Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 291841
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
241 PM EDT Mon May 29 2017

.SYNOPSIS...
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 Tuesday night and Wednesday night. Low
pressure approaches from the west this weekend.
&&

.NEAR TERM /THROUGH TUESDAY/...
The marine wedge has eroded across the forecast area, and
resides just across the eastern shore at this time. The surface
boundary extends down I-95. There is not much thermal contrast
anymore, but there is still weak surface convergence. That,
combined with modest instability (MLCAPE around 500 j/kg) and
good shear (40-50 kt effective bulk) has resulted in widely
scattered showers across southern Maryland and the
Fredericksburg area of Virginia. Would not rule out a
thunderstorm or two, but conditions do not appear favorable for
anything strong. These should dissipate by sunset.

With ample low level moisture (dewpoints in the 60s) along the
metro corridor and without anything impeding it, the marine
wedge should return tonight. The only questions are how soon it
will return and how far west it will make it. Current NAM/GFS
plus latest RAP/HRRR suggest at least to the Blue Ridge if not
I-81. Am taking an overcast layer to CHO-MRB. Believe the PoPs
in MOS data drizzle. Have not yet added it to weather, but will
advise evening forecaster.

Tuesday morning will be a struggle to erode marine air mass once
again. That will impede afternoon thunderstorm development.
However, the right rear quad of the upper jet, differential
heating, and another surface boundary supported by a 500 mb
shortwave should provide the catalyst for scattered if not
numerous showers. Guidance generating enough CAPE west of the
maritime layer to justify including chance thunder.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
We will have one more front with shortwave support to contend
with...this time late Wednesday. Have not made many changes to
the database in association with this feature (still chance
PoPs). In keeping with this pattern, have been reserved in use
of thunder. Clouds should be numerous in advance of the front.
One difference though is that this time it appears as though we
will have a decent push of drier air behind the front, to
support clearing skies Wednesday night.

Decent consistency in guidance temps. Have not strayed far from
an ensemble blend.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Thu is looking like a very pleasant day with Canadian high pressure
settling in over the region for the first time in quite awhile.
Plentiful sunshine and highs near or even slightly cooler than
normal for the first day of June. It will be short lived however. By
Saturday morning the next cold front is slowing as it approaches
us from the north early Saturday. It will likely stall over or very
close to us over the weekend, with low pressure riding along west to
east across it and over us Sunday.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
VFR for now, but aob IFR (low clouds and vsbys) should return
tonight as marine layer spreads inland again. High confidence in
its occurrence; questions revolve around timing. Current TAFs
on the conservative side, taking restrictions just for the
morning push; it could be sooner.

Erosion Tuesday will be a challenge as well. Restrictions will
into mid morning if not later. Then, afternoon showers and
thunderstorms a good possibility. These flight restrictions
should be brief.

Wouldn`t rule out more fog-related restrictions early Wed
morning, followed by another round of possible thunderstorms Wed
afternoon.

VFR likely for rest of week.

&&

.MARINE...
Flow pattern should be light for most of the week. Bigger
concern would be fog tonight into Tue morning. Lesser chance Tue
night into Wed morning. By late Wednesday, a cold front should
dry airmass out.

There could also be a few thunderstorms each afternoon through
Wednesday, although not likely strong.

&&

.TIDES/COASTAL FLOODING...
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 are in place for Washington DC and St.
Marys for this evening`s high tide cycle, which is the lower of the
next two cycles. The advisory for St. Marys continues through
Tuesday and additional advisories may be necessary at other sites
for Tuesday morning`s cycle. It is also likely that additional
advisories will be needed for high tide cycles through Wednesday for
the sensitive locations, and possibly elsewhere. By Thursday,
northwesterly flow as high pressure builds in should finally allow
anomalies to drop.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory from 11 PM this evening to 2 AM EDT
     Tuesday for DCZ001.
MD...Coastal Flood Advisory until midnight EDT Tuesday night for
     MDZ017.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...RCM
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...CAS
AVIATION...HTS
MARINE...HTS
TIDES/COASTAL FLOODING...MM



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