Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 230004

Area Forecast Discussion
National Weather Service Blacksburg VA
804 PM EDT Sun Apr 22 2018

Overnight through Wednesday, a slow moving area of low pressure will
progress from the Middle Mississippi River Valley northeast to over
the eastern Great Lakes region by Wednesday afternoon. The result
for our area will be widespread heavy rain for the whole region, and
gusty winds across the mountains. In the wake of this system, two
additional disturbances look to cross over or near our area Thursday
into Friday.


As of 745 PM EDT Sunday...

Right now, continuing to await the arrival of the rain and wind
from the approaching low. Newest short term guidance indicates
no need to re-invent the wheel on this update. Adjustment made
to temps and DP to reflect actual conds. Otherwise, previous
forecast holding up well.

As of 230 PM EDT Sunday...

A slow moving upper level low pressure system will progress from
just west of Memphis, TN tonight, to just northwest of Nashville, TN
by tomorrow afternoon. In advance of this low, winds across our
region will become southeast, and gradually increase in speed. The
result will be copious moisture drawn into the area from the
Atlantic getting lifted upslope into and across most of the area.
Also, by Monday afternoon, the influence of the left exit region of
the upper jet on the south side of the upper low will add a region
of strong diffluence aloft across the area. With this scenario, we
are expecting a heavy rain event for the area, with the
precipitation making its way into the area from southwest to
northeast late tonight through Monday. The amounts expected over a
multi-day period will prompt the issuance of a Flood Watch. Please
reference the HYDROLOGY section of this discussion for details.

The exception to the above pattern will be the far southwestern part
of our forecast area where a southeast flow yields downsloping
conditions. Areas such as Bluefield, WV, and Richland and Tazewell,
VA should have notably less precipitation, but much gustier winds.
The gusty winds will also be prevalent at the higher ridge tops of
the Northern Mountains of North Carolina, and the Mountain Empire
region of southwest Virginia. At these different locations, gusts of
35 to 45 mph are expected starting early Monday morning at the
highest elevations, and spreading to lower elevations by Monday
afternoon. A Wind Advisory will be posted to address these concerns.


As of 300 PM EDT Sunday...

Definitely looking at a wet period from Monday night into Tuesday
night. Will be monitoring flooding issues both stream flooding, to
potential main stem flooding. See hydro section below for details.

Southeast low level jet increases to 2 to 4 standard deviations above
normal Monday hence the wind advisory. Having rain falling may
hinder winds mixing down, but higher terrain from East River
Mountain along the Appalachian Trail to the NC mountains will be very
windy, with gusts to 45 to 55 mph not out of the question.

PWATs and moisture transport although high, not too much over 1 SD.
Expect the surface low over Georgia Monday evening to work with the
strong high off the New England coast to pull Atlantic moisture
right into the Blue Ridge. Although deep convection is not expected
some rainfall rates of one half inch for 3-6 hours may lead to

The low takes on a Miller B setup with one weakening over KY/TN
while secondary one strengthens over southeast NC by Tuesday
afternoon. The southeast flow weakens Tuesday afternoon so rainfall
will begin to wind down by evening.

The surface low should be off the mid-Atlantic Coast Wednesday, with
lingering showers as the upper trough stays to our west and a
northern stream front tracks toward us.

Temperatures staying very cool Tuesday with rain and east flow, with
highs running about 1- to as much as 20 degrees below normal, and
with a wedge in place, coolest highs will be along the Blue ridge
into the Alleghanys with upper 40s to lower 50s, while far southwest
VA toward Chilhowie will reach the upper 50s, and the piedmont
should reach the mid 50s.

The clouds and rain will keep low temps above normal Monday and
Tuesday night in the mid 40s to lower 50s.

With not much airmass change Wednesday with northwest flow
increasing along with a return of some sunshine east of the
mountains, highs will milder, from the upper 50s to lower 60s
mountains, to lower 70s east of the Blue Ridge.


As of 135 PM EDT Sunday...

Deterministic models are highly variable this period, especially
after Thursday with the pattern. The 00z ECMWF takes a front east
of us by Thursday morning with drier weather in place, followed by a
weak piece of energy  moving over the Gulf Coastal States into
Friday morning while surface high stays over our area. The 500 mb
pattern shows a broad trough over the eastern half of the nation,
with two distinct storm tracks into the weekend. Overall the ECMWF
has us mainly dry with a few showers possible with a front cutting
across Saturday.

Meanwhile, the 12z GFS shows a strong Gulf Coastal system Thursday
with better southwest flow into the southern Appalachians. Then it
develops a surface low across the central Carolinas Thursday
evening, tracking it to the mid-Atlantic coast by Friday afternoon
while a strong northern stream upper low forms over the lower Great
Lakes, with a secondary front and shot of colder air moving in
Friday night into Saturday.

Will lean toward the steadier ensemble means this period which for
now paints at least a small chance of showers Wed night-Friday, with
possibly cold enough air after the front Saturday morning to have
snow showers in the Alleghanys. Expect temperatures to run at or
just below normal this period.


As of 745 PM EDT Sunday...

VFR conditions look to continue overnight tonight while a slow
moving low pressure system pushes high clouds over the forecast
area. As increased moisture is introduced into the region under
southeast flow, sub-VFR ceilings will begin to build in from the
southwest to the northeast. Guidance continues to suggest a
broad swath of rain will move into the area, first affecting our
TAF sites in the early afternoon. This will likely cause
fluctuating cig/vis through the end of the TAf period.

As the pressure gradient increases ahead of the low, variable
winds will trend more toward the southeast, and some decent wind
gusts can be expected in many of the mountainous locations
overnight into tomorrow. BLF in particular will see some very
strong winds with occasional gusts to near 40 MPH possible.

Still plenty of uncertainty for exact timing of when sky
conditions will begin to deteriorate, however confidence is
medium to high that all TAF locations will still be VFR through

Extended Aviation Discussion...

Sub-VFR ceilings and visibilities and gusty winds in the west,
will continue through Monday night and early Tuesday.

Tuesday afternoon and night, the low pressure responsible for
the wet weather will shift east of the area. This will bring our
winds to the northwest, and allow for the precipitation to trend
showery in the west.

The weather pattern will remain unsettled Thursday into Friday
as a series of disturbances crosses the area, each with the
potential for providing showery precipitation and localized
sub-VFR conditions.


As of 300 PM EDT Sunday...

A prolonged period of strong upslope, moisture laden flow into the
region will produce a very generous rainfall for most of the region.
Amounts of one to two inches will be common for most the area.
However, across the area near the crest of the Blue Ridge from near
Floyd, VA southwest into the Northern Mountains of North Carolina,
amounts of three to four inches are more likely. It will be this
region, and downstream flow areas in parts of Patrick, Surry, and
Wilkes Counties, that have the greatest potential for flooding in
the nearer term. A flood watch will be in effect from 800 AM Monday
through 400 PM Tuesday. Confidence in a significant upslope rain
event is heightened by the Ensemble easterly wind component
anomalies of 2 to 4 standard deviations during the time of concern.

Looking a little bit further out in time, Model Ensemble River
Forecast are focusing on the Dan River as a location to watch for
possible minor river flooding once we reach the Tuesday into
Wednesday time frame.


VA...Flood Watch from Monday morning through Tuesday afternoon for
     Wind Advisory from midnight tonight to midnight EDT Monday
     night for VAZ007-009-015.
NC...Flood Watch from Monday morning through Tuesday afternoon for
     Wind Advisory from midnight tonight to midnight EDT Monday
     night for NCZ001-018.
WV...Wind Advisory from midnight tonight to midnight EDT Monday
     night for WVZ042.


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