Area Forecast Discussion
Issued by NWS Blacksburg, VA

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376
FXUS61 KRNK 240601 AAA
AFDRNK

Area Forecast Discussion...UPDATED
National Weather Service Blacksburg VA
201 AM EDT Wed May 24 2017

.SYNOPSIS...
Wedge of cool air will remain in place overnight as a weak wave
of low pressure slides east and offshore along a frontal
boundary just south of the area. A lull in the rain will occur
early Wednesday, before another area of low pressure arrives
Wednesday afternoon with more showers which may persist at times
through Thursday. Weak high pressure will follow this system
for Friday.

&&

.NEAR TERM /THROUGH TODAY/...
As of 200 AM EDT Wednesday...

Quick update to let the going flood watch expire given exodus of
more widespread rainfall. May still see a few bands produce a
quick shot of moderate rain over the region overnight but not
enough to cause flooding. Remainder unchanged for now.

Previous discussion as of 1130 PM EDT Tuesday...

Had some decent rainfall the past 6-12 hours but overall the
flooding threat is still confined to the Dan River, as the rain
rates have not been that high. Will keep the flood watch til
2am, as we are watching another area of convection over the
Smokys that may sneak into SW VA/NW NC by 1am. However, all
high-res models show this area dissipating to light showers or
drizzle after 2am, so thinking the expiration of the watch at
2am looks reasonable.

Otherwise, overnight we will see less moderate rain but enough
low level moisture convergence with southeast flow to keep
chancey pops around. Setup favors drizzle at times as well as
some fog, especially over the ridges.

Previous discussion from early evening...

Forecast still looks rainy though some breaks developing in the
piedmont though still drizzly/light rain. Shower more focused
over the mountains this evening, with high-res models shifting
this area northeast through midnight. Should see a drying trend
from southwest to northeast by late evening/midnight though not
completely dry, given depth of moisture. Will also have to watch
upstream convection over Al/northern GA as it moves toward the
NC mountains/foothills/piedmont after midnight. The high-res
models weaken it, but not so sure this will occur.

No changes to the flood watch, with minor flooding still
possible though focus looks to shift more toward the mountains.

Previous discussion from this afternoon...

Radar imagery shows that the shield of precipitation associated
with wave of low pressure sliding northward along the front
lingering in the east has spread across the piedmont, with a
secondary area of precipitation to the west advancing up the
mountains. Guidance seems to be handling the wave in the
piedmont fairly well, but not so much for the precipitation in
the west. Given very wet antecedent conditions in the mountains,
have expanded the flood watch to the western border. As dynamic
forcing pulls off to the northeast overnight we will see a
decrease in precipitation, though fully expect some degree of
shower activity to be around through the night.

The next wave will move out of the southern Appalachians as a
large closed low digs southward into the lower Mississippi
valley, with a trailing cold front not pushing through until
Wednesday night. These systems will be running into a solid
wedge east of the Appalachians, setting up another period of
rain with good isentropic lift over the wedge. However, there
are good dynamics in place and the wedge will be eroding,
allowing instability to creep in from the west and south in s a
highly sheared environment, opening a window for embedded
thunderstorms late tomorrow afternoon with possibly linear
organization to embedded convection heading into Wednesday
night. The current day 2 SPC outlook for a marginal risk of
severe across the south and west portions of our area looks
right on target and the situation will be watched closely.
Additionally, the expected precipitation tomorrow may warrant
another flood watch if later guidance continues to indicate
sufficient rainfall.

Expect temperatures to fall slowly tonight and remain well
below normal through Wednesday.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...
As of 330 PM EDT Tuesday...

A slow moving area of upper level low pressure will head eastward
through the Ohio Valley Wednesday night into Thursday.  A
substantial stream of moisture will be advecting into at least the
eastern portion of the area early Wednesday evening. As the evening
progresses, this moisture axis will shift eastward in concert with
the approaching upper level trough axis. Anticipate very good
coverage of moderate rain showers and scattered storms to give way
to decreasing coverage by late evening, and even less coverage after
midnight. The activity across the region after midnight will be
associated with cold pool lapse rate with the passage of the trough
axis.

On Thursday, this trough axis will still be passing overhead, have
daytime heating to help fuel additional showers and storms, all
while its associated surface front heads east of the region. Expect
good coverage of showers and storms during the day with west to
northwest wind becoming gusty by the afternoon.

Thursday night, northwest flow continues to increase on the backside
of the system. Precipitation will quickly decrease in coverage east
of the Blue Ridge thanks to increased subsidence. Upslope rain
scattered rain showers are forecast across parts of southeast West
Virginia during the overnight. Some of the gusts at the higher
elevations will be on the order of 30 to 40 mph.

On Friday, drier air will continue to work its way into the area,
but lingering upslope showers in the west will continue through at
least mid-day. Although, coverage will be on the decline.

By Friday night, the winds will have weakened as the pressure
gradient continues to weaken.  Just as conditions trend drier, we
will start to watch our next system developing in the mid-
Mississippi Valley by late Friday night, with associated leading
moisture reaching western parts of the area by sunrise Saturday
morning.

Temperatures during this portion of the forecast will start slightly
below normal but trend to readings near normal by Friday.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 330 AM EDT Tuesday...

Saturday into Sunday, our weather will turn wet again as low
pressure tracks through the Ohio Valley. In advance of this system,
a warm front will lift through the region on Saturday, followed by
an associated Sunday afternoon and evening.

Guidance differs on how promptly this front will exit the region,
along with any associated lingering showers. What is more uniform in
solution is the position of another closed upper low. Anticipate a
slow moving upper low just north of the Great Lakes region to linger
through at least Tuesday. Energy pinwheeling around this feature
will interact with the remnants of the cold front from the weekend.
Additionally, a cold front in association with this upper low will
be heading toward our area late in the day Tuesday.

The result will be a weekend that will have the potential to be on
the wet side, with Monday and Tuesday having scattered activity.

Temperatures during this portion of the forecast will average a
little over five degrees above normal.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
As of 1250 AM EDT Wednesday...

Expect overall poor flying conditions to prevail at all TAF sites
this period.

Confidence is high that most if not all sites will stay sub-VFR
with periods of IFR or worse through the period. Exceptions at
times may be KBLF/KDAN Wednesday afternoon as wedge/stable layer
shifts east with thunderstorms possible. At the moment will
keep sites on the IFR side with some MVFR at times overnight,
but not a big window.

Otherwise rain should become more spotty overnight with a
possible break in coverage to mainly drizzle/fog by daybreak on
Wednesday. Next round of showers and possible storms to return
by Wednesday afternoon from west to east with continued overall
sub-VFR into the evening. Will keep VCTS mention across the
south and west while edging east into KBCB although trends
appear slower.


Extended Aviation Discussion...

Rainfall should become a bit more showery by Thursday with
sub-VFR mainly associated with the showers while periods of
afternoon VFR possible outside of the convection. 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 Sunday.

&&

.HYDROLOGY...
As of 1130 PM EDT Tuesday...

Rainfall amounts the past 12 hours have averaged 0.50 to 1 inch
over most of the forecast area, with lower amounts in portions
of the NC foothills, and amounts approaching 2 inches along the
Blue Ridge near Blowing Rock, NC/Wilkes County, as well as the
Greenbrier Valley.

Appears threat of heavy rain will be over through the overnight,
and convection upstream is progged to weaken as it moves into
the stable airmass. Still the Dan River is in minor flood at
South Boston, and is expected to reach minor flood stage at
Danville and Paces.

Next round of convection/heavier rains will occur Wednesday
afternoon, with most models favoring the same areas to get the
higher amounts, with some minor differences, but generally along
and either side of the Blue Ridge south of Roanoke, east to
southside VA into the NC piedmont.

Flood or Flash Flood watches may be needed for these areas if
models stay consistent. This will be decided overnight or early
Wednesday morning.

&&

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

&&

$$
SYNOPSIS...MBS/WP
NEAR TERM...JH/MBS/WP
SHORT TERM...DS
LONG TERM...DS
AVIATION...JH/MBS/WP
HYDROLOGY...JH/MBS



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