Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 300803

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
403 AM EDT Tue May 30 2017

Upper level low pressure will drift east across the north side
of the Great Lakes this week. In the meantime, weak high
pressure will prevail over the Mid Atlantic outside of weak cold
frontal passages tonight and Wednesday night. Another cold front
will slip south into the region later Friday and stall over the
region this weekend.


It`s a little difficult to discern in surface obs, but a
boundary resides south of the area with a weak impulse around
it. This impulse combined with weak elevated instability has
allowed isolated showers to develop in southern MD. Onshore flow
is also causing low clouds, fog, and perhaps a little drizzle
to ease westward. It appears this will make it to the northern
Shenandoah Valley, and perhaps barely to Charlottesville. Once
it reaches its westward extent, it appears it will be very slow
to erode today as onshore or southeasterly flow continues.

HRRR has been the most aggressive model spreading showers with
southern impulse northward during the midday hours. Gradually
have POPs ramping up, but feel most precipitation through midday
will be drizzle.

Weak front approaches from the west this afternoon and evening.
SBCAPE of 1000-1500 J/kg will build west of the maritime layer
amidst deep layer shear of 40-50 kt. A few cells may develop
ahead of the boundary on the terrain, but it looks like most of
the activity would be near the actual front. With these
parameters, at least a few strong to severe storms will be
possible. Strong winds would be the main threat, though some
hail is also possible. What remains uncertain is the position of
the stable airmass. Majority of models keep the I-95 corridor
locked in with clouds until at least early afternoon. The
Baltimore area may struggle to reach 70 (areas which see sun
will reach the 80s). So while elevated instability may help
maintain storms into the I-95 corridor, their intensity should
wane. Sunset will also help the cause. Certainly a situation
that will need to be monitored today.

Low clouds and fog look probable to redevelop tonight,
especially from DC north and east. Lows will range from the mid
50s west to mid 60s metros.


Clouds and fog should be quicker to erode Wednesday with less
of an onshore component. Final front and shortwave will be
approaching, but likely not until late in the day into the
evening. Have timed precipitation chances accordingly.
Instability looks shallower and lower in magnitude, so thunder
may be more difficult to come by. However shear will be
plentiful so the situation will need to be monitored.

With the front clearing the area late at night, drier air will
arrive and likely last through Thursday night as surface high
pressure slides across the area. Temperatures will be


Next cold front will be approaching from the northwest on
Friday, with southwesterly winds bringing warmer and more humid
air northward into the region. Some guidance has slowed the
approach of the front, keeping the region dry thru the day,
while others bring the front close enough to get convection
going in the afternoon and evening. Regardless, it will feel
more summerlike. Saturday looks cooler as the front settles into
the area, but the same split in the agressiveness of the front
continues, with some guidance having it stall in the area while
others pushing it south. Either way, chance of showers and
thunderstorms will be present. Sunday, wave of low pressure
approaches from the west, with increasing risk of showers and
t-storms later in the day and at night. By Monday the speed of
this low is in question, with some guidance pushing it east of
us, while others keeping it nearby, so Monday could still be
wet. Either way, late weekend into early next week does not look
particularly warm.


Marine layer has been working inland with IFR to perhaps LIFR
conditions. MRB/CHO will have the shortest duration of reduced
conditions. However, it appears low clouds could hang on into
the afternoon across the metros, especially Baltimore area,
which may never see VFR conditions as low clouds are expected to
develop again tonight. Baltimore area is most likely to have
near-IFR conditions tonight.

Showers/storms develop around MRB by mid afternoon and push
eastward, although their intensity should decrease as they move
toward the I-95 corridor. Coverage is also uncertain.

Low clouds should be quicker to erode Wednesday. A front will
move through late in the day with a chance for showers and
perhaps a thunderstorm.

VFR conditions are expected late Wednesday night through
Thursday night as high pressure builds across the area.

VFR likely Friday, but showers and t-storms will be possible
later in the day thru Saturday as a front drops into the region.


We saw an enhancement of onshore flow in southern MD during the
evening, but that seems to be diminishing. Low clouds and fog
will be the story this morning and again tonight with light
onshore flow. A few showers or storms may reach the waters late
in the afternoon to evening. though intensity should be
decreasing. The upper Potomac has the best chance for gusty

A cold front will move through Wednesday night with another
chance for showers and perhaps a storm. At this time it appears
post frontal westerly flow remains below SCA. Light winds with
high pressure Thursday and Thursday night.

SCA not likely Friday or Saturday, but shower and thunderstorm
risk grows as we head into the weekend with another cold front
dropping down.


Water levels remain high and should continue to be above normal for
the next couple of days due to the persistent onshore flow.

Coastal Flood Advisories for most of our tidal shorelines with
the exception of the far northern part of the Chesapeake Bay,
where the easterly flow is actually keeping water levels a
little lower. Some will likely need to be extended further given
expectation of continued onshore flow into tonight.

By Thursday, northwesterly flow as high pressure builds in
should finally allow anomalies to drop.


DC...Coastal Flood Advisory from 10 AM this morning to 4 PM EDT
     this afternoon for DCZ001.
MD...Coastal Flood Advisory until 10 AM EDT this morning for MDZ018.
     Coastal Flood Advisory until 10 AM EDT this morning for MDZ016.
     Coastal Flood Advisory from 8 AM this morning to 2 PM EDT this
     afternoon for MDZ011.
     Coastal Flood Advisory until 3 PM EDT this afternoon for
     Coastal Flood Advisory until midnight EDT tonight for MDZ017.
VA...Coastal Flood Advisory until 10 AM EDT this morning for VAZ057.
     Coastal Flood Advisory from 10 AM this morning to 4 PM EDT
     this afternoon for VAZ054.
MARINE...Small Craft Advisory until 5 AM EDT early this morning for


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