Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 110213

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
913 PM EST Sun Dec 10 2017

Weak high pressure will slide by south of the area through
Monday. A low pressure system will cross the lower Great Lakes
on Tuesday, bringing a cold front through the region. High
pressure will build overhead Wednesday with another low
pressure area approaching from the west on Thursday.


A weak shortwave will pass just to our north tonight bringing
an increase in clouds, but not much precipitation with it. The
latest guidance even leaves the favored upslope areas dry, but
the pattern would suggest that at least some flurries likely.
Have lowered PoPs further in the chance/slight chance range,
and eliminated any mention of snow accumulations.

In term of temperatures, the advancing cloud shield, associated
with this wave, will reduce the extent of radiational cooling.
Thus, not much of a city-suburb spread. However, would expect
some advective cooling by dawn. As a result, guidance
temperatures not looking too bad. We`ll be below freezing
areawide (in many cases, we`re almost there). Favored a multi-
model mesoscale ensemble for low temps.


The shortwave will exit to our east Monday morning, with
clearing skies in its wake. Weak high pressure will move in from
the west, keeping winds light, and bringing mostly sunny skies.
This ridge will quickly move through our area, turning winds out
of the south by Monday afternoon. This will allow temperatures
to moderate a bit, topping out in the low to mid 40s. High
clouds will start to stream overhead during the afternoon hours
ahead of the next approaching shortwave over the lower Great

A stronger shortwave (compared to tonight) will near our area
late Monday night in to Tuesday morning. The associated surface
low pressure will pass to our north, dragging a cold front
through the area during the day on Tuesday. Dynamics appear
somewhat favorable with strong mid-level vorticity advection
and an upper jet nearby. Add to that low-level isentropic
upglide looks possible. Model guidance continues to keep the
bulk of the moisture north of the Mason Dixon line, with
upslope snow looking very likely Tuesday morning.

Temperature profiles hint at the possibility of light mixed
wintry precipitation along the northern portions of our CWA,
particularly near the MD/PA border. That being said, most of
the moisture and lift will remain to the north, so do not expect
much in the way of accumulation. The best chance for
accumulating snowfall will be along and west of the Allegheny
Front through early Tuesday afternoon, where advisory criteria
snow looks possible.

On the heels of this shortwave is another stronger mid-level
trough which will help push a cold front through the region,
bringing strong cold air advection in its wake and strong/gusty
northwesterly winds. Wind direction will be favorable for
continued upslope snow and snow squall potential. Temperatures
at the surface and aloft do not seem too supportive of snow
squalls making much impact east of the mountains. The
combination of strong gusty winds and the cold air filtering in,
wind chills may drop below zero over the western ridges of
Allegheny Front. Tuesday night, temperatures will fall in to the
teens across our western zones, lower 20s elsewhere, and the middle
20s in the metro areas.


A highly amplified upper level flow pattern with east coast
troughing will result in a wintry pattern through Friday. Wednesday
will be blustery and cold, with strong winds possible as low
pressure pulls away from Maine. High temperatures may fail to crack
the freezing mark across a large part of the area. With a slightly
faster exit to the upper trough, flurries are looking less likely on
Wednesday. Weak surface ridging will relax the winds by Wednesday
night, but lows will be in the teens and 20s.

There`s some model spread in timing on the next clipper system which
will arrive sometime between late Wednesday night and Thursday
evening. While the best chance for snow will be along the upslope
areas, it`s not out of the question some light snow could spread
east of the mountains as well, particularly near the Pennsylvania
border. The next, sharper shortwave trough will arrive on Friday,
prolonging chances for upslope snow. Instability through the snow
growth zone could allow snow showers to move east of the mountains
as well, although surface temperatures will likely rise above
freezing at lower elevations.

The upper level pattern will break down to more progressive flow
next weekend. Surface high pressure will arrive on Saturday and
temperatures will rise closer to normal. There`s still a large
amount of uncertainty regarding the next frontal system which may
arrive late in the weekend.


VFR conditions are expected at all the terminals through Monday
as weak high pressure traverses the area. Clouds will be mostly
mid deck, with sct-bkn stratocumulus around 040-050 by dawn.
These clouds will dissipate after sunrise.

A clipper system will pass to the north Monday night in to
Tuesday, bringing the possibility of brief flight restrictions
along and to the north of MRB/BWI/MTN. Behind a cold frontal
passage on Tuesday, gusty northwesterly winds will overtake the
terminals in to Tuesday afternoon and Tuesday night, gusting 25
to 30 knots.

Overall VFR conditions are expected Wednesday through
Friday. W/NW winds could gust over 30 kt on Wednesday. There are
a couple disturbances which could bring snow showers Thursday
and Friday, but impacts are uncertain at this time.


Winds greatly diminished near sunset. However, a 25-kt low-
level (near 2000 ft) jet ahead of a shortwave traversing the
waters at this time. Since water temperatures are warmer than
air temps, momentum transfer has been favorable. Multiple
reports of 20 kt gusts noted from meso sites. Consequently,
reissued a Small Craft Advisory for most of the waters through
daybreak. Left out the upper Tidal Potomac, Patuxent, Baltimore
Harbor, north of Pooles Island, and Chester.

Weak high pressure moving through on Monday will bring a return
of light winds (less than 10 kt). Ahead of an approaching cold
front Monday night and Tuesday, winds may reach SCA level, with
SCA likely behind the front Tuesday afternoon and night with
strong gusty northwesterly flow. Gale conditions look possible
behind the front, and through Tuesday night.

Strong winds will continue into Wednesday behind departing low
pressure. Small Craft Advisory conditions are likely, and gales are
possible early in the day. High pressure will bring lighter winds by
late Wednesday night. Low pressure will cross near the area on
Thursday, but winds may not increase appreciably at the surface
until a second disturbance moves through on Friday.


MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ531>534-536-


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