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000
FXUS61 KLWX 271448
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1048 AM EDT Thu Jul 27 2017

.SYNOPSIS...
An upper-level disturbance will pass through the area this
afternoon into 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 /THIS AFTERNOON-TONIGHT/...

Surface analysis shows high pressure off the southeast coast
with a cold front extending from Quebec to Illinois. Southerly
flow will usher in a return of seasonably warm and humid
conditions. An upper-level disturbance will pass through the
area this afternoon into this evening. 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. Max temps
will be in the mid and upper 80s.

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 middle 70s near Washington and
Baltimore.

&&

.SHORT TERM /FRIDAY 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. This is because the closed upper-level low cuts off from
the jetstream.

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. We are considering issuing a flood...possibly flash
flood...watch with a later forecast package. 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.
There is some uncertainty with some guidance showing that band
a bit farther south toward central Virginia and other guidance
showing it closer to the Mason-Dixon line. 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. Rain may also be
locally heavy with the upper-level low overhead. This may
produce additional flooding concerns. The trend over the past
couple days has been slower and farther south/west with this
system for Saturday through Saturday night. Therefore...the
forecast has been adjusted to account for this.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Guidance has slowed considerably with the departure of upper-level
low late in the weekend as it remains cutoff from the main belt of
westerlies to its north. Thus, unsettled weather may now remain into
Sunday and perhaps even Monday as the upper-level low only gradually
pulls away from the area. Precipitation potential during this time
will be tied to position of the low -- the closer it remains to our
area, the greater the potential. Low would become quasi-vertically
stacked, which would generally limit precipitation intensity.
Though, antecedently we may not be able to handle much extra
rainfall after previous days rainfall.

Low will eventually move east of the area with drying conditions
behind it.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...

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. Will continue with the SCA for middle
portions of the Bay and the lower Tidal Potomac River through
tonight...but it will be marginal.

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.

Rainfall and gusty winds AOA Small Craft Advisory criteria may
remain Sunday into Monday as low pressure remains nearby.

&&

.HYDROLOGY...
Heavy rain is becoming increasingly likely later Friday through
Saturday night. There is still some uncertainty as to where the
heaviest rain is to setup...but based on the latest guidance the
most likely scenario is across northern Virginia...northern and
central Maryland...eastern West Virginia and toward the
Washington and Baltimore Metropolitan 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. Latest MMEF shows minor flooding possible along
portions of the Potomac River Basin later Saturday into Sunday.
Confidence remains low at this time due to some uncertainty as
to exactly where the heaviest rain will setup.

&&

.TIDES/COASTAL FLOODING...
Persistent southerly flow has allowed tidal anomalies to
increase to over a foot with Coastal Flood Advisories
currently in effect for this morning at DC, Straits and
Annapolis and then again 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. Confidence is timing and areal
extent of the threat is currently low due to uncertainty on the
track/timing of the low.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 2 PM EDT this afternoon for
     DCZ001.
MD...Coastal Flood Advisory until noon EDT today for MDZ014.
     Coastal Flood Advisory until 9 PM EDT this evening for MDZ017.
VA...Coastal Flood Advisory until 2 PM EDT this afternoon for
     VAZ054.
WV...None.
MARINE...Small Craft Advisory until 2 AM EDT Friday for ANZ532>534-537-
     540-541-543.

&&

$$
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