Flooding and Heavy Rain in West Virginia, April 15 to 17, 2018

Area Forecast Discussion
National Weather Service Blacksburg VA
725 AM EDT Sun Apr 15 2018

.SYNOPSIS...
High pressure offshore will give way to a strong cold front that
will approach from the west today before passing across the
region this evening, and offshore by early Monday. A line of
showers and thunderstorms will precede the boundary later today
into tonight before drier air arrives overnight. Much colder
air on strong northwest winds including mountain snow or rain
showers follows the front for early next week.
&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM EDT Sunday...

Very dynamic system still on track to affect the region mainly this
afternoon and evening as a closed upper low becomes negatively tilted
to the west while swinging a surface cold front in from the west
overnight. Strong moisture transport along with deep shear and moisture
convergence likely to lead to a double threat of severe and flooding
before the main moisture axis exits east after midnight. Latest
guidance continues to show a multiple band type scenario starting in
the far west by early afternoon as a pre-frontal band of deep
convection heads east reaching the Blue Ridge by late in the day. Short
term solutions show this to be the main QLCS type feature including
embedded bowing segments and possible discrete cells both within and
just ahead of the main line. Second broken line likely to arrive with
the actual front during the evening with another round of heavy rain
possible. However some of the latest guidance has shifted the better
severe potential east of the mountains where a little more instability
likely with better rotational threat with perhaps a weak meso-low over
southern sections per latest upgrade to an enhanced severe threat.
Heavy rain also a concern given such high level to Pwats and possible
training espcly west from a couple bands before lift shifts east of the
ridges. This may be enough given potential higher rainfall rates
southwest and moist soils northwest to prompt a bit higher flash flood
threat. Thus hoisting a watch across the far west for this
afternoon/evening. Does appear that convection passing to the south
could initially limit northward moisture advection but think that the
Cstar effect will be overcome as the flow aloft turns more
south/southeast under the negatively tilted system.

Appears FFG high enough to limit the flash flood threat out east but
still will keep heavy rain mention. Otherwise slowed pops down to start
this morning given current trends and very compact nature to the band
of showers to the west. Then blended an area of categorical pops east
with damaging wind wording espcly south/east this afternoon/evening.
Highs tricky pending clouds and timing of showers with 60s/70s west and
mostly 70s to near 80 east, where if get more heating then will
see a much higher severe threat.

850 mb front punches in from the west overnight leading to drying out
east and onset of upslope far west as temps aloft fall below 0C.
However will take a while for the boundary layer to cool enough to
support snow, so thinking only light accumulations on the ridges at
best. Will turn blustery espcly west by morning as deepening cold
advection kicks in with this perhaps pushing added showers east to the
Blue Ridge late. Winds may be close to advisory levels late but since
likely only on the far western ridges will keep added wind headlines
out for now. Lows ranging from mid/upper 30s far west to around 50
southeast.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 445 AM EDT Sunday...

The line of strong to severe thunderstorms will quickly shift
off to the east of the region after midnight Sunday night,
generally exiting the eastern part of the CWA Monday morning.
Strong cold advection will ensue through the day Monday on the
heels of increasingly gusty northwest winds. With the upper low
and surface low becoming vertically stacked, it will deepen and
move very slowly northeast away from the region. This will
result in an extended period of gusty northwest to north winds
that could last Wednesday, at which point an upper-level
disturbance will moving toward the region from the west. MAV MOS
guidance has been highlighting and extended period of 20+
sustained winds in the Tuesday through Wednesday period for
several runs now, especially from Roanoke to Jefferson to Boone.
A High Wind Warning may be needed in a few spots through this
corridor, but a Wind Advisory certainly seems likely across much
of the CWA west of the Blue Ridge.

Another issue, mainly across western Greenbrier, but potentially
areas further south as well toward Jefferson and Boone, may be
candidates for a Winter Weather Advisory Tuesday. With the upper
low deepening and slowing its forward progress and linger across
PA Tue-Wed, this will bring a prolonged period of upslope
northwest flow to the Alleghanys. Snow accumulations at the
higher elevations may be in the 2-3 inch range with locally
amounts up to 4 inches, with 1-2 inches possible south toward
Boone over a two day period. A Winter Weather Advisory for part
or all of this area, but especially western Greenbrier, is not
out of the question at all.

Temperatures are expected to be well below normal through the
entire period with highs in the 40s mountains to the 50s
Piedmont with lows in the 30s to lower 40s. With the
frost/freeze program already in place across the Piedmont, we
may need to consider a Freeze Advisory for parts of the Piedmont
during this time frame. There appears right now to be too much
wind for the need of any Frost Advisories.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 510 AM EDT Sunday...

Active weather pattern expected to continue through the period
with at least two potential active weather systems to impact
the area during this time frame.

The late weekend/early week deep vertically stacked upper low
will continue to linger across the eastern Great Lakes and
PA/NY into Wednesday before finally shifting slowly off toward
the Canadian maritimes by Thursday. Meanwhile, a fast moving
relatively zonal mid-level short wave will be tracking across
the mid-latitudes and reaching the Ohio Valley late Thursday,
quickly moving into the prior position of the early week upper
low. Moisture appears limited with this system and remains
mostly north of our area. Two concerns will be an increase in or
renewed period of gusty winds as low pressure deepens across
PA/NY with this secondary upper low and the possibility of more
snow showers across the Alleghanys as this upper low skirts to
our north. Nonetheless, most of the precipitation with this
system should remain north of I-64 and points further north
toward the WV/MD/PA border.

Finally, a much stronger southern stream Pacific-based system
will move through the south central U.S. during the weekend and
reach the southeast U.S. by Sunday. This looks to be a much more
potent system for our area and again has the potential to bring
healthy rainfall, possibly thunderstorms, and perhaps even some
threat of winter weather to western higher elevations.

The pattern in place next week will leave much of the time frame
with below normal temperatures with a brief warmup noted between
the departing early week system and the fast moving system
approaching late Wednesday. Look for lows in the 30s and 40s and
highs in the 50s west to 60s east much of the week, or roughly
about 10 degrees below normal.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
As of 710 AM EDT Sunday...

Mix of MVFR to VFR cigs will prevail this morning with periods
of low clouds including spotty showers pushing north ahead of
the cold front to the west. Then expecting deteriorating
conditions from west to east this afternoon as the leading band
of showers/storms works east. Appears best timing for heavier
rainfall arrival will be around KBLF/KLWB in the 19-20z time
frame, KBCB/KROA 20-22z, and KLYH/KDAN 21-23z.

Thunder is possible but not expected to be widespread so
maintained heavier showers including VCTS at all sites for now.
Gusty winds with the leading edge of showers/embedded storms
may have a quick shift from south- southeast to west-southwest
for less than an hour with gusts over 40 kts possible.

Moderate to heavy showers expected as well, so mainly looking at
sub-VFR visibilities with localized IFR within the heavier
embedded storms along the line(s) through this evening.

Expect the majority of the showers/storms to have exited eastern
sections shortly after midnight with upslope driven rain/snow
showers returning over the far west late tonight. Appears a
return to VFR will be possible within downslope flow east of the
mountains by early Monday with MVFR to IFR cigs lingering espcly
KBLF/KLWB. Northwest winds to also become quite strong late
tonight over the mountains behind the front with gusts to 25-30
kts possible by daybreak.

Sub-VFR to persist across the southeast West Virginia corridor
Monday with lingering snow/rain showers making for MVFR to IFR
vsbys at times. Otherwise looking at mostly VFR elsewhere but
under strong northwest winds that could gust to 25-35 kts over
the region during the afternoon.

Extended Aviation Discussion...

Gusty northwest winds will continue Monday night through Tuesday
along with mountain sub-VFR in low cigs espcly KBLF/KLWB.

Finally looking at a return to overall VFR on Wednesday with a
gusty southwest wind likely ahead of yet another approaching
cold front.

Next front arrives Wed night-Thu with chance of showers, with
gusty winds and sub-VFR cigs again Thursday in the mountains.

&&

.HYDROLOGY...
As of 350 AM EDT Sunday...

Main concern with possible training of multiple convective bands
over the mountains this afternoon and early evening where
guidance has between 1-3 inches of rain in a 6-8 hour window.
Rainfall rates could also be better than a couple inches per
hour if convection does indeed deepen enough especially to the
south. Since this similar scenario has lead to flash flooding
with past events, decided in coordination with surrounding
offices to hoist a flash flood watch for parts of the west for
this afternoon into early this evening when guidance indicates
the best potential for heavy rain. Some concern for convection
to perhaps limit moisture transport early on before the system
negatively tilts allowing for deepening south/southeast flow to
win out. However this only supported by limited number of
solutions with the latest GFS/ECMWF more in support of the
heaviest rain near the watch area. This could lead to some river
flooding as well into early next week so something to watch
pending locations of heavy rainfall and overall totals at this
point.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flash Flood Watch from noon EDT today through this evening for
     VAZ007-009-010-012-015-016.
NC...Flash Flood Watch from noon EDT today through this evening for
     NCZ001>003-018>020.
WV...Flash Flood Watch from noon EDT today through this evening for
     WVZ042>044-507-508.

&&

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

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