Area Forecast Discussion
Issued by NWS Blacksburg, VA

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286
FXUS61 KRNK 241037
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
637 AM EDT Wed May 24 2017

.SYNOPSIS...
Wedge of cool air across the region will gradually erode today
as a strong wave of low pressure slides east, pushing a trailing
cold front through the area this evening. An upper level area of
low pressure will then cross the Mid-Atlantic region Thursday
resulting in more showers and storms. Weak high pressure will
follow this system for Friday.
&&

.NEAR TERM /THROUGH TONIGHT/...
As of 310 AM EDT Wednesday...

Brief reprieve from the rain will be short lived as a digging strong
upper low across the Midwest comes in tandem with surface low pressure
sliding through the Ohio Valley by this evening. This will push a
trailing cold front toward the region today and to the east late
tonight as the system occludes. Very strong southwest flow aloft ahead
of the upper cold pool to aid a swath of upper diffluence that will
arrive from the southwest this afternoon along a possible triple point
secondary wave. This basically overtop the residual wedge will help
redevelop strong isentropic lift that should result in another axis of
showers initially before the actual pre-frontal convergence arrives.
Strong dynamics and deepening southwest flow should also help erode the
wedge as the residual front to the south lifts north. However just how
far any surface based instability gets still iffy with potential to see
much higher severe potential south/southwest zones with more elevated
storms farther north, espcly north of BCB/ROA to LYH and points north.
Models also a bit slower to push main axis of lift east which could
allow more erosion of the cool pool with strong shear developing over a
strong CAPE gradient just south of Highway 460 per latest Cam/Nam
solutions.

Other issue remains with rainfall given wet soils and possible higher
rates later on given more convective nature. Thus plan to hoist a flash
flood watch for a few counties along the VA/NC border and far southwest
where flash flood guidance is quite low and any downpour resulting in
an inch or more in an hour could cause problems. This may have to be
expanded on a short term basis farther north during the day pending
trends. Otherwise slowing down high pops a bit through early afternoon
before slinging in categorical coverage from southwest to northeast
into this evening. Highs again tricky with potential to see northern
sections stay in the 60-65 range while southern and southwest
sections zoom up into the 70s pending where the front ends up as
the convection arrives. For now bumped up values a notch given
a warmer start in spots.

Should finally see the shield of showers/storms swing northeast and
mostly out of the area after midnight as a dry slot wraps in from the
southwest. However some wrap around behind the pivoting upper low to
the northwest may keep showers going espcly northern sections through
the overnight but more spotty in nature by then. Otherwise remaining
mostly cloudy with showers diminishing and a bit cooler west with lows
mainly in the 50s with some lower 60s southeast.
&&

.SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/...
As of 400 AM EDT Wednesday...

Really only one mostly dry day this period as a very active
weather pattern continues across the region. The first upper low
that has kept the wet/stormy weather in place for the first half
of the week will be located over the region Thursday. The wedge
will be gone and well to the northeast leaving the forecast area
in the warm sector. As the core of the upper low moves over the
region with cold -20C or better 500mb temps providing steep 7C
1000-500mb lapse rates, any pockets of stronger surface heating
will quickly yield instability thunderstorms and the potential
for mainly marginal severe hail. SPC has outlooked the eastern
portions of the CWA Thursday for the hail threat as indicated,
but would not be surprised with later outlooks to see this
expanded further west as really feel much of the CWA will be
under the threat for strong discrete hail producing cells
Thursday afternoon. However, other severe weather threats should
be less of an issue and the threat for thunderstorms should
diminish quickly with the loss of daytime heating. Rainfall
issues should be less of a concern with lower PWATs and a more
scattered nature to the activity overall.

Friday has the potential to offer one dry day to the area,
however, it may not be totally dry in all areas. While the first
upper low will lift out of the area, another one is quickly on
its heals digging southward through the Midwest. This upper low
will take several days to get here in full form, but will
result in increasing lift and instability as we progress through
the weekend. This next upper low will not become as deep as the
former one and thus result in a baroclinic zone/frontal boundary
setting up across the area through the weekend serving as a
focus for convection. More on that in a minute. For Friday, very
brief upper ridging is indicated, however, convection developing
across the Ohio Valley could well reach the western part of the
CWA toward evening and especially during the overnight hours.
All synoptic-scale models now indicating quite a bit of
convection reaching the area before daybreak Saturday. So, have
had to introduce pops late Friday and especially into the
overnight hours. This may include thunder, especially in the
west.

Saturday, convection during the afternoon has the potential to
become quite vigorous along/near the baroclinic zone. The GFS
seems a bit overdone with LIs near -8C, CAPES in the 2000-3000
J/kg range, with other models showing such values about half of
this. There is also decent shear across the area with upper
diffluence also a factor with a broad upper ridge to our south.
The potential for severe appears to be fairly decent on
Saturday. Location may be the main issue, that is will it be
mainly along and north of the I-64 corridor, or perhaps a tad
further south. An average location would put the maximum core of
severe weather basically between I-64 and U.S. 460. All threats
of severe weather appear possible with the advertised scenario
and flash flooding may also be a concern once again given the
nature of strong convective cells and the extensive areas of
saturated ground in our CWA at the current time. Convection
would well continue into the evening Saturday across parts of
the CWA.

Temperatures Thursday will remain cool with scattered convection
and abundant cloud cover under the cold core upper low. Look for
highs in the 60s west to 70s east with lows in the 50s. Temperatures
Fri-Sat will be notably warmer as 850mb temps surge toward +20C
across the area. This very warm air will further fuel the
potential severe storm environment Saturday afternoon. High
temperatures Friday should be in the 70s west to the lower 80s
east rising even further Saturday to the 80s across much of the
forecast area, except 70s mountains.

&&

.LONG TERM /SUNDAY THROUGH TUESDAY/...
As of 430 AM EDT Wednesday...

Continued unsettled and wet, unfortunately. The Midwest/Ohio
Valley Upper Low will continue to slowly deepen and shift
further east through the time frame, but remain upstream of the
area. A potent short wave will move across the area Sunday. The
baroclinic zone should be located near the NC/VA border as this
occurs, so look for another good chance of widespread showers
and thunderstorms once again. Instability will remain a factor
and shear/dynamics will be even better than Saturday, so once
again there is a continued threat for severe Sunday along with
locally heavy rainfall. The axis of the upper low will remain
west of the area through the period and beyond keeping an active
weather pattern in place. Expect scattered to numerous showers
and thunderstorms each day until potentially late week when the
upper trough may shift far enough east to reduce the threat for
such some. Temperatures will be seasonably warm with no wedges
indicated and 850mb temps mostly +10C or better until after the
trough moves east of the area late week. This will yield maximum
temperatures mostly in the 70s west to 80s east with minimum
temperatures mostly in the 50s mountains to the 60s Piedmont.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
As of 635 AM EDT Wednesday...

Expect overall poor flying conditions to prevail at most 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 this afternoon as the ongoing wedge/stable
layer shifts east with thunderstorms possible. At the moment
will keep sites on the IFR side with some MVFR at times
today.

Next round of showers and possible storms to return by
this afternoon from west to east with continued overall sub-
VFR into tonight. Will keep VCTS mention across the south and
west while edging east into KBCB although trends appear slower.

Showers exit to the north after midnight with some lingering
rainfall possible across northern/western sections late. However
combination of developing stratus/fog once the rain tapers and
upslope flow develops over the west to keep overall IFR in place
until early Thursday.

Extended Aviation Discussion...

Rainfall should become a bit more showery by Thursday with
sub-VFR mainly associated with the showers/thunderstorms 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 408 AM EDT Wednesday...

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.

Next round of convection/heavier rains will occur this
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 southwest of Roanoke, east to
southside VA into the NC piedmont.

Given wet antecedent conditions in much of the southern and
southwest sections, hoisting a Flash Flood watch given potential
for higher rate rainfall over wet ground into this evening. This
may need to be expanded north and east later pending trends.
&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flash Flood Watch from noon EDT today through this evening for
     VAZ007-009-010-012-015-016-032-043.
NC...Flash Flood Watch from noon EDT today through this evening for
     NCZ001>006-018>020.
WV...None.

&&

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



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