Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 120223

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

Weak high pressure will slide offshore tonight. An arctic cold
front will pass through the area Tuesday. Weak high pressure
will build overhead Wednesday. An Alberta Clipper will pass
through early Thursday. Developing low pressure will move up the
east coast late Friday.


After a period of mid deck clouds, skies have cleared earlier
this evening. In spite of that, a light south breeze has kept
temperatures from falling far. Additional warm advection clouds
beginning to overtake the area. Anticipate that the overnight
hours will be cloudy. So, do not believe that temperatures
outside of the Appalachians will be able to drop below freezing.
Have modified hourly temps based on a multi-model consensus.

That minimized thermal profile issues previously discussed. But,
am hard pressed to find a guidance solution that suggests that
precip poses a credible threat during the overnight hours. As a
result, have backed off PoP forecast considerably. The
exception will be the favored upslope areas,as snow showers
will arrive there predawn.

On Tuesday, the warm front lifts northward east of the
mountains and we could get pretty mild ahead of the cold front.
Raised our highs a bit and they may need to come up a bit more
given how mild it got today. During the afternoon the strong
cold front will blast across the area, with temps likely falling
during the afternoon. With strong forcing and a little
instability, would not rule out some showers with or just behind
the front itself, so maintained some slight chance pops, with
any rain showers transitioning back to snow. Winds will blow at
least up to 30 mph with the front, and its not impossible we
touch wind advisory criteria, though at this time its not our
official forecast. Will let next shifts re- evaluate this

Along the Allegheny Front itself, the story will be quite
different. Upslope flow will allow snow showers to develop late
tonight and continue through the day on Tuesday. Accumulations
during the day look like a general 3-6 inches, so have issued a
winter weather advisory for the most likely locations (western
Pendleton and western Grant). This may be a bit conservative as
accumulations could touch advisory levels in western Mineral,
western Allegany and western Highland. Its also possible that
given the strong upslope flow on Tuesday, we could reach warning
criteria in western Pendleton and western Grant, particularly
our sweet spot in Bayard. However, the latest guidance suggests
that our forecast may be on the high end of solutions. Based on
the pattern, do not mind being high; but also do not have
confidence to raise anyone to a warning. However, did add
extreme western Allegany and Mineral to the Winter Weather

Final issue which occurs mainly Tuesday night but starts late
Tuesday afternoon is the wind chill. Most of Tuesday will stay
above criteria all zones, but by late Tuesday afternoon, the
strong gusty winds plus falling temps well into the teens will
bring wind chills down into the single digits along portions of
the Allegheny Front, so started the advisory for wind chills
late in the afternoon in these areas.


The cold is the main story for the short term period as a brutal
air mass moves down from Canada. Winds will gust all night long
and into Wednesday morning as high pressure briefly builds into
the region. Probably stay shy of wind advisory most areas, but
it may be close. Either way, with the strong cold advection,
wind chills will fall into the single digits across the board,
and across most of the far western counties it will reach wind
chill criteria, so expanded wind chill advisory across western
zones. May need to add Blue Ridge zones as well.

Other story is the continued upslope snow along the Allegheny
Front, which will continue through the night into early
Wednesday morning before tapering off as high pressure builds.
As mentioned above, we may approach warning criteria in our
advisory zones, and advisory criteria in some of the zones
adjacent to the current advisory, so future shifts may further
expand/upgrade the existing headlines.

As weak ridge of high pressure builds across Wednesday, winds
will slack off and upslope snow will end by afternoon, but cold
air mass will keep temps mostly below freezing through the day -
coldest day of the year.

Then a clipper system starts moving in Thursday night. Still
some uncertainty with this system as it crosses the area, but
with the low expected to track south of at least part if not
most of the CWA, the odds of some snow is certainly significant.
Accumulations however will likely not be much. Best odds of snow
however are northern areas near PA, locations where the low is
most likely to stay south of. Lows will be in the 20s Wednesday


Not as chilly Thursday with a chance for snow showers, mainly
in the west and north, as the next low pressure system arrives
from the west. The chance for snow showers linger Thursday night
and Friday, as well, due to the upper level cold air support
behind the surface storm.

A modifying area of high pressure is expected to build eastward
behind the low pressure system Friday night through Saturday
night, bringing milder temperatures and temperate southwesterly

By Sunday, the next threat for rain or snow showers will come
with a cold front sagging southward across the region. High
temperatures, next weekend, could reach the 50s. A low pressure
system could attach to the cold front and bring added lift of
support for rain or snow showers Sunday and Monday.


VFR expected overall through Thursday night. But, an Arctic
Front may bring a brief opportunity for snow showers/squalls
late Tuesday into Tuesday night, which could reduce cigs and
vis below VFR. Additionally, a clipper system may bring light
snow to the region late Wednesday night into Thursday, which
would do likewise. Otherwise, main story is the northwest wind
later Tuesday into early Wednesday, with 30+ knot gusts likely
at times. A few 40 knot gusts can`t be ruled out either.

VFR conditions Thursday night through Friday night. Winds
southwest at 5 to 10 knots Thursday night, becoming northwest
around 10 knots Friday and Friday night.


South flow ahead of approaching cold front will increase
tonight, with SCA slowly spreading across the waters as we head
into Tuesday. Then strong cold front is likely to bring Gales
late Tuesday into early Wednesday, and a Gale Warning is in
effect for this period. Have extended the Gale Warning for the
main channel of the Chesapeake Bay, lower Tidal Potomac
River, and eastern tribs into Wednesday before winds diminish.
Otherwise, Small Craft Advisories will be required that period.
SCA may also be needed through Thursday as another, much weaker
low moves across the region.

No marine hazards Thursday night through Friday night. Winds
southwest around 10 knots Thursday night, becoming northwest
around 10 knots Friday and friday night.


Elevated water levels from earlier have dropped 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

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


MD...Winter Weather Advisory from 7 AM to 7 PM EST Tuesday for
     Wind Chill Advisory from midnight Tuesday night to 6 AM EST
     Wednesday for MDZ501.
VA...Wind Chill Advisory from midnight Tuesday night to 6 AM EST
     Wednesday for VAZ504.
     Wind Chill Advisory from 6 PM Tuesday to 6 AM EST Wednesday
     for VAZ503.
WV...Winter Weather Advisory from 7 AM to 7 PM EST Tuesday for
     Wind Chill Advisory from midnight Tuesday night to 6 AM EST
     Wednesday for WVZ503-506.
     Winter Weather Advisory from 7 AM Tuesday to 7 AM EST
     Wednesday for WVZ501-505.
     Wind Chill Advisory from 4 PM Tuesday to 6 AM EST Wednesday
     for WVZ501-505.
MARINE...Small Craft Advisory until 4 PM EST Tuesday for ANZ531>534-537-
     Gale Warning from 4 PM Tuesday to 6 PM EST Wednesday for
     Small Craft Advisory from noon to 4 PM EST Tuesday for ANZ535-
     Gale Warning from 4 PM Tuesday to 6 AM EST Wednesday for
     Small Craft Advisory from 6 AM to 4 PM EST Tuesday for ANZ530.


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