Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 151442 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
942 AM EST Wed Nov 15 2017

High pressure will depart to the northeast today. A cold front will
cross the region early Thursday. High pressure will traverse the
area Friday, then a strong cold front will cross the region late
Saturday. High pressure will return by early next week.



Updated sky cover grids and MaxT. Skies will become increasingly
cloudy through the day due to dense cirrus coming in from the
west and stratus moving in from the southeast. All areas should
be completely overcast by 21Z if not earlier. Temps were
adjusted a bit upward by a deg or two, but in general, should be
similar to yesterday.

By this evening, low-level moisture will be deep enough to support
occasional drizzle and at least spotty light rain along what looks
to be an inverted trough-like surface feature across northern
Virginia east into central and eastern Maryland including the
DC/Baltimore areas. Exact timing of drizzle and rain depends on what
model you look at. Some guidance (i.e. NAM) starts around sunset (5
PM) and other models (i.e. GFS) do not start until mid-evening (9
PM). Current forecast splits the difference.

The aforementioned cold front will bring showers into western parts
of the CWA this evening, but as they move east they should begin to
die out due to a lack of substantial upper forcing.
After midnight, showers to the west and light rain/drizzle to the
east should simultaneously dissipate as the front moves to the east
and drier air moves in.


The surface cold front will have departed to the east of the area by
daybreak Thursday, but the upper trough axis will lag behind. As a
result, expected scattered to broken cumulus to develop. Strong
cold/dry advection on NW flow will lead to breezy conditions, with
wind gusts to around 30 mph likely.

Upslope showers along the western ridges of the Allegheny Front will
be slow to dissipate, and should mix with snow by mid-morning
Thursday, then change to all snow Thursday afternoon as cold air
rushes in. Model soundings show low-level moisture in tact through
about midnight Thursday night, so have extended at least low end
PoPs to account for this. Precipitation looks to be relatively
light, so accumulations should be minimal.

Winds will be slow to subside as high pressure continues to surge
into the area Thursday night. Cooler than normal temperatures are
expected Friday as the high moves overhead, and the forecast leans
on Superblend for highs which splits the difference between the
warmer statistical guidance and cooler raw model 2-meter

The high quickly moves offshore by Friday evening with clouds
quickly filling in from the west ahead of the next approaching storm
system. Given strong warm/moist advection and isentropic lift,
nudged PoPs upward more in line with the SREF by daybreak


Changeable weather can be expected this weekend as a strong cold
front crosses the region. Low pressure will be deepening as it lifts
northeast from the Great Lakes on Saturday. Gusty southwesterly
winds will develop during the day, which should help propel
temperatures into the 50s, if not 60s in some locations. At the
moment, the best chance for warm advection showers will be the
northwestern half of the CWA. Models are beginning to converge on a
Saturday evening frontal passage. There`s a possibility showers
along the front may break apart somewhat crossing the mountains.
Post-frontal showers will also be possible as the sharp mid
level trough will lag the surface front. Any precipitation
should clear the area by Sunday morning except along the
Allegheny Front, where snow showers will be occurring by that
time. There is still some uncertainty in the wind forecast (both
ahead of and behind the front) lying in the amount of mixing,
but expect breezy to windy conditions which will persist into

In addition to the wind, strong cold advection Sunday will lead
to temperatures in the 40s much of the day. A secondary trough
axis may swing across the area, which could enhance upslope snow
showers and allow some precipitation to spill east of the
mountains -- especially eastern WV and northern MD. Surface wet
bulb temperatures will be low enough that snowflakes are
possible in these areas as well. Northwest flow at the surface
and aloft will continue into Sunday night, although moisture
begins to decrease, so upslope snow showers should be

00Z operational GFS and ECMWF are in better agreement on high
pressure building in from the southwest on Monday and sliding off
the Carolina coast on Tuesday. However, much greater spread is noted
in the ensembles. Overall the beginning of the week should be dry
though, with temperatures below normal (but modifying by


VFR through through early afternoon with light and variable

MVFR stratus develops/advects in from east to west starting this
morning, reaching the Blue Ridge by early afternoon. Drizzle
develops this evening (shortly after sunset). IFR possible for a
time this evening, mainly between 7 PM and midnight. Conditions
should improve markedly after midnight as drier air moves in behind
a cold front. NAM is more pessimistic but has had a better handle on
low level moisture/clouds in this setup, so gave it a little more
weight for this forecast.

As the front departs to the east Thursday, expected gusty NW winds
15-20 kts with gusts 25 to near 30 kts. Winds subside by midday
Friday as high pressure moves across.

Showers will become increasingly likely Saturday afternoon and
evening as a strong cold front crosses the area. Duration of impacts
is uncertain at this time, although a period of sub-VFR conditions
will be possible. Strong winds will also be possible Saturday
through Sunday (southwest shifting to northwest with the front).


Winds will diminish through the day into the evening with winds
less than 10 kts by evening.

Light winds tonight will veer to the NW and become gusty
Thursday with solid SCA gusts of 25- 30 kts expected. Gale force
gusts are possible Thursday night (925 mb winds 35-40+ kts in
most guidance in cold NW flow over relatively warmer water).

Winds abate by midday Friday as high pressure moves across, then
increase again late Friday night ahead of the next cold front.

Southwesterly winds will continue to increase on Saturday. It`s
not out of the question winds could approach gale force. The
cold front will likely cross Saturday evening with a wind shift
to the northwest. Gales will also be possible behind the front
and could last into Sunday. Winds will decrease some by Sunday
night, although Small Craft Advisory conditions will likely


Tidal anomalies of around 1 foot above normal are lurking in the
southern waters of the Chesapeake Bay. A brief period of light
onshore flow is expected this evening into early Thursday which
should allow water levels to rise, and could lead to near minor
flooding at Strait`s Point and Annapolis this evening.


MARINE...Small Craft Advisory from 10 AM Thursday to 6 AM EST Friday
     for ANZ530>543.


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