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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 120125 AAA
AFDLWX

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
925 PM EDT Tue Aug 11 2020

.SYNOPSIS...
High pressure will settle to the south of our area through
tonight. A cold front will approach late Wednesday into
Wednesday night and remain stalled near our area through the
end of the week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

T-storms are still ongoing this evening over portions of the
Shenandoah Nat`l Park. Have received reports of trees down in
southeast Rockingham County earlier around 815 PM. Storms should
dissipate by midnight with another warm and muggy night ahead
with lows in the 70s. Areas of dense fog are possible.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/...
An active stretch of weather will be in store across the area for
Wednesday and Thursday. During the day tomorrow, a cold front will
drop southward from Pennsylvania into our area, before eventually
stalling out overhead. The day will start out dry, with ample
sunshine expected during the morning hours. This sunshine coupled
with moisture transport into the area will allow instability to
build, with 2000-3000 J/kg of MLCAPE in place to the east of the
Blue Ridge by peak heating. Flow will be weak throughout the column,
and deep moisture will be in place, so the threat for damaging winds
is expected to remain low, but still non-zero, given the highly
unstable atmosphere in place.

The much greater threat tomorrow will be flash flooding. The
combination of a stalled frontal boundary, precipitable water
values in excess of two inches, deep warm cloud layers, slow
storm motions and antecedent wet conditions suggests that
instances of flash flooding are likely across the area tomorrow.
As a result, we have issued a Flash Flood Watch for all areas
to the east of the Blue Ridge, as well as Rockingham, Augusta
and Page counties to the west from noon until 11 PM. Storms may
initiate as early as noon, and could linger through the
overnight.

Thursday will be another very active day, with a similar overall
setup to Wednesday. The main difference will be the presence of a
shortwave tracking to our northwest through the Ohio Valley. The
surface front will remain stalled over our area, but the shortwave
may induce a bit more lift/overrunning over the boundary, as flow at
850 hPa turns southerly in response to the shortwave. Deep moisture
will remain in place with precipitable water values holding over two
inches, but instability values may be lesser compared to Wednesday.
Nonetheless, the threat for flash flooding looks to remain elevated
on Thursday, especially given that the ground will likely be
saturated from rains on Wednesday.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
A very active period is expected during the long term period from
Friday through Monday. A near stationary boundary will remain
stalled near to over top our region through Saturday. Multiple lows
will form and move along this boundary bringing continued showers
and thunderstorms capable of producing heavy rainfall. A flow
out of the south to southwest will lead to warm and humid air
being advected into our region. The presences of a stalled
boundary coupled with high PW`s will lead to a continued risk
for flash flooding Friday through early Sunday. During this same
period, warm and humid conditions will lead to modest CAPE with
decent shear which could lead to a strong to severe
thunderstorm threat, but that all depends on the positioning of
the frontal boundary.

There remains some uncertainty on whether the extended rain threat
will continue through Sunday. The rain and flood threat for Sunday
will depend on the upper level trough that approaches from the west
and kicks the stalled boundary out of our area. If the trough shifts
further south, the trough would kick off a new low on Sunday that
brings further rain to our potentially water logged region, but
if it stays further north rain threat shifts further south with
the boundary. Another front will drop down into our area on
Monday bringing another threat for rain and thunderstorms.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
The main aviation concern over the next couple days will be
thunderstorm coverage. Overall coverage of storms today should
remain isolated to widely scattered in nature. Coverage of
storms is expected to increase moving into Wednesday and
Thursday. For tomorrow, have introduced VCTS at all terminals
due to high expected coverage of storms. At the three major
hubs, tried to target a four hour window with the highest
coverage, but storms could be possible starting as early as noon
and could potentially linger through the night and right into
Thursday.

An extended period of rain and a south to southeasterly flow will
likely lead to subVFR conditions Friday through Sunday with low
ceilings being the main threat with periods of reduced visibility.
Strong gusts will be possible with thunderstorms that move over
terminals.

&&

.MARINE...
No significant synoptic wind issues expected through the next
few days, but southerly channeling could cause winds to approach
SCA levels this afternoon into tonight. Current thinking is
that gusts will remain below SCA levels, but will continue to
monitor winds through the remainder of the day. Otherwise, the
main concern through Thursday will be thunderstorms. Coverage
should remain isolated to widely scattered today, but will
increase significantly Wednesday and Thursday, with gusty winds
a threat from any storm. Best timing is afternoon and evening,
though the risk may linger through the night Wednesday and
Thursday nights. Special Marine Warnings may be needed for any
stronger storms.

The overall flow will lead to SubSCA conditions Friday through this
weekend but thunderstorms could bring periods of gusty winds over
our marine areas.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from Wednesday afternoon through Wednesday
     evening for DCZ001.
MD...Flash Flood Watch from Wednesday afternoon through Wednesday
     evening for MDZ004>006-011-013-014-016>018-503>508.
VA...Flash Flood Watch from Wednesday afternoon through Wednesday
     evening for VAZ025-026-029-036>040-050>057-501-502-
     505>508.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...LFR
NEAR TERM...LFR
SHORT TERM...KJP
LONG TERM...JMG
AVIATION...KJP/JMG
MARINE...KJP/JMG