Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250717

Area Forecast Discussion
National Weather Service Blacksburg VA
317 AM 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 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 1245 AM EDT Thursday...

Poor flying conditions to prevail overnight as a complex low
pressure system and associated cold front swing through from the
west. Expecting showers to slowly wane through the early morning
hours as deeper moisture lifts northeast. However still
expecting sub-VFR cigs overnight with MVFR to IFR vsbys in fog.
High confidence in timing of the models bringing the cold front
through the region after midnight.

Downslope winds on the east side of the Blue Ridge will erode
clouds and fog once mixing begins Thursday morning. Low level
moisture and VFR stratocumulus will remain over the mountains.
Scattered showers and isolated thunderstorms will develop in the
afternoon. Some of the stronger thunderstorms may have small
hail and MVFR visibilities along with heavy rain.

Extended Aviation Discussion...

Friday will be drier with a better chance of VFR ceilings and
visibilities. Another front reaches the area for Saturday with
more showers and thunderstorms including sub-VFR conditions
into Monday.


As of 255 AM EDT Thursday...

Showers continue in bands across the region early this morning
leading to high creek and stream levels with minor flooding in
spots. As rainfall rates slowly decrease expecting some
improvement although may take until midday for levels to start
receding while continuing to rise on mainstem rivers.
Significant rises will also occur on mainstem rivers as well
with both the New and Greenbrier likely reaching bankfull

Minor Flooding is expected to continue on the Dan River at
South Boston and is forecast on the Dan River at Paces and on
the Roanoke River at Randolph.

Another round of showers and storms will occur again this
afternoon which given low flash flood guidance values could
lead to added flooding issues. At this point will hold off on
any added watch headlines given uncertainty in locations of
heavier rainfall including rates later today.




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