Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 090233

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
933 PM EST Fri Dec 8 2017

Low pressure system will impact the area overnight through
Saturday. High pressure will return Sunday into Monday. Another
low pressure will impact the area on Tuesday. Weak high pressure
builds over the area for the second half of next week.


Previous forecast looks pretty much on track. Have adjusted
snowfall totals up to the highest advisory threshold 3-5" for
the Baltimore and Washington Metro areas. Have expanded the
Winter Storm Warning to include Spotsylvania and Stafford
Counties. Prince Georges, Anne Arundel were on the cusp of
getting upgraded to a warning too, but will wait for 00Z models
for further evaluation.

12Z ECMWF has 850-700 thickness of 1555 bisecting St. Mary`s
County at 06Z-12Z. This should delineate the rain/sleet/snow
line. Looking for a local max snow accumulation band to extend
through Calvert/Charles/King George Counties with lesser amounts
to the southeast. To the northwest to the city centers, lesser
QPF forecast but colder boundary layer will lead to near warning
level snowfalls. Expecting a tight gradient to the northwest of
the city centers, and currently putting that gradient through
central Loudoun/northern Montgomery/central Frederick counties.

Snowfall should get going in earnest after dawn and last through
early afternoon, as two upper jet structures merge over the
mid-Atlantic. We will be in the right rear quad of upper level
jet centered over New England and the left front quad of upper
level jet approaching from southeast US. They should merge by
12Z Saturday overhead, causing enhanced upward motion. The
boundary layer will become much colder after noon Saturday, with
another inch or two of fluffier snowfall before tapering to
flurries at sunset.


Low pres moves farther offshore with precip ending but upper
level trof crossing the area will result in snow showers
over the mtns with a couple of inches of accumulation likely.
Turning colder and windy late Sat night and Sun.


A highly amplified upper level flow pattern will continue to
dominate the long term period with west coast ridging and east coast
troughing. We`ll be in between shortwave troughs on Monday, so dry
but chilly weather is expected. A surface low will be moving through
the Great Lakes Monday night. Model consensus has the warm advection
arm of this system dry except along the Appalachian crest, but if
this changes, there could be mixed precipitation potential. The
associated cold front and upper trough axis will swing through the
area Tuesday and Tuesday night. Winds could be quite gusty behind
the front. Upslope snows look likely, while there could be some snow
showers or even squalls east of the mountains. Surface temperatures
will likely remain above freezing (40s for early highs) until very
late in the day though.

The core of the cold air will be over the area Wednesday into
Wednesday night. It`s possible some areas could fail to reach
freezing for highs on Wednesday, then fall into the teens Wednesday
night. There could be additional opportunities for flurries during
this time. High pressure will nudge into the area from the south on
Thursday, but the next shortwave trough will approach on Friday,
likely driving another surface low through the Great Lakes. There`s
some additional spread in the guidance by this point, although there
will be a chance of wintry precipitation across parts of the area.


Developing IFR/LIFR conditions in light/moderate snow late
tonight into Sat. Precip ends by 21Z Sat with improving
conditions Sat evening. Winds strengthen from the NW Sat night
with gusts up to 30 kt possible into Sun. Winds diminish Sun

An interval of sub-VFR clouds is possible Monday night. Brief
visibility impacts in scattered snow showers will be possible
Tuesday into Tuesday night. Wind gusts over 30 kt possible Tuesday
through Wednesday.


Light and variable winds increasing late tonight across the
southern waters. SCA issued earlier. Winds strengthen across all
waters Sun with solid SCA conditions everywhere. Chance of
gales appears relatively low attm.

Light southerly flow is expected Monday with weak high pressure.
Small Craft Advisory conditions could begin Monday night, but become
more likely with winds out of the west-northwest Tuesday into
Wednesday after a cold front sweeps across the area. A period of
gale force winds will also be possible.


DC...Winter Weather Advisory until 4 PM EST Saturday for DCZ001.
MD...Winter Weather Advisory until 4 PM EST Saturday for MDZ006-011-
     Winter Weather Advisory from 4 AM to 4 PM EST Saturday for
     Winter Storm Warning until 4 PM EST Saturday for MDZ016>018.
VA...Winter Weather Advisory until 4 PM EST Saturday for VAZ025-026-
     Winter Weather Advisory from 4 AM to 4 PM EST Saturday for
     Winter Storm Warning until 4 PM EST Saturday for VAZ057.
     Winter Storm Warning until 4 PM EST Saturday for VAZ055-056.
MARINE...Small Craft Advisory from midnight Saturday night to 6 AM EST
     Sunday for ANZ530>533-538>542.
     Small Craft Advisory from 5 AM to 4 PM EST Saturday for ANZ533-
     Small Craft Advisory until 6 AM EST Sunday for ANZ534-537-543.


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