Area Forecast Discussion
Issued by NWS Sterling, VA

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471
FXUS61 KLWX 220845
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
345 AM EST Wed Nov 22 2017

.SYNOPSIS...
A cold front will pass through the area early this morning and
high pressure will build overhead later today through Friday.
High pressure will move offshore Friday night and a cold front
will pass through late Saturday into Saturday night. High
pressure will return for early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cold front is located over southwest Pennsylvania into West
Virginia as of 3 am this morning. The cold front will pass
through our area over the next few hours from west to east. At
the same time...southern stream moisture is advecting north into
our area. The southern stream moisture is most likely to make
it into areas near and east of Interstate 95. Therefore...a
period of rain is most likely across these areas early this
morning with the frontal passage.

The front will move off to the south and east by mid-morning
and high pressure will approach from the Midwest and Great
Lakes. A potent shortwave will pass by to our north during this
time...but with most of the dynamics remaining to the north it
should not have an impact on our weather. Therefore...increasing
sunshine and gusty northwest winds are expected ahead of the
building high. Max temps are a little tricky to forecast since
there will be cold advection behind the cold front...but that
will be offset by increasing sunshine and mixing along with a
downsloping flow. Did tweak the hourly trend in temps a little
away from the typical diurnal trend. Feel that max temps will
occur late this morning to around noon before holding steady and
even falling during the middle afternoon hours. Max temps are
most likely to range from the lower to middle 30s along the
ridge tops of the Allegheny Highlands...to the mid and upper 40s
for most of northern Maryland into the Shenandoah Valley...to
the middle 50s near Washington and Baltimore...to the upper 50s
and even near 60 in central Virginia into southern Maryland.

Also of note...there will be a period of snow showers through
mid-morning for locations along/west of the Allegheny Front.
Moisture will be limited...but an upslope component along with a
higher subsidence inversion will cause some accumulation above
2000 feet. Accumulations around an inch or less are most likely
before snow showers taper off by late morning.

High pressure will build overhead tonight and winds will
decrease around sunset for most areas. Winds will become light
overnight and with dry air and mainly clear skies this will
provide a great setup for radiational cooling. Min temps will
range from the upper teens and lower 20s in sheltered valleys
and rural areas west of the Blue Ridge Mountains...to the mid
and upper 20s across most other locations to the lower and
middle 30s in downtown Washington and Baltimore.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
High pressure will remain overhead Thanksgiving Day and
Thanksgiving night...bringing dry and chilly conditions along
with mainly clear skies. High pressure will shift to the east
coast Friday and move offshore Friday night. A southerly flow
will return during this time. Max temps will be a bit warmer
Friday...but still near or even slightly below climo. More dry
conditions are expected during this time. Friday night will be
seasonably chilly...but not as cold as Thanksgiving night.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Long term period starts out Saturday with a ridge west/trough
east pattern, but by the end it completely flips, with trough
west/ridge east by Tuesday. At the surface, this will translate
into a cold frontal passage on Saturday, followed by high
pressure progressing slowly east across the region Sunday
through Tuesday. In many ways, surface features Saturday will
mirror what is happening tonight. As with tonight, one thing to
watch Saturday is a coastal low pressure which will also be
moving northeast along the Carolina coast. Should this system
end up a bit further northwest, it could spread some rain into
the region, but unlike tonight, most guidance keeps this system
well to the southeast, completely separated from the
approaching northern stream shortwave and surface cold front.
This means that right now, we think the cold frontal passage
should be dry for most of the CWA. We will likely moderate
thanks to southwesterly flow just ahead of the cold front,
resulting in Saturday being rather like yesterday was... breezy
with some clouds and temps well into the 50s, perhaps even low
60s. Along and behind the front, some showers will affect the
northwestern and western portions of the CWA, primarily the
higher terrain, and much like today, these will change to snow
showers behind the front, which in this case will occur on
Saturday night. However, the system is fast moving, so do not
expect significant accumulations even in the favored upslope
regions. Chilly and breezy but mostly dry Sunday with high
pressure starting to build in...any lingering upslope snow
showers will be winding down. Most areas will not get out of
the 40s. On Monday, high pressure crosses the region, but temps
remain chilly with the trough just starting to exit aloft, so
only slight warming. By Tuesday, however, southwest flow will
start again in earnest as the high slides off the coast, and
moderation will become more apparent, with highs returning to
the 50s most likely.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
A period of rain is possible across the eastern
terminals...KDCA...KBWI and KMTN through about 12z. Brief MVFR
conditions are possible during this time...but a majority of the
time should be VFR.

Gusty northwest winds will develop behind a cold front between
13z and 15z. The gusty winds will continue through the afternoon
with frequent gusts around 20 to 25 knots with a few gusts up
to 30 knots. The winds will diminish around sunset. High
pressure will build over the terminals tonight through Thursday
night...bringing VFR conditions and lighter winds. Winds will
turn to the south later Friday and Friday night as high pressure
shifts offshore.

Saturday and Sunday look mainly VFR for now, with a gusty wind
(perhaps up to 30 knots at times) starting southwest, then
switching to northwest behind a cold front later Saturday. These
winds likely persist Sunday. A coastal low will be sliding
northeast off the coast on Saturday, but right now effects look
minimal to non-existent. If it ends up closer, a period of sub-
VFR would be possible on Saturday due to rain, but odds of this
look low right now.

&&

.MARINE...
A cold front will pass through the waters early this morning.
High pressure will approach from the north and west today before
building overhead tonight through Thanksgiving night. Gusty
northwest winds are expected behind the cold front today. Gusts
of 20 to 30 knots are most likely. An SCA is in effect for the
waters. The winds will gradually diminish tonight...but an SCA
is still in effect for most of the Bay and lower Tidal Potomac
River. Winds will continue to diminish for Thanksgiving as the
high settles overhead.

High pressure will shift offshore Friday through Friday night
and a southerly flow will return...but winds should remain below
SCA criteria for most of the time since the gradient will be
weak.

Cold front crossing the area Saturday will bring southwest
winds which then switch northwest by Saturday night, lingering
Sunday. These are likely to reach SCA criteria and could be near
gale, but confidence is not high on this.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 AM this morning to 6 AM EST
     Thursday for ANZ531>534-537-539>541-543.
     Small Craft Advisory from 6 AM this morning to 6 PM EST this
     evening for ANZ530-535-536-538-542.

&&

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



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