Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 210847
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
447 AM EDT Sat Oct 21 2017

.SYNOPSIS...
The influence of high pressure over the area will continue to
maintain dry conditions across the region this weekend. A strong
cold front will approach 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 TONIGHT/...
As of 230 AM EDT Saturday...

Surface high pressure was centered over the eastern panhandle of
West Virginia. An upper level ridge of high pressure extended from a
parent high near Charleston, SC, north-northwest over our region,
and farther to over Michigan and western Ontario. Over the course of
the next 24 hours, both the surface high and upper ridge will make
gradual progression eastward, placing the centers of both off the
coast of the Carolinas by late Saturday night.

The result across our region will a trend toward increasing
southerly flow across the area. This flow will help advect milder
air with higher dew points into the region such that high
temperatures today will be slightly, by a degree or two, milder than
those realized on Friday, especially western sections of the area.
Low temperatures tonight will average a few degrees milder than
those expected this morning, especially across western sections.

While we are expecting some short-lived patchy mountain and river
valley fog this morning, the potential increases tonight with the
increase in dew point temperature and winds still remaining on the
light to calm side.

Sunshine today will be abundant again, but with increasing potential
for some cirrus clouds to work their way into the area, especially
in the west, from mid-day and onward. This trend will continue
through tonight across a greater expanse of the region.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 415 AM EDT Saturday...

The strong upper ridge that has been over the area for the past
several days will finally shift east to along the NC coast by
the end of the day Sunday. 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 U.S. Although clouds will be on the increase as
Gulf moisture is drawn northward as well as high clouds from the
rapidly increasing PVA/WAA aloft, temperatures will still be
warm with southerly flow and 850mb temps hovering in the +14C to
+16C range.

Monday, will bring a marked deterioration in the weather as the
southern stream upper low/trough takes on a negative tilt,
begins to lift northeast and phase with the northern stream
trough approaching from the northeast. The ECMWF phases these
two together as the upper low moves out of the southeast U.S,
while the GFS is a bit slower, still indicating that the systems
will not phase until they reach the Mid-Atlantic. While some
light rain will develop during the day Monday, especially along
the southern Blue Ridge thanks to increasing moist southeast
upslope flow, a marked increase in showers will begin around or
just after sunset Monday evening, tracking across the CWA from
southwest to northeast overnight and into Tuesday morning. In
fact, the heaviest precipitation should be during the early
morning hours Tuesday. 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 1.5 to
2.0 inch range for much of the CWA. Only brief, minor flooding
problems are expected given the recent long dry spell, even
though WPC has outlooked our NW NC mountain counties for a
slight risk of excessive rainfall and much of the remainder of
the CWA west of the Blue Ridge for a marginal risk of excessive
rainfall.

With respect to the thunderstorm threat, there is a small window
of opportunity for thunderstorms early Tuesday morning into
mid-morning Tuesday, 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 afternoon
leaving scattered showers in its wake as the northern stream
upper low digs southward into the area. A dry slot may limit the
bulk of the showers to areas west of the Blue Ridge and north of
I-64 at this point.

Temperatures will continue well above normal again Sunday, but
that will be the last day of highs in the 70s as cloud cover,
precipitation, and eventually strong cold advection bring more
seasonal conditions to the area. On the other hand Monday
morning and Tuesday morning will, however, be very mild and on
the order of 20-25 degrees above normal thanks to the cloud
cover, showers, and strong southerly flow. Afternoon
temperatures will be held mostly in the 60s, or near normal.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 445 AM EDT Saturday...

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, all of the extended
models have trended much more progressive with the upper trough,
lifting it out of here as quickly as Thursday while another
upper trough amplifies and moves east from the Midwest. West to
southwest flow aloft will return to the area and allow
temperatures to begin to moderate as quickly as Thursday.

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-Friday will see an end to the precipitation and a
moderation in temperatures as the upper trough quickly lifts out
of the region. Despite the intrusion of cold air, 850mb temps
generally remain above zero through the period, a widespread
freeze is still not anticipated.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
As of 125 AM EDT Saturday...

High pressure across the area will help yield primarily VFR
conditions through the 24 hour TAF forecast time period
concluding at 06Z/2AM Sunday. The exception will be a few hours
around daybreak this morning of some mountain and river valley
IFR/MVFR fog and associated stratus. Any sub-VFR conditions will
quickly improve to VFR a couple of hours after sunrise.

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 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.

&&

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

&&

$$
SYNOPSIS...DS
NEAR TERM...DS
SHORT TERM...RAB
LONG TERM...RAB/WP
AVIATION...AMS/DS/MBS


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