Flash Flooding in Virginia, May 26, 2018

Area Forecast Discussion
National Weather Service Blacksburg VA
1013 AM EDT Sat May 26 2018

.SYNOPSIS...
High pressure centered offshore will maintain a moist southerly
flow of air into the region for the next several days.
Subtropical Storm Alberto moving into the Gulf of Mexico this
weekend will move ashore and into the lower Mississippi Valley
early next week. Tropical moisture will stream north into the
region making for very rain efficient showers and thunderstorms.
&&

.NEAR TERM /THROUGH TONIGHT/...
As of 950 AM EDT Saturday...

Seeing slow moving convection across the far west within an axis
of weak convergence aligned along the western mountains between
upstream troffiness and return of deeper tropical moisture per
morning soundings. Latest analysis also showing a weak mid level
wave riding up from the southwest to the north of the residual
front and surface wave to the south. This supports some slow
expansion of convection to the north and east with coverage
perhaps slowed somewhat by lack of instability due to cloud
cover. However latest short term solutions show quite a bit of
shra/tsra expanding east under increasing southwest flow so
keeping likely/cat pops through the afternoon. Weak steering
still of concern with potential flash flood issues but given
more localized nature and uncertainty including how far
north/east heavier rainfall might materialize wont go with with
a watch unless things become more widespread. Lowered highs a
little in the west/south per more clouds/showers while keeping
mainly 80s in the east.

Previous discussion as of 430 AM EDT Saturday...

Unsettled weather is expected through the Holiday Weekend as
southerly wind flow brings increasing moisture to the
Appalachians. Surface dewpoints in the 60s to near 70 along
with PWAT values in excess of 1.50 inches will favor August-
like conditions with high humidity and rain efficient
showers/storms. The key word is showers, as it will not rain the
entire time, but when it does rain, it will pour.

Shower coverage today will be the greatest during the peak
heating of the day with convergence along the mountains driving
much of the lift to create the showers and afternoon
thunderstorms. There is a subtle shortwave embedded within the
southerly flow that may provide enough lift to help focus some
of the deep convection into clusters, so will have to monitor
this closely as this may help provide a focus for an area of
localized heavy rain. The high resolution models suggest threat
for the heaviest rain today will be just north of our CWA where
the westerlies across the Ohio Valley and northern Mid-Atlantic
converge with the southerly winds coming up from the southern
Mid-Atlantic. For this reason WPC has highlighted areas north
of I-64 within a Slight Risk for excessive rainfall in their
Day-1 Outlook.

There is a weak upper level trough moving east across the Great
Lakes and Ohio Valley. This feature will maintain enough lift
for showers to persist across the Virginias tonight, and
especially north of I-64 where wind convergence will continue
between the two flow regimes.

Temperatures will trend 10-15 degrees above normal for the next
several nights per the significant uptick in dewpoints.
However, daytime highs will not be as extreme per the abundant
cloud cover and shower coverage...yielding Tmax values of no
more than about 5 degrees above the norm.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 400 AM EDT Saturday...

The pattern will remain blocky and sluggish into the first part of next
week with a shallow trof sliding by to our north and Alberto moving
slowly along the eastern Gulf coast to the Mississippi delta by Monday.
This will keep the Appalachians and central mid Atlantic region in a
warm and very moist airmass for the next several days.

Sunday looks to feature ample instability in a relatively weak wind
field as the upper trof tries to push a washed out frontal boundary
into the region from the northwest. This will add some synoptic scale
lift to air convection and should yield good coverage of showers/storms
especially across the northwestern two thirds of the area. Any severe
threat will be small with more of a focus on potential hydro issues
from training storms in an environment of precipitable water values
in the upper percentiles.

Sunday night into Monday will see some subsidence around the periphery
of Alberto push in from the south along with slightly lower theta-e
air. While there will still be showers/storms around, believe these
limiting factors will suppress activity a bit and keep the preferred
area for convection across the southern half of the area. Steering flow
becomes very weak with little to move precipitation along and will have
to watch for areas of prolonged persistent rainfall. A better push of
moisture arrives on Tuesday as Alberto will be moving up into the lower
Mississippi valley and throwing waves of energy in our direction, but
the best forcing with this system remains well off to our
southwest.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 400 AM EDT Saturday

By mid to late week, remnants of the tropical system could bring
tropical moisture and heavy rains to our region. Flooding potential
will depend on the track of the system and how much rainfall is
accumulated from the showers and thunderstorms earlier in the week.

Expect Alberto to make its way into the Tennessee valley by Wednesday.
The timing of the ECMWF was quicker and further north and west (Ohio
Valley) compared to the GFS which was slower and more compacted (along
the Appalachians. By Thursday and Friday, the low will open up and
weaken as it moves northeast. There is a continued threat for showers
and thunderstorms into the end of the week. Urban and small stream
flooding risk will be highest early in the week with river flooding
possibly by the end of the week depending on actual rainfall totals.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
As of 800 AM EDT Saturday...

Increasing moisture from the south will bring increasing
cloudiness to the Mid-Atlantic Region with finger of MVFR Cigs
creeping north along spine of Appalachians. This moisture will
combine with the heating of the day to produce numerous showers
and embedded thunderstorms...esp during the afternoon and
evening. Other than convergence from the mountains,
showers/storms will lack any sort of organization and are
expected to dissipate after sunset. Mean steering wnd aloft is
fm 240 deg at 12 kts. Low level winds will be out of the SSE at
less than 10 kt.

.Extended Aviation Discussion...

Wet pattern anticipated through the upcoming week with periods
of at least diurnally driven sub-VFR conditions associated with
deep convection. Moist low level southerly winds will also favor
lower layers of cloudiness at night, especially along the spine
of the Appalachians with potential for both MVFR Cigs and early
morning MVFR visibilities from mist/haze. The tropical system
over the Gulf of Mexico may begin to impact the region mid-week.

&&

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

&&

SYNOPSIS...PM
NEAR TERM...JH/PM
SHORT TERM...MBS
LONG TERM...MBS
AVIATION...PM

Hazardous Weather Outlook...UPDATED
National Weather Service Blacksburg VA
1018 AM EDT Sat May 26 2018

NCZ001-002-018-VAZ007-009>020-022>024-WVZ042>044-507-508-271430-
Ashe-Alleghany NC-Watauga-Tazewell-Smyth-Bland-Giles-Wythe-Pulaski-
Montgomery-Grayson-Carroll-Floyd-Craig-Alleghany VA-Bath-Roanoke-
Botetourt-Rockbridge-Mercer-Summers-Monroe-Eastern Greenbrier-
Western Greenbrier-
1018 AM EDT Sat May 26 2018

This Hazardous Weather Outlook is for northwest North Carolina,
southwest Virginia, west central Virginia and southeast West
Virginia.

.DAY ONE...Today and tonight.

Bands or clusters of slow moving showers and storms will result in
periods of heavy rainfall. Localized flooding or flash flooding could
occur into this evening.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Showers and thunderstorms will keep a threat for heavy rain with
localized flooding possible into next week. The track of a tropical
system may have major impacts on how much rain we get and the overall
flood threat next week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

NCZ003>006-019-020-VAZ032>035-043>047-058-059-271430-
Surry-Stokes-Rockingham-Caswell-Wilkes-Yadkin-Patrick-Franklin-
Bedford-Amherst-Henry-Pittsylvania-Campbell-Appomattox-Buckingham-
Halifax-Charlotte-
1018 AM EDT Sat May 26 2018

This Hazardous Weather Outlook is for north central North Carolina,
northwest North Carolina, central Virginia, south central Virginia,
southwest Virginia and west central Virginia.

.DAY ONE...Today and tonight.

There is a low probability for widespread hazardous weather.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Showers and thunderstorms will keep a threat for heavy rain with
localized flooding possible into next week. The track of a tropical
system may have major impacts on how much rain we get and the overall
flood threat next week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.