Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 232357

Area Forecast Discussion
National Weather Service Blacksburg VA
757 PM EDT Mon Apr 23 2018

Low pressure will slowly move from the Tennessee Valley to the
Carolinas through Tuesday. Prolonged southeast winds will keep
the area wet through Tuesday night, when the low lifts to the
Delmarva. As this system departs on Wednesday, two additional
disturbances look to cross over or near our area Thursday into


As of 225 PM EDT Monday...

Southeast flow accelerating this afternoon, with gradient tightening
between low pressure over Western TN and high pressure off the mid-
Atlantic/Northeast coast. Getting some gusts over 40 mph in the
higher terrain of NC, with 25 to 35 mph further north. Models are
indicating stronger part of low level wind field arrives around 00z-
04z this evening, and have added western Greenbrier to the wind
advisory. Also strong winds should linger past midnight, so extended
the advisory til 2am Tuesday.

The flood watch still looks good, see hydro discussion below for

Looking at periods of rain working in from the south as plume of
deep moisture drives northwest from the Atlantic through the
overnight. Models in reasonable agreement in placing emphasis of
higher rates over the southern Blue Ridge with light-moderate rain
surround this. Latest high-res models are keeping the northeast
piedmont dry until overnight/after midnight and leaned pops toward
that solution with categorical over the southern CWA to chance in
the far northeast and northwest.

Toward morning, southeast flow at 8h continues but better lift
starts shifting to the northeast with potential rain shadow effect
taking shape over the mountain empire of SW VA into southern WV.
Seems enough moisture transport to keep pops greater than 50 percent
across the west, with 100 pops in the east. As the 8h low moves over
the Mountain Empire in the afternoon, a pocket of lower rainfall
works from southwest VA into central WV, while upslope flow in the
east persists but weakens somewhat in the afternoon. Still looks
like a good period of rain along/east of the Blue Ridge Tuesday.

Some SBCAPEs greater than 500 J/KG start to get close to our NW
piedmont of NC in the afternoon, and includes isolated thunder
threat here, but overall looks too cloudy and wedged in for deeper

Kept lows tonight similar to previous forecast with mid to upper 40s
across much of the area. Highs Tuesday will be cool again especially
across the Blue Ridge, and Alleghanys, with upper 40s to mid 50s,
with upper 50s to lower 60s expected west of I-77 in WV, and over
the upper TN VAlley in SW Va, as well as southeast of a Lynchburg to
Stuart line.


As of 200 PM EDT Monday...

The center of the slow moving upper low that would have been
associated with our rainfall Monday and Tuesday, is expected to
start Tuesday evening over eastern Kentucky and progress due east
across the forecast area by early Wednesday morning. Showers will
prevail through the night in association with the system`s steeper
cold pool lapse rates. Early in the evening, the far southeast
portion of the area will be very close to region of weak CAPE. SREF
numbers keeps any amounts greater than 500 J/Kg just outside the
forecast area. At this point, am leaning toward keeping the forecast
simply as showers.

On Wednesday, the upper low will begin a turn northeast in response
to yet another shortwave trough heading southeast into NY/PA. The
first upper low will lose its definition as it becomes enveloped
within the second. Flow across our area will turn northwest on the
back side of the system, trending the precipitation to be focused in
two regions. The first will be an area of upslope showers across
southeast West Virginia. The other will be showers on the western
flank of the upper trough axis, still influenced by the steeper
lapse rates of the associated cold pool. Close to adding thunder in
the east on Wednesday, but the best lift looks to remain just east
of the forecast area.

As we progress Wednesday night through Thursday night, our region
will remain within a progressive pattern, influenced by both the
northern and southern stream jets. First there is the question as to
how quickly the NY/PA upper low will head northeast, taking its
influence with it. While there are some minor differences within the
deterministic models, a clear consensus is that by daybreak Thursday
all should be dry, at least for a little bit of time.

As early as Thursday afternoon, we will need to start examining our
next potential weather maker. This one is another shortwave trough,
but within the southern stream this time. Guidance sweeps the system
from the Lower Mississippi River Valley on Thursday afternoon, to a
location near GA/SC by daybreak Friday. The path and timing vary
such that either this feature will have no influence on our weather,
or the southern half or third of the region will experience showers
on its northern flank Thursday night, potentially starting as late
Thursday afternoon across the far southwest section of the forecast
area. Our forecast will reflect a toned down version in terms of
POPs that the wetter solutions have for our region.

During this portion of the forecast, high temperatures will trend
milder to readings near normal for this time of the year. Low
temperatures will trend slightly cooler to readings slightly below


As of 335 PM EDT Monday...

On Friday, we will still have the question as to the degree of
influence the southern stream shortwave trough has for our area. A
lot will depend upon how quickly yet another northern stream system
progresses eastward, inviting a turning northward of the southern
system. If in fact it does turn north Thursday night, scattered
showers will continue to impact the area on Friday.

The aforementioned northern stream system is progged to move through
the area Friday night into Saturday morning. Models differ as to
whether this will be a quick moving clipper, or be a bit beefier
with an actually associated closed or open low that crosses our
region. Either way, the the forecast detail this far out in time
will be the same, one of forecast scattered showers.

Once this system clears the area by Saturday night, the overall
synoptic pattern changes. For Sunday into Monday, expect an upper
level ridge to approach, amplify, and then be centered over the
area. This will result in a forecast that has two consecutive days
of no precipitation.

Temperatures will start this portion of the forecast a few degrees
below normal, but trend milder, especially for Sunday and Monday. By
Monday temperatures are forecast to average about five degrees above


As of 750 PM EDT Monday...

With wedge of high pressure in place east of the mountains and
isentropic lift increasing, expect showers to continue to spread
into the region from the south with cigs trending toward IFR
conditions. With slow motion of the closed low moving out of the
Mississippi valley expect rain and IFR conditions to linger
through through Tuesday.

Believe the wedge barrier jet will keep winds a bit gusty at
KLYH and KDAN through the period. While the southeasterly low
level jet running over the top of the wedge is quite robust,
very moist and stable boundary layer conditions will inhibit
winds mixing down. The most likely locations for stronger wind
gusts will be at the higher elevations in the west to include
the airfield at KBLF.

Extended Aviation Discussion...

Tuesday afternoon the low will be situated overhead so rainfall
may start to fade in the southwest around BLF after 18z Tuesday
then shifting northeast through Tuesday evening. Expect sub-VFR
through the afternoon, and into Tuesday night/early Wednesday.
Winds turn to the northwest, by late Tuesday night into
Wednesday 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 240 PM EDT Monday...

No changes need to the flood watch, which remains in effect
through 400 PM Tuesday. Forecast amounts from this morning
through Tuesday showing 2-4" along the southern Blue Ridge from
near Meadows of Dan to Blowing Rock. Looks likely that the areas
around the Watauga River in NW NC as well as streams flowing
off the Blue Ridge are going to need to watched. Ensembles for
the rivers are still lagging though the SREF is a bit robust in
bringing flooding threats later on to the Dan River, as well as
portions of the Greenbrier and New. Other ensembles as well as
the RFC are keeping all rives at or below flood stage. Something
to watch once we see how much falls, but rises along the main
stems, including the New and Dan Rivers are likely, as well as
portions of the Roanoke.

Given lack of deep convection, flash flooding appears less likely.


VA...Flood Watch through Tuesday afternoon for VAZ015>017-032.
     Wind Advisory until 2 AM EDT Tuesday for VAZ007-009-015.
NC...Flood Watch through Tuesday afternoon for NCZ001>003-018-019.
     Wind Advisory until 2 AM EDT Tuesday for NCZ001-018.
WV...Wind Advisory until 2 AM EDT Tuesday for WVZ042-043-508.


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