Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 211127 AAA
AFDLWX

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
627 AM EST Sat Jan 21 2017

.SYNOPSIS...
A front will remain stalled over the region through today. Low
pressure will impact the area Sunday through Monday before moving
away Tuesday. A cold front will pass through the area Wednesday
into Wednesday night and an upper-level trough may build overhead
late next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Low clouds and areas of dense fog will continue through early this
morning. A Dense Fog Advisory is in effect for northern and
central Maryland...valleys west of the Blue Ridge Mountains...and
portions of central Virginia. The Dense Fog Advisory may need to
be expanded farther south and east...but confidence is too low to
do so at this time due to a stratus deck.

A warm front will stall out over the are today. The boundary is
most likely to stall out across the northern Shenandoah Valley
into the Virginia Piedmont. For locations north and east of the
boundary...low clouds will likely hang around throughout the
day...but dense fog and drizzle should dissipate later this
morning. For locations south and west of the boundary...there will
be some breaks of sunshine and it will be unusually mild for this
time of year. Max temps are forecast to be in the lower to middle
50s across Maryland as well as the Washington and Baltimore
Metropolitan areas...extreme northern Virginia and the eastern
Panhandle of West Virginia. Went a few degrees below guidance due
to the anticipated cloud cover. However...max temps are forecast
to be in the lower to middle 60s in the central Shenandoah Valley
where more breaks of sunshine are expected.

The boundary will remain nearly stationary tonight and an upper-
level disturbance will pass through the area during this time. A
period of light rain is possible as the upper-level disturbance
passes through. The best chance for rain will be from central
Virginia into southern Maryland...but rainfall amounts will be
light. More low clouds and areas of fog and drizzle are expected
to develop tonight due to plenty of moisture trapped underneath
the nocturnal inversion. Locally dense fog is possible.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Cutoff low pressure in the southern stream of the jet will track
through the Gulf Coast States Sunday before tracking northeast
into our area Sunday night through Monday. An easterly flow will
increase ahead of the low on Sunday and this will allow for
moisture to move in from the Atlantic Ocean. At the same
time...warm and moist air from the Gulf of Mexico will overrun the
surface cooler air in place. The overrunning will allow for plenty
of clouds along with period of rain. The best chance for rain will
be during the afternoon.

More rain is expected Sunday night. A soaking rain is possible as
the low-level jet increases ahead of the approaching low. As the
low tracks overhead Monday...the dry slot associated with this
system will move overhead. Details and timing of the dry slot are
uncertain at this time...but this may allow for a lull in the
precipitation. As of now...it looks like a soaking rain is
possible across all areas Monday morning...but rain may taper off
across southern and eastern areas Monday afternoon.

The surface low will begin to move away from the area Monday
night...but the upper-level low associated with this system will
remain overhead. More unsettled conditions are likely. Rainfall
amount Sunday through Monday night are forecast to be around
1-2"...but confidence is low due to the uncertainty with the
potential dry slot on Monday. Should the dry slot move in
quicker...which some of the latest guidance is suggesting...then
rainfall amounts will be less.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Primary area of wraparound precipitation from low pressure system
will be tapering off by Tuesday morning...with at most a few
lingering light showers from DC northeastward. Breezy W/NW winds
and relatively warm conditions will remain Tuesday as heights
rise over the area. Remaining dry Wednesday as high pressure
slowly weakens over the area in advance of next system. Cold front
poised to cross the area Wednesday into Thursday. Best forcing for
ascent passes by to our north...while better moisture resides
across the Carolinas...leaving us in between. Thus, precipitation
chances will be rather minimal for our area (except along the
Allegheny Front where upslope precipitation is expected)...with
only light amounts anticipated. Main impact will be a drop in
temperatures back closer to seasonable averages by Friday.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Low clouds and areas of dense fog will continue through mid-
morning. SubIFR conditions are expected during this time. Vsbys
and cigs may improve a bit this afternoon...but IFR conditions are
still expected. There is a chance that MVFR cigs could develop for
a brief period later this afternoon.

Clouds will lower again tonight into Sunday morning and areas of
dense fog and drizzle are expected. There may even be a period of
steadier light rain as an upper-level disturbance moves through.
SubIFR conditions are forecasted during this time.

Steadier rain may develop later Sunday into Monday. IFR conditions
are possible during this time as the cloud deck remains low. A dry
slot may improve conditions a bit for later Monday into Monday
night...but more showers are possible.

Becoming mostly VFR Tuesday morning as low pressure system moves
away...with only a few lingering light showers remaining. Though,
breezy conditions will continue through Tuesday...before subsiding
Tuesday night. VFR Wednesday with dry weather...as high pressure
briefly nudges into the area.

&&

.MARINE...
An onshore flow will continue through tonight...but wind speeds
should remain light. Low pressure will track through the Gulf
Coast States Sunday. An easterly flow will strengthen during this
time. A Small Craft Advisory is in effect for the Bay and lower
Tidal Potomac River Sunday afternoon.

Low pressure will approach the waters from the south and west
Sunday night before moving overhead Monday. The pressure gradient
will continue to strengthen ahead of the low. A Small Craft
Advisory will be needed for the waters Sunday night into Monday
and a Gale Warning may be needed for portions of the waters during
this time.

Gusty NW winds exceeding SCA criteria remain Tuesday behind
departing low pressure system. Winds slacken late Tuesday as
influence from low wanes. Mostly dry cold front crosses the waters
late Wednesday into Thursday...with gusty winds possibly to SCA
criteria behind it Thursday afternoon.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will continue through tonight...but it should be
light. Therefore...elevated water levels are expected but tidal
flooding is not expected.

An easterly flow will strengthen Sunday into Monday. This may
cause anomalies to increase during this time. Minor tidal flooding
is possible near night tide...especially late Sunday through
Monday. An offshore flow will develop behind the departing low
Monday night into Tuesday. Minor flooding is still possible Monday
night since exact timing of the offshore flow is still uncertain
as of now.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Dense Fog Advisory until 9 AM EST this morning for MDZ003>006-
     502-503-505-507.
VA...Dense Fog Advisory until 9 AM EST this morning for VAZ025>031-
     036>040-050-051-501-504-505.
WV...Dense Fog Advisory until 9 AM EST this morning for WVZ050>053-
     055-502-504-506.
MARINE...Small Craft Advisory from noon to 6 PM EST Sunday for
     ANZ530>534-537>543.

&&

$$
SYNOPSIS...MSE
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...MSE
AVIATION...BJL/MSE
MARINE...BJL/MSE
TIDES/COASTAL FLOODING...BJL



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