Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 121944

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
244 PM EST Mon Feb 12 2018

High pressure will build north of the area tonight through
Tuesday before pushing offshore Tuesday night. A warm front
will then cross the region during midweek, followed by a cold
front late in the week.


A few lingering rain showers remain across portions of southern
Maryland and central Virginia early this afternoon, but these
too will push eastward and out of the region by late in the day
as the upper level energy and shortwave trough pull away. Low
level clouds have been decreasing through the day and this trend
will continue with variable amounts of mid/high clouds
remaining, more south/east, less north/west.

Strong high pressure (1045mb) will then build to our north
through the night and into Tuesday morning, leading to
significant drying and skies becoming mostly clear. Some high
clouds will continue to move across the region in the
west/southwest flow aloft but they should be pretty thin.
Northerly flow will persist through the night, but winds should
gradually lessen. Thus, temperatures should be able to drop into
the 20s for much of the region, except around 30F in the urban
centers and some teens in the coldest locations.


Surface high pressure will move eastward across upstate NY and
New England, moving offshore of southern New England by the
evening. A ridge axis will also wedge itself southward across
our area down into the Carolinas, as is typical of cold air
damming scenarios. At the same time, warm air advection will
begin aloft atop the cooler low levels, which will lead to
mostly sunny skies to start the day seeing an increase in clouds
from the south/west during the day. Some light precipitation is
also possible (<20%) across portions of the lower Shenandoah
Valley, central Blue Ridge, and Highland County VA during the
day Tuesday, and coverage will be spotty at most, however it is
possible for some light freezing rain with temperatures near or
below freezing for the first half of Tuesday. By later in the
afternoon, high temperatures will reach into the upper 30s to
mid 40s.

Warm air advection will continue Tuesday night, and that
combined with added cloud cover will keep temperatures mainly in
the 28-35F range. Some light precipitation becomes possible
along and west of the Allegheny Front late Tuesday night and
Wednesday morning. Temperatures will be rising, but if
precipitation does arrive early enough, there is a low potential
again for a bit of very light freezing rain (<25%). Moisture and
warm air advection will also act to increase shower chances
through the day, although mainly west of the Blue Ridge
Mountains. High temperatures Wednesday should be able to rise
through the 50s.

A more organized area of rain then becomes increasingly likely
Wednesday evening and Wednesday night as a weak system passes
north and a warm frontal zone crosses the region. No mixed
precipitation issues are expected with lows Wednesday night in
the 40s to around 50F.


One shortwave scoots off to the east Thursday morning which should
leave most of the day dry. Mild temperatures will continue,
especially if we can develop breaks in the clouds given deep SW flow
and an anomalously strong (590 dm 500 hPa) ridge centered over
Florida. Another wave of rain looks to accompany a cold frontal
passage Friday, though with more of a westerly component to the wind
amounts may tend to be on the lighter side.

A much cooler (seasonable) airmass will take over for the weekend. A
weak wave of low pressure will likely develop along the
aforementioned front to our south Saturday. One GEFS member and
two Canadian ensemble members/operational GGEM (from 00z runs
last night) are a little more amplified and bring precipitation
into the area, but this looks like a very low probability
solution at the present time. Strong building high and NW flow
should push any area of low pressure to the south of the area.

Temperatures should gradually moderate by early next week as high
pressure moves offshore.


VFR is expected through at least Tuesday evening. Northerly
winds around 10-12 knots with gusts up to 20 knots are expected
this afternoon under gradually clearing skies. Winds diminish
tonight and stay light on Tuesday with VFR continuing. There is
potential for some lower ceilings at times Tuesday night
and Wednesday with warm moist air returning, but chances are
low (<30%) at this time. Higher probabilities exist for some
MVFR ceilings Wednesday night with areas of showers.

Mainly VFR Thu-Sat. Some MVFR possible in -RA first thing Thu AM and
again Fri PM. Winds generally SW around 10 kts.


SCA remains in effect for all waters through the rest of today
as cold air pushes back into the region behind a cold front.
Winds have lessened this afternoon, but still seeing scattered
gusts into the 18-23 knot range. Most locations then drop below
SCA criteria this evening and remain that way through tonight.
However, as high pressure does build in and flow becomes more
northerly, channeling is likely to lead to another surge of
winds up to about 20 knots tonight into early Tuesday morning.
Thus, have extended the SCA for portions of the central
Chesapeake Bay and lower Tidal Potomac. Sub-SCA winds return for
much of Tuesday through Wednesday. Winds may then reach SCA
criteria as southwest flow increases Wednesday night.

Despite poor mixing over cooler water on SW flow, the gradient
should be strong enough to result in SCA gusts on at least portions
of the waters Thursday into Thursday night. SCA gusts are more
likely in NW flow behind a cold front Friday into Saturday. Winds
should go light by Saturday night as high pressure moves overhead,
then remain light through the remainder of the weekend.


MARINE...Small Craft Advisory until 6 PM EST this evening for
     Small Craft Advisory until 9 AM EST Tuesday for ANZ533-534-537-


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