Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 041541
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1041 AM EST Sun Dec 4 2016

.SYNOPSIS...
An upper-level disturbance will approach the area today, passing
through the region tonight. High pressure will briefly return for
Monday. Coastal low pressure will impact the area Tuesday before
high pressure briefly returns again for Wednesday. A potent cold
front will move through late in the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Southwest flow aloft will increase through this afternoon ahead
of a northern stream trough moving out of the central plains. This
will help to usher in warm air advection and moisture aloft,
leading to thickening and gradually lowering clouds through the
afternoon. As system pushes eastward, some light precipitation
will begin to approach the region late this afternoon, but most
places remain dry through the afternoon. Temperatures will be on
the cool side with highs in the 40s for most locations, except
some 30s across the higher terrain.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Northern stream trough will push across the region later this
evening and overnight, picking up some energy/moisture from the
southern stream, but largely remaining separate and unphased. This
will lead to a period of precipitation this evening and overnight,
but keeping amounts on the light side. With southwest flow and
marginally cool atmosphere in place to start, the majority of the
area will see rain as the predominant precipitation type. As wet-
bulb cooling occurs aloft when precipitation overspreads the area,
there will likely be a period when sleet mixes in with the rain,
but with little to no sleet accumulation. However across the
higher elevations of the Alleghenies and possibly the Blue Ridge
that reach into that colder layer, some freezing rain is expected.
Thus have issued a Winter Weather Advisory from 5pm this evening
to 6am Monday morning for a light ice accumulation less than a
tenth of an inch.

In addition, there may be some snow across portions of
northern/western Maryland and into northeastern West Virginia, but
any accumulations will be light and less than one inch, and
mostly confined to the higher elevations.

As warming occurs late at night will see most locations warm above
freezing and precipitation should end as some light rain by
morning. Lows in the 30s to around 40F.

High pressure will build in behind the system on Monday, leading
to a drying trend. Highs back up into the upper 40s to mid 50s.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Low pressure will track through the Gulf Coast States into the
Tennessee Valley Monday night. The surface ridge from high
pressure over New England will continue to extend southwest into
our area. Clouds will lower and thicken as the low approaches from
the southwest. Latest guidance has slowed the onset of
precipitation...so most areas will be dry. However...rain/mixed
precip may approach central Virginia toward daybreak.

The low will slowly move northeast into the Ohio Valley Tuesday
while it transfers its energy to a coastal low along the North
Carolina Coast. Warm and moist air from the Gulf of Mexico and
Atlantic Ocean will overrun the surface cooler air in
place...likely resulting in a soaking rain for most areas.
There will be plenty of dry air ahead of the system and as
precipitation into the dry air...it will lead to insitu cold air
damming from evaporative cooling. The cooling effect may be enough
for a cold layer cold air to remain trapped near the surface. This
may cause freezing rain...especially for the higher elevations in
the Allegheny and Potomac Highlands. Elsewhere...evaporative
cooling may lead to some sleet at the onset but surface temps
should be above freezing. Any ice amounts from freezing rain should
be light with temperatures near 32...but that can still cause
slippery conditions and it will have to be monitored.

Coastal low pressure will slowly move off to our north and east
Tuesday night. Rain in the evening should give way to drying
conditions overnight. Exact timing of precipitation ending is a
bit uncertain at this time. High pressure will briefly build
overhead for Wednesday. A potent cold front will pass through
Thursday and much colder conditions are expected behind the
boundary for the end of the week.

&&

.AVIATION /16Z SUNDAY THROUGH THURSDAY/...
VFR conditions will continue through today with a gradually
thickening and lowering cloud deck ahead of an approaching low
pressure system. Precipitation in the form of rain and possibly
some sleet will overspread the TAF sites tonight bringing
widespread sub-VFR conditions overnight and into early Monday
morning. Conditions will then improve back to VFR during the day
Monday.

Winds light northerly this morning become southerly by this
afternoon before turning back to the northwest on Monday.

VFR conditions expected Monday night. Rain will overspread the
terminals Tuesday morning and it will continue through Tuesday
afternoon before gradually ending Tuesday night. Subvfr conditions
are expected during this time. Rain may start off as a period of
sleet Tuesday morning. Rain may also freeze on some surfaces for a
brief period Tuesday morning around KMRB...but confidence is low
at this time.

&&

.MARINE...
Winds area finally below SCA and should remain below through
Monday.

Coastal low pressure will develop to the south Monday night and it
will pass by to the east later Tuesday into Tuesday night. High
pressure will briefly build toward the waters Wednesday...but a
potent cold front is expected to pass through Thursday. A Small
Craft Advisory may be needed for the waters Tuesday into
Wednesday. A Gale Warning may be needed for the waters Thursday
night and Friday behind the cold front.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST
     Monday for MDZ501.
VA...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST
     Monday for VAZ503-504.
WV...Winter Weather Advisory from 5 PM this afternoon to 6 AM EST
     Monday for WVZ501-503-505-506.
MARINE...None.

&&

$$
SYNOPSIS...BJL/RCM
NEAR TERM...MM/RCM
SHORT TERM...MM
LONG TERM...BJL/MM
AVIATION...BJL/MM/RCM
MARINE...BJL/MM/RCM



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