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Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia
955 PM EDT Sat Jul 4 2015

Synopsis...
a cold front will stall to our south tonight into Sunday. Weak
high pressure will build overhead early Sunday before moving off
the coast Sunday afternoon and evening. An upper level
disturbance impacts the area Sunday night through Tuesday bringing
unsettled weather.

&&

Near term /through Sunday night/...
..update to include dense fog advisory for the Virginia Piedmont.
Previous discussion follows...

A weak boundary stretches from the Shenandoah Valley into the
Virginia Piedmont this evening. An upper-level trough axis is
moving off to our east during this time...and surface low pressure
that was along the boundary is also moving away from the area. The
showers and thunderstorms that developed earlier today have moved
out of the County Warning Area this evening. Subsidence behind and upper-level
disturbance will keep most areas dry this evening. However...an
isolated shower cannot be completely ruled out with the boundary
nearby. For any areas that do get a shower it should only last
about ten to fifteen minutes before moving away...and again most
areas will not see any precipitation.

Dry conditions will persist overnight as weak high pressure
settles over the region. The boundary will dissipate and shift
south during this time. There is little dewpoint or temperature
gradient associated with the boundary...so low-level moisture will
hang around. The combination of low-level moisture...light winds
and breaks in the clouds will result in areas of fog. Some fog may
be dense overnight and this will have to be monitored. A dense fog
advisory may be needed for portions of the area late
tonight...especially for locations that received rainfall and also
sheltered valleys/rural areas.

High pressure will dominate Sunday with partly to mostly sunny
skies for much of the day especially over the metropolitan.
However...further southwest...a new closed low will develop aloft
tonight over the Tennessee Valley and move northeast toward our
region. Clouds will begin increasing again...and showers and
thunderstorms are expected to develop and advect northeastward
starting in our southwestern zones during the afternoon. These
will overspread most of the County Warning Area by later Sunday night. After highs
in the 80s Sunday...lows will drop back into the 60s on Sunday
night.

&&

Short term /Monday through Tuesday/...
middle/upper level disturbance that is cutoff from the main belt of
westerlies to the north begins to impact the area in earnest
during the day Monday. Combination of surface warm front moving north
over area...upper level diffluence...and very moist deep layer
with precipitable waters prognosticated to be near 2 inches suggest the potential for
efficient rain producers with moderate to heavy rain possible.
Shear remains weak (0-6 km ebs at or below 20 kts) and instability could
be limited due to extensive cloud cover...which would limit
overall storm organization/intensity. Temperatures might need to be
adjusted a little further downward in subsequent forecasts...as
aforementioned cloud cover could limit insolation.

Bulk of activity should move away from the area during the
overnight Monday as the disturbance is engulfed into the northern
stream. Likely a period of subsidence behind disturbance providing
some clearing during the early part of Tuesday. Little in the way of
forcing mechanism Tuesday...though could get some activity moving east
off higher terrain or along Lee trough. Starts getting a little
warmer Tuesday...with temperatures getting back to near normal (u80s).

&&

Long term /Tuesday night through Saturday/...
disturbance lingers in the area Tuesday night into Wednesday as a cold
front approaches late on Wednesday. Cold front slowly drifts south over
the County Warning Area and lingers through the end of the week...drier conditions
arrive for the weekend. Uncertainty on location of the front may
affect probability of precipitation...but diurnal effects allows for the risk of
isolated/scattered showers and/or thunderstorms each day. High temperatures will
be in the 80s...low temperatures in the 70s for most of the County Warning Area...and in
the middle to upper 60s at higher elevations.

&&

Aviation /02z Sunday through Thursday/...
the storms have moved away from the terminals this evening. A
stray shower cannot be ruled out this evening...but most areas
will be dry. Areas of fog are expected late tonight into Sunday
morning. Fog may be locally dense with IFR/subifr conditions
possible. Sunday after any fog breaks should be mostly
VFR...though late in the day another round of storms looks to move
into cho and then northeast toward the rest of the terminals
Sunday night...with more concerns about gusty winds and reduced
visible. Outside of storms...winds should remain below 10 knots
through Sunday night.

IFR ceilings appear possible through the day Monday in moderate/heavy
rain showers and storms. Improving to MVFR late Monday and VFR by
Tuesday morning as upper level disturbance pulls away from the
area.

VFR conditions expected for Tuesday night and Wednesday. Sub-VFR due to
possible showers and thunderstorms Wednesday afternoon.

&&

Marine...
thunderstorms will move through the lower tidal Potomac River and
Maryland Chesapeake Bay south of Drum Point early this evening.
Elsewhere...dry conditions are expected most of the time but a
stray shower cannot be ruled out. North to northeast winds will
remain below Small Craft Advisory criteria this evening. Winds will be light
overnight as weak high pressure builds overhead. The high will
move off the coast Sunday and an east to southeast flow will
develop. Wind speeds will remain below Small Craft Advisory criteria...but showers
and a few thunderstorms are possible Sunday night.

Winds expected to remain light and variable Monday....but could
strengthen some on Tuesday due to channeling srly flow. Could be some
isolated thunderstorms Monday and again Tuesday afternoon.

Sub-Small Craft Advisory conditions expected for Tuesday night and Wednesday. Possible showers
and thunderstorms across the waters on Wednesday afternoon.

&&

Tides/coastal flooding...
the flow will remain light through Sunday. No tidal flooding is
expected...despite anomalies being a bit above normal.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...dense fog advisory until 10 am EDT Sunday for vaz039-050>052-
055>057-502.
WV...none.
Marine...none.

&&

$$
Synopsis...mse
near term...bjl/rcm
short term...mse/rcm
long term...imr
aviation...bjl/imr/mse/rcm
marine...bjl/imr/mse/rcm
tides/coastal flooding...

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