Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 271852
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
252 PM EDT Thu Jul 27 2017

.SYNOPSIS...
An upper-level disturbance will pass through the area this
evening. Upper level low pressure system will intensify as it
moves into the area Friday. This will slowly pass through the
area Friday night into Saturday before stalling near the Mid-
Atlantic coast through Monday. High pressure is expected to
build overhead during the middle portion of next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Surface analysis shows high pressure off the southeast coast
with a cold front extending from Quebec to Illinois.

Isolated to scattered showers and thunderstorms are possible.
Shear profiles will strengthen a bit in response to the
shortwave energy moving through and this may cause a few
thunderstorms to become severe. However...widespread severe
weather is not expected since the forcing will be weak and shear
profiles will be marginal. Locally damaging wind gusts are the
primary threat.

The shortwave energy and surface trough will move to the east
tonight while a stronger upper-level low digs through the Great
Lakes. Our area will remain in between these systems...so any
showers will likely remain isolated to scattered. Min temps will
range from the 60s in most areas west of the Blue Ridge
Mountains to the lower and mid 70s near Washington and
Baltimore.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/...
Upper-level low will track from near Lake Winnepeg into the
Ohio Valley Friday, and into our area Friday night. This system
will interact with a cold front to our north, causing surface
low pressure to develop along the boundary. The upper-level low
and surface low will strengthen as it moves into our area Friday
night. Coastal low pressure is expected to develop near the
Delmarva Peninsula Friday night. This system is expected to
remain nearly stationary around this area through Saturday
night.

As the low approaches the area Friday...showers will become more
widespread along with some thunderstorms. Deep moisture in place
suggests that showers and thunderstorms will be locally heavy.
The widespread showers and thunderstorms are expected to
continue through Friday night as the low pressure strengthens
overhead. A flash flood watch has been issued for the northern
2/3 of the forecast area. The heaviest rain is likely to be
just north of the surface low and near the 850mb warm
front...where the most frontogenetical forcing is expected.


Latest guidance seems to paint this area across northern and
central Maryland...northern Virginia...eastern West Virginia and
toward the Washington and Baltimore metropolitan areas.
Most likely rainfall amounts for Friday through Friday night
are around 1 to 3 inches...again with the best chances of the
highest amounts being across the areas mentioned.
However...locally higher amounts are possible due to the
convective nature of the precipitation.

The low is expected to stall near the Delmarva Peninsula
Saturday through Saturday night. A gusty north to northeast flow
is likely along with more rain. Unusually cool conditions are
expected due to the cloud cover and rain. The trend over the
past couple days has been slower and farther south/west with
this system for Saturday through Saturday night.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
The cut-off low will likely be moving off the east coast Sunday
while coastal low pressure will be near Delmarva. Showers will
be tapering off from west to east across the Mid- Atlantic
region and northerly flow will keep temperatures on the cool
side for late July. The cut-off low oscillates along the Mid-
Atlantic coastline through Monday resulting in cool and cloudy
conditions across portions of the Mid-Atlantic region. This
system finally moves out to sea Tuesday and high pressure moves
into the region.

Temperatures will slowly climb back to near normal by next Wed-Thu.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
Although plenty of clouds around celings are VFR. Isolated to
scattered thunderstorms are expected this afternoon and evening.
A couple storms may be severe...with damaging winds the primary
threat.

Low pressure will approach the terminals Friday before moving
overhead and intensifying Friday night. Widespread showers and
thunderstorms are expected to develop and rain may be locally
heavy. A few storms may also be severe. More rain is likely
Saturday and Saturday night along with Subvfr cigs and vsbys as
the low stalls out near the Delmarva Peninsula. Gusty north to
northeast winds are also possible during this time.

Rainfall may linger Sunday (and perhaps into Monday) as low
pressure meanders near the area. Improving conditions expected
once low moves further out to sea by the middle of the week.

&&

.MARINE...
High pressure will remain offshore through tonight and an
onshore flow is expected. Marginal SCA is effect.

Low pressure will approach the waters Friday before
strengthening overhead Friday night. The low will likely stall
out near the Delmarva Peninsula Saturday through Saturday night.
Gusty winds are possible with the low nearby...especially Friday
night through Saturday night.

Northerly winds continue on the waters Sunday-Monday. SCA
may be needed.

&&

.HYDROLOGY...
A flash flood watch has been issued for Friday afternoon through
Saturday afternoon for the northern 2/3 of the forecast area.

Most likely rainfall amounts during this time will be around 3
to 5 inches. This may cause minor flooding...particularly across
the Potomac River Basin.

&&

.TIDES/COASTAL FLOODING...
Persistent southerly flow has allowed tidal anomalies to
increase to over a foot. Coastal Flood Advisories this evening
at Straits Point.

Low pressure is expected to impact the area Friday into the
weekend, bringing the potential for elevated tidal anamolies
with a risk of coastal flooding.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for DCZ001.
MD...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for MDZ003>006-011-013-014-016>018-501>508.
     Coastal Flood Advisory until 9 PM EDT this evening for MDZ017.
VA...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for VAZ027>031-040-051>055-057-501-502-505>507.
WV...Flash Flood Watch from Friday afternoon through Saturday
     afternoon for WVZ050>053-055-501>504.
MARINE...Small Craft Advisory until 2 AM EDT Friday for ANZ532>534-537-
     540-541-543.

&&

$$
PRODUCTS...WOODY!/HSK



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