Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 050238

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
938 PM EST Sun Dec 4 2016

An upper-level disturbance will pass through the region tonight.
High pressure will briefly return for Monday. Coastal low pressure
will impact the area Tuesday before high pressure briefly returns
again for Wednesday. A potent cold front will move through late in
the week.


Forecast is largely on track this evening outside of minor
details. 00Z IAD sounding shows a shallow sub-freezing layer
between approximately 2400 and 3600 ft. This layer can be seen
with some of the high elevation mesonets (i.e. Wintergreen) below
freezing, with some reports of freezing rain glaze at those
elevations as well. Winter Weather Advisory covers these areas. As
the column saturates, temperatures should gradually rise through
the night, at least on the ridges. Doubt much if any snow is
falling. However, some ice pellets are being reported all across
the region on the leading edge of precipitation due to the
evaporative cooling aloft. There shouldn`t be enough sleet to
cause any issues.

Low pressure centers to our north and south as well as a shortwave
trough will continue to progress east tonight. Rain will continue
at times. Cooling will be limited as dew points/wet bulb temps rise,
so p-type issues should dwindle with time. Back edge of precip
will progress east quickly late tonight, and most areas may be dry
by daybreak except communities near the Chesapeake Bay.

The system quickly heads east Monday morning and a weak ridge
returns. This should allow clouds to break for some sun by
afternoon. Highs will return to the upper 40s and lower 50s.


Closed low over northern Mexico will eject eastward into Texas on
Monday, then open up and head northeastward towards us Monday
night and Tuesday. In response, surface low pressure will develop
and head northeastward along the Atlantic seaboard. A primary low
will lift north west of the Appalachians while a secondary will
redevelop in the Carolinas. In between, a wedge of colder air is
likely to hold firm east of the Allegheny Front. In short...this
spells trouble...though the extent of it remains uncertain.

Skies may still be mostly clear early Monday night, allowing for
some radiational cooling. However, clouds are likely to build in
by midnight and put a stop to that, perhaps even allowing temps to
rebound a bit. That said, the further inland one is, the less of a
bump one might expect. Warm advection precip will start moving
northeast across the region late Monday night with precip likely
to arrive during rush hour in the metro, perhaps near the end of
the rush up towards Baltimore. Dry air again will be in place so
even though much of the I-95 corridor may still be above freezing,
some snow and sleet may initially make their way to the surface.
No impacts are currently anticipated in this corridor, however, as
the moistening of the already warm atmosphere should cause any
sleet and snow to go over to rain with little accumulations.

Further inland, especially across the Shenandoah Valley, cold air
may be trapped for the duration. These areas may get significant
icing if the colder guidance is to be believed. After also
possibly starting as snow/sleet, freezing rain is likely to be the
predominate precip type most of the morning, perhaps changing to
rain as temps only very slowly warm up in the afternoon. Given its
usual good handling of cold air damming, the NAM temps were
favored. This suggests that some areas could stay freezing rain
nearly all day on Tuesday. However, since temps will be marginal,
and most of precip will fall during daylight, we are skeptical about
a warning level ice event, so for now still will advertise
potential advisory. Highs will be stuck in the 30s in the
SHenandoah Valley, while struggling to reach 40 further east. As
the system weakens and lifts north, expect rain to taper off
Tuesday night, though if cold air damming is as persistent as some
guidance suggests, there could still be a little freezing rain
ongoing in the Shenandoah Valley. Temps will likely move little
Tuesday night.


High pressure will build in on the west side of the coastal low
Wednesday and Wednesday night. Drier air will filter in from the
northwest and temperatures will remain seasonably chilly. A few
isolated rain showers cannot be ruled out near the Chesapeake Bay,
while some snow showers could evolve over the Potomac Highlands.

The next storm system will approach from the southwest Thursday
and Thursday night. A chance of rain showers exist across the
region each period. We can`t rule out some wintry precipitation as
well, depending on the track of the storm system and whether or
not there is enough cold air at the ground surface to support it.
Uncertainty remains moderate to high.

A chilly dome of high pressure is expected to build in from the
west Friday through Saturday night. This high could bring the
chilliest air so far this season. Some upslope snow showers could
also occur in the Potomac Highlands.

The high pressure will move offshore the East Coast Sunday ahead
of the next approaching trough of low pressure Sunday night into


Conditions will continue to deteriorate tonight as a weak storm
system crosses the area. Lower cigs to the south will spread
north, reaching IFR after midnight and likely staying in that
category through morning. Periods of rain will continue until late
tonight. Sleet should only be an issue with this initial onset,
with temperatures expected to remain above freezing at the
terminals. The rain may be clear of the terminals by the morning

Conditions improve on Monday with VFR returning later in the
morning. Going back down late Monday night and Tuesday with a
slightly more wintry system, but still more plain rain than not
along I-95 - however, significant icing is possible at MRB. Icing
or not, IFR cigs and vis likely Tuesday and Tuesday night.

MVFR conditions possible in any rain showers near MTN or BWI early
Wednesday. VFR conditions elsewhere and rest of time. Winds
northwest 5 to 10 knots Wednesday becoming light and variable
Wednesday night. Gusty NW winds expected behind strong cold front
later Thursday into Friday.


Winds are below SCA and should remain below through Monday night.
Southerly winds are picking up to 15 kt tonight, and northwest
winds may do the same Monday afternoon. Will have to keep an eye
out for a potential marginal SCA.

Coastal low pressure will develop to the south Monday night and it
will pass by to the east later Tuesday into Tuesday night. As it
passes, SCA winds may redevelop on the bay as the system passes.
High pressure will briefly build toward the waters Wednesday, with
a potent cold front expected to pass through Thursday. A Gale
Warning may be needed for the waters Thursday night and Friday
behind the cold front.


MD...Winter Weather Advisory until 6 AM EST Monday for MDZ501.
VA...Winter Weather Advisory until 6 AM EST Monday for VAZ503-504-
WV...Winter Weather Advisory until 6 AM EST Monday for WVZ501-503-


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