Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 120828

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
328 AM EST Tue Dec 12 2017

An arctic cold front will pass through the area today. Weak
high pressure will build overhead Wednesday. An Alberta Clipper
will pass through Wednesday night through early Thursday.
Developing low pressure will move up the east coast late Friday
and high pressure will return for Saturday. Low pressure may
impact the area early next week.


Arctic front and potent upper level shortwave are poised to cross
the region during the day today and into tonight, and will bring
a drastic change in sensible weather.

As of 3 AM, surface low pressure is moving northeastward through
the eastern Great Lakes with the trailing surface front moving
towards the Appalachians. Out ahead of the front, widespread
cloud cover and increasing southerly flow is being observed.
Echoes are increasing as well on radar ahead of the front, and
some light rain showers are possible prior to frontal passage as
it moves across the region. A few sleet pellets are also
possible early this morning. The front will push across the
region during the morning and early afternoon hours. High
temperatures will be reached prior to passage, rising into the
40s to low 50s east of the Blue Ridge, with 30s to around 40F
west where passage occurs earlier.

Behind the front, temperatures will fall quickly and winds will
turn to the northwest and increase. Along and near the
Allegheny Front, this will lead to upslope snow showers/squalls
and a Winter Weather Advisory remains in effect for amounts in
the 3-8" range through tonight. Downwind of the Allegheny Front,
with increased northwest flow and potent shortwave moving
across the region, isolated to scattered snow showers and
possibly even a few squalls are possible this evening. This is
highlighted by the NAM12 Snow Squall Parameter as well as the
surge in the Froude Number, indicative of favorable conditions
for convective snow showers to make it downwind of the highest

Wind gusts this evening and tonight are expected to frequently
reach into the 30-40 mph range. Its possible some of the
higher terrain may approach 45-50 mph, and a Wind Advisory may
be needed later tonight.

Overnight, temperatures will plummet into the teens to around
20F for most locations with the higher terrain dropping into
the single digits. This combined with the wind will lead to wind
chill values dropping into the single digits for the majority
of the region, except as low as the teens below zero across the
higher terrain. Therefore a Wind Chill Advisory is in effect.


A small axis of surface ridging will move overhead Wednesday.
This will lead to a gradual relaxation of the winds through
the day and any remaining upslope snow showers will come to an
end. Temperatures will still be quite cold with highs possibly
not breaking the freezing mark area-wide. Wind Chill Advisories
run through Noon. Skies should become partly-mostly sunny, but
this will be short- lived as the next system will already be on
the doorstep by late in the day.


An Alberta clipper system will pass through the Ohio Valley
Wednesday evening before passing through our area overnight
Wednesday into Thursday morning. This system will bring a period
of snow with it...but there is still some uncertainty as to
exactly where with the gradient between accumulating snow vs.
little or no snow setting up over our CWA. Typically with these
systems...most of the snow falls north of the track of the
surface low where the frontogenetical forcing and warm advection
line up best. Latest guidance has the low tracking just south
of the Mason-Dixon Line. This means that the best chance for
snow will be near the Mason-Dixon line into Pennsylvania...and
this has been reflected in the latest forecast. However...a
slight shift southward and the accumulating snow will make it
into the Metro areas and possibly points south. Therefore...this
will have to be monitored closely over the next couple days.
One other thing to note is that an upslope flow and cold
advection behind the clipper system will cause snow showers for
locations along/west of the Allegheny Front and accumulating
snow is likely across these areas as well.

The clipper system will pull away from the area later Thursday
and high pressure will briefly build into the area Thursday
night...bringing dry and chilly conditions.

The high will move offshore Friday and an upper-level trough
will swing through the area later Friday into Friday night.
Latest guidance keeps the northern stream energy and southern
stream energy separate. This means that a coastal low with the
southern stream energy would move out to sea. However...some
southern stream moisture may still get drawn into the area ahead
of the trough axis...and this would result in a period of snow
or rain/snow mix. Will continue to monitor...because it may
affect the evening rush across most areas. Also...should these
systems phase a bit sooner than expected that would have a
significant impact on the forecast with more significant snow.

High pressure will build overhead Saturday before moving
offshore Sunday. Low pressure may impact the area later Sunday
into Monday...bringing a chance for precipitation.


Primarily VFR is expected for the region through Wednesday. A
strong cold front will cross the region today. Strong low
level southwesterly flow is overhead early this morning and this
is leading to some low level wind shear with FL020 winds about
40 knots. After frontal passage, winds will shift to the
northwest with gusts up to 30-35 knots developing by late in the
day and tonight. Isolated to scattered snow showers and their
associated restrictions are possible as well, especially MRB,
but they will be brief. Winds will gradually relax on Wednesday.

A clipper system may bring a period of snow Wednesday night into
early Thursday. The best chance for snow will be across the
northern terminals...but confidence still remains low with a
tight gradient between snow vs. no snow overhead. IFR conditions
are possible in any snow that develops.

High pressure will return for later Thursday through Thursday
night before moving offshore Friday morning. VFR conditions are
expected during this time. Low pressure will move out to sea
later Friday...but some moisture may get drawn into our area as
an upper-level disturbance moves through. This means that a
period of snow or rain/snow mix is possible later Friday into
Friday night. High pressure will return for Saturday.


SCA is in effect through 4 PM today with gusty southerly flow
ahead of a cold front. Winds will shift markedly to the
northwest following frontal passage this afternoon, and increase
to Gale Force by this evening. Therefore a Gale Warning is in
effect from 4 PM through 6 AM Wednesday for all waters. Winds
will gradually lessen on Wednesday, but Gale/SCA winds will be
around for at least a portion of the day.

Low pressure will pass through the waters later Wednesday night
into Thursday. A Small Craft Advisory will likely be needed for
the waters during this time. High pressure will build over the
waters Thursday night...but another area of low pressure may
impact the waters Friday before high pressure returns Saturday.
A Small Craft Advisory may be needed for the waters behind the
departing low later Friday night through Saturday.


Anomalies have spiked up this morning to around three quarters
to one foot above normal due to a southerly flow. However...the
flow should gradually turn west of south later this morning
through midday. Therefore...minor flooding is not expected at
this time.

A strong northwest flow will develop behind a cold front later
this afternoon through Wednesday. Tidal blowout conditions are
possible tonight through Wednesday.


MD...Wind Chill Advisory from 6 PM this evening to noon EST
     Wednesday for MDZ501.
     Winter Weather Advisory from 7 AM this morning to 7 PM EST
     this evening for MDZ501.
VA...Wind Chill Advisory from 6 PM this evening to noon EST
     Wednesday for VAZ503-504-507-508.
     Winter Weather Advisory from 7 AM this morning to 7 PM EST
     this evening for VAZ503.
WV...Wind Chill Advisory from 6 PM this evening to noon EST
     Wednesday for WVZ503-506.
     Winter Weather Advisory from 7 AM this morning to 7 PM EST
     this evening for WVZ503.
     Winter Weather Advisory from 7 AM this morning to 7 AM EST
     Wednesday for WVZ501-505.
     Wind Chill Advisory from 4 PM this afternoon to noon EST
     Wednesday for WVZ501-505.
MARINE...Small Craft Advisory until 4 PM EST this afternoon for
     Gale Warning from 4 PM this afternoon to 6 PM EST Wednesday
     for ANZ530>534-537-539>541-543.
     Small Craft Advisory from 6 AM to 6 PM EST Wednesday for
     Gale Warning from 4 PM this afternoon to 6 AM EST Wednesday
     for ANZ535-536-538-542.
     Small Craft Advisory from noon today to 4 PM EST this
     afternoon for ANZ535-536-538.
     Small Craft Advisory until 4 PM EST this afternoon for ANZ530.


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