Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 251753

Area Forecast Discussion
National Weather Service Blacksburg VA
153 PM EDT Thu May 25 2017

A weak cold front will pass east of the region this morning.
An area of upper level low pressure will then swing across the
area this afternoon before exiting tonight. Weak high pressure
spills in on Friday with a brief return to drier weather before
more showers and storms arrive this weekend.


As of 1250 PM EDT Thursday...

Updates for this afternoon include a lowering of forecast high
temperatures across the far western sections of the region where
cloud cover has been ongoing, and is expected to remain that way
the remainder of the afternoon. Also, have refreshed the timing
and location of after showers and storms. Plenty of sunshine has
been across central and eastern portions of the area, allowing
for surface warming. Anticipating convection to become more
robust across the area with both the upper cold pool lapse rates
passing overhead in alignment with the surface heating. Small
hail is still possible with almost any thunderstorm, and even
the stronger showers given the relatively lower freezing level
this afternoon.

Issuing a wind advisory for the mountain region for late this
afternoon through noon Friday. 850mb flow around the backside of
the upper low is expected to increase to around 40kts this
afternoon and remain that way through the overnight hours.
Translated surface wind speeds and gusts are not expected to
reaching official wind advisory thresholds, at least not many.
However, this advisory will be impact based upon the concerns
the gusty winds will be encountering trees with plenty of leaves
on them resting in a very saturated soil. Trees are expected to
come down with wind speeds lower than those typically expected
to do so.

As of 955 AM EDT Thursday...

Forecast update this morning will reflect the ongoing
precipitation coverage across the region. Some scattered
showers and isolated thunderstorms are between roughly
Martinsville, VA and Ellisboro, NC and moving northeast. Patchy
light rain showers extended north of this primarily showers
into parts of the Alleghany Highlands. This activity will
gradually head towards the northeast as the morning progresses.
We are still anticipating greater development as we head into
the afternoon during the heating of the day as an elevated cold
pool with steep lapse rates shifts eastward across the area
today. With a freezing level around 9000 feet MSL, even some of
the stronger showers may grow tall enough to produce some small
hail, especially across central and eastern parts of the area.

Updated hourly temperatures, dew points, and sky cover grids to
reflect the latest observations and expected trends into the
early afternoon.

As of 245 AM EDT Thursday...

Ongoing showers producing heavy rainfall slow to exit as waiting
for the upstream dry slot seen on water vapor to kick through
which guidance shows taking shape by daybreak. Thus will
continue high pops north-east early on before seeing a brief
lull as we slide in between deep moisture with the front and the
upper low to the west. Core of this cold pool aloft then looks
to pivot east through the far western zones around midday with
the associated vort lobe swinging across the east during the
afternoon. This should bring another round of showers to the
western mountains by early afternoon before clusters of deeper
convection develop on the leading edge and points east during
the afternoon. Secondary wave sliding around the base should aid
lift with perhaps some organization into a broken line over the
piedmont by late in the day as hinted at by a couple short term
solutions. Main aspect will be with cooling aloft and subsequent
steep lapses capable of producing hail and even some downburst
potential given lingering shear aloft. Also given a bit slower
scenario, have pushed thunder chances back to at least the Blue
Ridge where will mention small hail per latest SWODY1 with all
seeing a period of likely pops this afternoon.

Other issue with possible heavy rain and added flooding given
saturated conditions and high water levels. However at this
point appears far western coverage may be more showery with
lower rates while stronger storms to the east will be more
capable of producing at least isolated flash flooding. Since
still dealing with the current event and given uncertainty with
coverage this afternoon, opting not to include another flood
watch headline at this point and give the dayshift a chance to
issue a short term FFA if needed. Otherwise thinking will see
some sun espcly Blue Ridge east as the wedge breaks down and
downsloping kicks in. This should help push highs above 70 out
east while 60s prevail elsewhere over the west.

Upper low finally exits to the northeast this evening with
subsidence spilling in on a rather strong 850 mb jet at 30-40
kts that will make for a gusty evening/overnight. However latest
soundings suggest speeds to stay below advisory levels over the
ridges at this point. Otherwise appears any residual pops will
be confined mainly to upslope areas across the northwest where
some wrap around showers may persist. Thus keeping in chance
pops northwest third with a gradual decrease in coverage late.
Elsewhere expecting downslope drying to limit much in the way of
added showers with clearing skies east. Lows on the cool side
with most seeing 50s overnight if not a few 40s in the valleys.


As of 305 AM EDT Thursday...

On Friday, we will gradually lose the upslope scattered
showers, and associated cloud cover, as low level winds start to
back more westerly through the day with the approach of an
upper level shortwave ridge. The arrival of drier air will also
yield a decreasing trend in coverage. The best chances will
still be over parts of southeast West Virginia, mainly during
the morning hours. The vast majority of the forecast area will
experience limited cloud cover and weak winds.

Our period of drier weather will be short lived. Friday night into
Saturday we will be watching a cold front approach and then enter
the region. Isolated showers may develop by sunrise Saturday in
the northwest portions of the area, but the better chances will
be during the day Saturday with the arrival of the front
combined with daytime heating.

Guidance varies on what extent the front makes it through the region
before stalling. Model are leaning towards a thunderstorm complex
potentially riding along this stalled front late Saturday night and
entering the area by Sunday morning. This supported by the 00z
ECMWF which has multiple complexes riding the boundary from late
Saturday through Sunday.

Temperatures during this portion of the forecast will start near
normal, but trend to a little over five degrees above normal by


As of 205 PM EDT Wednesday...

During this part of the forecast, a broad, slow moving upper low
will transition from the Great Lakes region to over southeast
Canada.  This will keep our region in a pattern dominated by a
persistent trough of low pressure across the eastern portion of the
U.S.  Within this trough pattern a series of cold fronts are
expected to enter, and then move through the forecast region.
Guidance offers the first of these two to have a Sunday night into
Monday time frame, with a second during late Tuesday night into
Wednesday. Both of these time frames will be periods with a greater
potential for showers and storms, with the first being more robust
than the second. However, while the area is within the overall
trough pattern, daytime heating will be enough to produce at least
isolated showers during other parts of this time period.

Temperatures during this portion of the forecast will start the
period about ten degrees above normal. By Wednesday, readings within
a degree or two of normal are forecast.


As of 140 PM EDT Thursday...

Upper low was making progress eastward across northern Virginia.
Associated cold pool lapse rates and daytime heating will lead
to convection this afternoon, mainly in areas along and east of
the crest of the Blue Ridge. Broad area of IFR/MVFR cloud cover
over KY/WV will advance into the region from the west, and
impact areas in the west late this afternoon through the
overnight hours. Visibilities will mainly be VFR, but pockets of
MVFR visibilities are possible across the mountains.

Low level winds will become northwest by this evening and allow
for subsidence clearing in areas east of the Blue Ridge. Wind
gusts across the higher elevations will be in the neighborhood
of 25 to 35 kts. These prevailing winds allow for a continuation
of upslope rain showers in the west, and these will continue
into the day Friday. However, coverage during the day Friday
will be on the decline.

Extended Aviation Discussion...

VFR conditions are expected on Friday as a shortwave ridge
builds into the region.

We will transition back into a wet pattern over the weekend as a
frontal boundary enters and stalls over the area. A series of
disturbances will bring showers and storms to the area.
Anticipate at least some pockets of sub-VFR conditions.

The coverage of showers and storms decrease early next week, but
remain at least scattered in coverage.


As of 100 PM EDT Thursday...

Small streams and creeks have been receding and most are below flood
stage. We are in a river flooding scenario for portions of the
Roanoke and Dan River basins. The New River and Greenbrier river are
expect to stay below flood stage.

The upper level system moving in from the west will bring scattered
to numerous showers and possible thunderstorms. Overall rainfall
amounts on average will be under a quarter to half inch, in this
scenario. Will have to watch for any training or heavier downpours,
as it will not take much to cause flooding again given the saturated
grounds and swollen creeks and rivers.

Currently, we are not expecting enough for a flood/flash flood watch
and activity should diminish/weaken after sunset.


VA...Wind Advisory from 6 PM this evening to noon EDT Friday for
     Wind Advisory until noon EDT Friday for VAZ009-015.
NC...Wind Advisory until noon EDT Friday for NCZ001-002-018.
WV...Wind Advisory from 6 PM this evening to noon EDT Friday for


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