Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 110843

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
343 AM EST Mon Dec 11 2017

Weak high pressure will slide offshore through tonight. An
arctic cold front will pass through the area Tuesday and weak
high pressure will build overhead Wednesday. Weak low pressure
will pass through Thursday and another one will pass by to the
east Friday. High pressure will return for Saturday.


Upper level shortwave is passing across the region early this
morning with some clouds and perhaps a few scattered flurries.
Behind it, we`re seeing a shift to westerly winds and some terrain-
driven cumulus. A ridge of surface high pressure will build in
over the region by this afternoon, which will lead to dry
conditions and relatively light winds. Aside from the departing
clouds associated with the shortwave and the cumulus over the
terrain this morning, mostly sunny skies are expected through
the first half of the day, before high clouds increase during
the afternoon hours, out ahead of the next fast-approaching
shortwave trough.

Temperatures will nudge back up towards climatological normals,
with highs in the 40s.


A clipper low will dive through the Great Lakes region later
today, before tracking up the Saint Lawrence Valley tonight
and Tuesday. Warm air advection will overspread the area
tonight, followed by a cold frontal passage from west to east
beginning along the Allegheny Front early Tuesday morning, and
pushing eastward towards the Chesapeake Bay by Tuesday
afternoon. A potent upper level shortwave trough will also move
eastward into the region by Tuesday afternoon.

While much of the forcing and therefore precipitation is
expected to remain north of the region tonight into early
Tuesday morning, a few areas of light precip are possible late
tonight and early Tuesday morning, mainly north towards the PA
border. With marginally cold temperature profiles, there is the
potential for some very light mixed precip, but coverage and
intensity will be limited. Precip chances then increase along
the Allegheny Front along and behind the cold front as northwest
flow begins. Lows tonight in the 30s.

On Tuesday, there may be a few light showers along and ahead of
the front, but as temperature profiles crash behind the front
and forcing increases as the strong upper level trough moves
towards the region, chances for convective snow (and possibly
rain to start) showers increase. Along and near the Allegheny
Front, with strong northwest flow and favorable Froude numbers
and moisture profiles, upslope snow showers and squalls are
expected. Some of these may spill east of the Allegheny Front
with the deep mixing, strong northwesterly flow, and strength of
the trough. Highs Tuesday will be met prior to frontal passage,
generally in the 30s/40s, but near 50F across parts of central
VA and southern MD.

Upslope snow showers will continue into Tuesday night with
continued northwest flow. Will currently show total snowfall
accumulations in the 3-7" range along the Allegheny Front, and
Winter Weather Advisories will likely be needed. In addition, a
very cold air mass is expected to move overhead, with 850mb
temps from -15C to -18C by Wednesday morning. This will allow
for lows to bottom out in the teens to around 20F. That coupled
with a strong wind field will likely lead to wind chills in the
single digits for most locations and near -10F for the higher

A brief window of calm weather will move overhead by Wednesday
afternoon, but this will be short-lived, as another weak clipper
system will approach the region Wednesday night, with additional
chances for some light snow by Thursday morning. Highs Wednesday
may not break freezing area-wide, with lows Wednesday night


A weak clipper low will pass through the area Thursday. Guidance
has diverged on the track with solutions ranging from this
system tracking well to our north vs. to our south. The ECMWF
has been consistent on keeping the low farther south with other
guidance having it pass farther to the north. However...the 00Z
GFS has trended a bit farther south with the track of the low.

Should the low track to the north...dry conditions would most
likely occur but should the low track to our south...then a
period of light snow could affect the morning rush Thursday.
Temperatures would be cold enough for light snow to stick as
well. This will have to be closely monitored over the next
couple days. Will carry a chance for snow during this time.

High pressure will briefly build into the area late Thursday
into Thursday night...but another northern stream system will
dig through the Great Lakes Thursday night and then through our
area Friday. A southern stream system will also pass through the
southeastern CONUS and then offshore during this time. It is
unlikely that these systems will phase in time for strong
coastal low pressure to develop...but some of the southern
stream moisture could get drawn into our area bringing the
chance for rain or snow.

High pressure will briefly build into the area for
Saturday...bringing dry and seasonably chilly conditions. The
high will move offshore Saturday night and cold front will
approach Sunday. A southwest flow in between these systems will
allow for milder conditions Sunday...but showers cannot be ruled
out ahead of the cold front.


VFR is expected area-wide today and tonight with some high
clouds and relatively light winds. A patchy area of light
precipitation is possible towards Tuesday morning, but
coverage/impact low. A cold front will then cross the region on
Tuesday, which will lead to an increase in winds by Tuesday
afternoon. Winds may gust near 30 knots out of the northwest by
Tuesday afternoon. A few rain/snow showers are also possible,
especially at MRB. Gusty northwest winds will continue Tuesday
night into Wednesday morning, but conditions expected to be VFR.
The next chance of sub-VFR conditions will then come late
Wednesday night into Thursday morning with potential for light

A clipper low will pass through the area Thursday. A period of
light snow is possible Thursday morning and light accumulations
are possible. Confidence in this occurring is low at this time.
SubVFR conditions are expected in any snow that does develop.

Another area of low pressure will pass by to the southeast
Friday. There may be enough moisture for a period of rain or


SCA in effect until 7 AM this morning, with winds diminishing
for the rest of the morning and afternoon. Tonight, winds will
increase out of the south ahead of an approaching frontal
system, and SCA is in effect for portions of the waters. SCA
then goes into effect for all waters on Tuesday for southerly
winds ahead of a cold front Tuesday morning, as well as
northwest winds behind the front. Gale conditions are then
possible late Tuesday through Wednesday morning. SCA conditions
may continue into Wednesday night.

A clipper low will pass through the area Thursday and another
low will pass by to our southeast Friday. Winds may approach SCA
levels during this time.


Elevated water levels are occurring this morning due to a
southwest flow. Anomalies should drop some today as the flow
goes west to northwest...but anomalies will likely increase as a
southerly flow picks up for tonight into Tuesday. At this does not appear that there will be minor flooding but
it will have to be monitored closely for sensitive areas.

Strong northwest winds are expected later Tuesday and Tuesday
night. Tidal blowout conditions are possible Tuesday night into


MARINE...Small Craft Advisory from midnight tonight to 6 PM EST Tuesday
     for ANZ531-532-538>543.
     Small Craft Advisory until 7 AM EST this morning for
     Small Craft Advisory from 7 AM to 6 PM EST Tuesday for ANZ530-
     Small Craft Advisory from 9 PM this evening to 6 PM EST
     Tuesday for ANZ533-534-537.


TIDES/COASTAL FLOODING...BJL is the U.S. government's official web portal to all federal, state and local government web resources and services.