Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 220450 AAB
AFDLWX

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1150 PM EST Sat Jan 21 2017

.SYNOPSIS...
Low pressure will impact the area tonight 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 SUNDAY/...
Dense Fog Advisory expanded further to the south and east to
account for observations of visibilities of a quarter mile or
less. Still unclear if further expansion to the SE will be
necessary...as light rainfall is moving into these areas...which
could limit spatial coverage of dense fog.

Previous Discussion...
Areas of dense fog have been developing across north central MD,
along the western shore of the Chesapeake Bay and portions of the
eastern WV panhandle. Every piece of model guidance shows
visibilities lowering overnight across these areas and expanding
southward in coverage. A dense fog advisory has been issued until
10Z and may need to be expanded southward and extended in time.

Rain is now moving into the cntrl Shenandoah valley and the VA
foothills. This is associated with a developing sfc wave across
North Carolina. Brought a chance of rain farther north into ncntrl
MD to the PA border based on 12Z ECMWF ensembles and NSSL WRF-ARW.

Low clouds and areas of dense fog are expected again this evening
as low-level mixing shuts off. The next wave will cross the
region tonight and rain will move into the region. Model guidance
is depicting most of the rain to be across the southern half of
the outlook area however some hi-res guidance is showing the rain
may move further north. The best chance for rain overnight will
be across the southern half of the region including DC due to the
vicinity of a forming sfc low. Areas across the northern half
have the potential to get widespread dense fog due to the absence
of rain and an advisory may be needed.

Rain will move east of the Mid-Atlantic region by Sunday morning.
Another dreary day is expected Sunday as northeast winds keep low
level moisture in place. Mostly dry conditions are expected Sunday
morning before more rain arrives Sunday afternoon. Moisture
advection will increase Sunday afternoon and periods of rain are
expected into Sunday night. Temperatures will be the 50s Sunday.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
As low pressure takes shape across the TN valley/Appalachians
Sunday night into Monday a low level jet is expected to
strengthen over the Mid-Atlantic region. Moderate to heavy rain is
expected Sunday night into Monday. Rainfall totals between 1-2
inches are expected across the region. Widespread rain will end as
the dry slot moves into the Mid-Atlantic region Monday afternoon.
Can not rule out convective showers along the leading edge of the
dry slot Monday afternoon. A rumble of thunder is possible mainly
across Central VA to Southern MD. Occational rain showers are
expected through Monday night as an upper level low continues to
spin over the region.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Surface low pressure continues to moves northeast away from the Mid-
Atlantic coast on Tuesday as showers start tapering off, but breezy
conditions pick up. Upper level ridge builds Tuesday night into
Wednesday... and a cold front moves across on Wednesday night with
limited precipitation associated to it. Upper level trough behind it
could bring some precipitation Thursday through Saturday... mainly
over upslope areas, but uncertain at this moment.

Breezy conditions anticipated during parts of this period...
especially over coastal areas and higher elevations. High
temperatures will be in the 40s and 50s Tuesday and Wednesday,
decreasing to the 30s and 40s. Low temperatures will be in the 20s
and 30s.. with some 40s Tuesday and Wednesday night.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
-RA is likely across DCA/IAD/BWI tonight and may improve vsbys.
-RA is possible further north however confidence is low. Dense fog
is also possible across the terminals overnight. Low clouds and
fog will likely linger into Sunday. Another system will approach
the region Sunday and periods of RA are expected Sunday afternoon
into Monday night. RA may be heavy at times reducing vsbys. IFR
conditions expected during this time.

Surface low pressure moves away from the Mid-Atlantic region on
Tuesday. Some showers possible over the terminals. Dry conditions
expected Tuesday night into Wednesday night, before a cold front
moves across. Upper level disturbance will affect us on
Thursday... with possible showers. Breezy conditions expected for
parts of this period, gusting up to 20 kt. Sub-VFR conditions
possible mainly early on Tuesday and Wednesday night.

&&

.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 is in effect for all the waters Sunday evening and for
portions of the waters into Monday afternoon. A Gale Warning is
in effect for the northern waters from the Patapsco River north. A
Gale warning may be needed further south when confidence is
higher.

Surface low pressure moves away from the Mid-Atlantic region on
Tuesday. Some showers possible over the terminals. Dry conditions
expected Tuesday night into Wednesday night, before a cold front
moves across. Upper level disturbance will affect us on
Thursday... with possible showers. Breezy conditions expected for
parts of this period, gusting up to 20 kt. Small craft advisory
conditions expected for most of this period. &&

.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.

&&

.TIDES/COASTAL FLOODING...

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Dense Fog Advisory until 10 AM EST Sunday for MDZ003>006-011-
     014-502>508.
VA...Dense Fog Advisory until 10 AM EST Sunday for VAZ027>031-040-
     501-505-506.
WV...Dense Fog Advisory until 10 AM EST Sunday for WVZ050>053-055-
     502-504.
MARINE...Small Craft Advisory from 6 PM Sunday to 1 PM EST Monday for
     ANZ535-536.
     Small Craft Advisory from noon Sunday to 1 PM EST Monday for
     ANZ532>534-537-540>543.
     Small Craft Advisory from noon Sunday to 1 AM EST Monday for
     ANZ530-531-538-539.
     Gale Warning from 1 AM to 1 PM EST Monday for ANZ530-531-538-
     539.

&&

$$
SYNOPSIS...HSK
NEAR TERM...MSE/LFR
SHORT TERM...HSK
LONG TERM...IMR
AVIATION...IMR/LFR
MARINE...IMR/LFR
TIDES/COASTAL FLOODING...RM



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