Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180854

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
354 AM EST Mon Dec 18 2017

Warm front will lift northward away from the region today, but a
pressure trough will linger over the Appalachians. High
pressure will return Tuesday before a cold front crosses the
region Tuesday night. A storm system will pass to the south
Wednesday. High pressure will return Thursday before a storm
system approaches from the west at the end of the week.


A surface warm front presently resides north of the forecast
area, and it will continue north today. There will be a pressure
trough along the spine of the Appalachians that will be
responsible for clouds, light rain, drizzle, and fog in the
mountains. Otherwise, broad ridging will be affecting the
southeastern United States today.

Present cloudiness will break apart somewhat, especially across
Virginia/southern Maryland where low-mid level moisture a bit
more sparse. Aside from that, its tough to pinpoint any forcing
features. As 850 mb temperatures rise to +2 to +4C, anticipate
high temperatures to be able to exceed 50F just about
everywhere. Since insolation will be best in Virginia, matched
MaxT accordingly.

A similar setup exists tonight, with zonal flow and subtle warm
advection. There is a shallow layer of moisture around 925 mb,
so I`m not sure how clear it will be. Am keeping temps above
freezing areawide. Winds don`t fully decouple either, so even if
there were clearing, radiational cooling may not be that ideal.


Broad ridging and zonal flow will be the key features for
Tuesday and Wednesday as well. A northern stream system will
drag a cold front through Tuesday night. Expect enough moisture
for a period of clouds but no precipitation. An impulse in the
southern stream will then ride along this boundary Wednesday
afternoon-evening. However, the consensus solution is to keep
most of the associated precip south of the forecast area. Will
entertain chance PoPs south of Harrisonburg- Fredericksburg.

Given the sun and shot of warm advection, highs near 60F
reasonable on Tuesday. The frontal passage will carve 10 to
maybe 15 degrees off that for Wednesday. Although Wednesday
night lows likely to be subfreezing, any precip would exit
before temps drop, so p-type issues not anticipated. Will be
maintaining just rain as precipitation type for Wed PM.


While the long term starts out tranquil, the general theme is
"un"... "un"settled and "un"certain.

On Thursday, we start out with a broad flat ridge over the
eastern half of the country, while a closed low is sitting near
the four corners. By Friday, this closed low opens up into a
short-wave trough extending from Minnesota to western Texas,
while the ridge over the east strengthens ahead of it. On
Saturday, the shortwave weakens and moves northeast across our
region, beating the ridge back, while another shortwave digs
over the west, such by this time a broad trough dominates much
of the country. By Sunday, the new shortwave digging out west
starts to push east towards the Plains, while the ridge regains
a bit of strength over the Southeast.

The details are where the trouble lies. To start, high pressure
at the surface will dominate on Thursday, with temps fairly
close to normal and mostly sunny skies. Things change Friday as
warm advection/isentropic lift gets underway ahead of the
approaching trough and surface front, with increasing clouds and
a growing chance of rain, though still slight during the day
Friday. We will need to keep an eye on potential for a little
freezing rain if precip arrives early enough, particularly west
of the Blue Ridge/Catoctins, but expect all areas to end up
above freezing by afternoon.

Saturday and Sunday are where the real uncertainty lies. The EC
and GGEM keep the Southeast ridge just strong enough such that
the approaching cold front stalls out just to our southeast. The
GFS, on the other hand, pushes the front off the Southeast coast
before stalling it. With the front stalling fairly close by on
all models, the chance of precipitation is elevated through the
weekend. However, the GFS would suggest enough cold air moves in
behind the front such that we could have some mixed
precipitation or snow issues, particularly Christmas Eve into
Christmas Day. Meanwhile, the ECMWF and GGEM keep the area mild
with just plain rain expected. In fact, by Monday, EC guidance
is over 20 degrees warmer than the GFS. Model blending has
suggested that a solution closer to the EC is more likely than
the GFS, and this is also suggested by the digging trough out
west which would likely cause the Southeast ridge to bring the
front to a halt faster than the GFS would suggest. That said,
there remains a fair amount of run to run inconsistency.
Therefore, anyone with travel plans during the holiday weekend,
particularly Christmas Eve and Christmas Day, should keep
abreast of the latest forecast, in case a colder and perhaps
snowier/icier outcome starts to look more likely. This is
especially true if those travel plans are north and west of


VFR conditions expected today. Believe the layer of moisture
around 040-050 will be rather stagnant, although some thinning
will be possible. If there were any flight restrictions in the
next three days, it would come late tonight as this layer of
clouds lower. There is a chance at cigs dropping to or just
below 030. Have that in TAFs at this time, but not completely
confident in it.

Clouds will mix out Tuesday, with a dry frontal passage Tuesday
night and increased wind Wednesday. Cloud bases Tue-Wed will be
scattered stratocumulus, although brief periods of bkn clouds

Generally VFR Thursday under high pressure with light winds.
Friday clouds thickening and lowering as next system approaches,
but chance of sub-VFR is still fairly low.


Light winds across the waters at this hour (3am) will continue
through the daylight hours. Southwest winds will pick up Tuesday
ahead of a cold front. However, warm air over relatively cool
waters should impede good mixing. Am capping daytime wind gusts
near the sustained speeds-- near 15 kt. Thus, Small Craft
Advisory not anticipated at this time for Tue. Not sure the same
can be said Tue night near frontal passage, and especially
Wednesday in the wake of the front with favorable mixing.

Likely sub-SCA Thursday under high pressure. Southerly flow
increases the risk of SCA gusts Friday as the next system
approaches, but cold air damming probably going to keep gusts
below threshold.




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