast. The bulk of the heavier precipitation and any threat
for severe weather in our area will occur Tuesday morning before
shifting well east/northeast of our area. A dry slot will
quickly spread into the region from the southwest by
afternoon/evening, confining the better threat for showers to
the western mountains at that time.

On Wednesday, the upper trough will begin to shift east of the
region, but keep cold advection and upslope 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, namely Thursday,
all of the extended models have trended much more progressive
with the upper trough, lifting it out of here as quickly as
Thursday as another fast moving trough moves across the northern
U.S. West to southwest flow aloft will return to the area and
allow temperatures to begin to moderate by the end of the week.

Temperatures Tuesday will still average above normal as the
front/cooler air mass moves slowly across the area under the
high amplitude/parallel flow aloft. Wednesday will be a rather
chilly raw day, especially across the mountains where rain/snow
showers and gusty northwest winds are expected as the upper
trough reaches its deepest point. Thursday will see an end to
the precipitation and a moderation in temperatures as the upper
trough quickly lifts out of the region.

Look for minimum temperatures in the 50s Tuesday dropping into
the 30s by Thursday. Maximum temperatures will still be above
normal Tuesday in the 50s and 60s west to the lower 70s east,
albeit with somewhat of a non-diurnal trend as temperatures fall
during the afternoon behind the front. Wednesday will be the
coldest day overall with clouds, showers, gusty northwest winds,
and CAA. Look for highs in the 40s west to the lower 60s east
with lows in the 30s west to the 40s east. Thursday, with
greater amounts of sunshine, after a chilly morning with lows
largely in the 30s, afternoon temperatures will warm into the
50s west to the 60s east.


As of 130 AM EDT Friday...

Mainly VFR conditions are expected through the 24 hour TAF
forecast period concluding at 06Z/2AM Saturday thanks to high
pressure remaining over the region.

The exception to this rule will be some IFR/MVFR fog/stratus
development later tonight into early Friday morning mainly with
in the mountain and river valley regions. Any sub-VFR conditions
will improve to VFR by the mid to late morning Friday.

Extended Aviation Discussion...

High pressure will remain over the region through Sunday with
generally VFR conditions expected, outside the usual late
night/early morning patchy river and mountain valley fog.
A deep upper trough and associated strong cold front will move
into the region from the west by Monday promising a period of
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 as an upper trough cuts off across the central





Forecast Discussion from: NOAA-NWS Script developed by: El Dorado Weather