Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 250737

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
337 AM EDT Sun Sep 25 2016

High pressure will build in for today before moving off the coast
Monday. A cold front will pass through the area overnight Monday
into Tuesday. An upper-level low will likely impact the area
during the middle portion of next week.


High pressure located over eastern Ontario early this morning will
build southeastward over the Northeastern US through today. Abundant
low level cloud cover currently exists over southern and western
portions of the region due to trapped low level moisture under the
developing subsidence inversion in combination with light upslope
flow into the terrain. Some very light rain/drizzle/patchy fog also
possible this morning across these areas.  Extent of cloud cover not
expected to change a whole lot today, although some shrinking of the
coverage is likely later today. Thus a mostly sunny day expected
north/east and mostly cloudy south/west. Highs generally 70-75
degrees, although in locations that remain cloudy through the day,
temperatures may be stuck in the 60s.


The high will continue to shift eastward into eastern New England
tonight, promoting persistent light easterly flow. This will
likely lead to the expansion of a low stratus deck of clouds again
tonight, especially in western areas. Again some patchy
fog/drizzle or an isolated shower possible west of the Blue Ridge.
Lows mainly in the 50s.

A frontal system will then approach on Monday and cross the region
late Monday night into early Tuesday morning. Clouds will thicken
through the day Monday across the entire region with chances for
some showers increasing, especially west and late in the day. A
thunderstorm or two also possible in the evening as the front
approaches. The most widespread rain showers will overspread the
area Monday night, out ahead of the front. Again a thunderstorm or
two is possible as well. Highs Monday in the 70s, with lows Monday
night a bit milder ahead of the front, in the 50s/60s.

Front will push eastward Tuesday, slowing as it does so though as
associated upper low closes off and shifts southward through the
Great Lakes. Thus chances for rain showers continue into Tuesday,
mainly in the morning, and east of the DC/Baltimore metro areas.
Highs Tuesday will be similar to Sunday/Monday, generally 70-75F.
Lows Tuesday night 40s/50s.


A cutoff upper-level low over the Great Lakes early Wednesday
will drop south and east toward our area for Thursday and Friday.
The cutoff low may even hang around for Saturday as well.

Exact details in the forecast for Wednesday through Saturday
remain highly uncertain because guidance continues to diverge with
exactly how far south the upper-level low will drop before
eventually moving away to the north and east. Should the low drop
far enough to the south...then coastal low pressure will develop
along the Mid- Atlantic Coast and widespread rain will occur
across our area along with the chance for thunderstorms.
However...should the low stay farther north...then precipitation
amounts will be light and coverage would remain scattered.

Guidance over the last 24 hours has trended a bit farther south
with the cutoff low...and the forecast will reflect that trend.
However...the ensemble forecasts also show high uncertainty and
the latest GEFS and EPS mean indicate that the upper-level low
will remain just to our north. Therefore...the latest forecast
will also contain an element of continuity. Basically...will
allow for more clouds along with a chance for showers.


VFR at all sites today except CHO, which will experience MVFR
ceilings through about 15z or so, before lifting to VFR. Could also
be a few periods of patchy drizzle during that time period. Winds
generally northeast, less than 10 knots.

Additional development of sub-VFR conditions (primarily ceilings)
expected tonight, mainly CHO, although also possible at MRB. A few
areas of patchy fog/drizzle also possible again.

Sub-VFR conditions possible Monday/Monday night as clouds and
chances for showers increase. Will see some improvement on Tuesday.

Light south winds develop tonight and increase on Monday/Monday
night ahead of frontal passage. Winds will turn light westerly on

An upper-level low may impact the area Wednesday and Thursday.
Details remain uncertain...but subvfr cigs and vsbys are possible
along with showers.


Marginal SCA remains in effect across the northern Chesapeake Bay
early this morning with expected surge of northerly surface winds.
Some gusts to about 18-20 knots expected. Sub-SCA conditions then
return later today and continue into tonight. Southerly winds will
then be on the increase Monday and especially Monday night ahead of
an approaching frontal system when additional Small Craft
Advisories will likely be needed. Winds gradually lessen on
Tuesday/Tuesday night.

An upper-level low will drop south toward our area for Wednesday
through Friday. Details are uncertain...but if the upper-level low
drops far enough to the south. This will cause coastal low
pressure to develop along the Mid-Atlantic Coast. A small craft
advisory may be needed for the waters Wednesday through Friday.


High pressure will build toward the coast today before moving
offshore tonight through Monday. An onshore flow will persist
today and this will cause tidal anomalies to increase.
However...the onshore flow should be light. The onshore flow
should be light so water levels should remain below minor flooding
thresholds. Water levels will be close to minor flooding
thresholds in sensitive areas during the high tide cycle tonight
into early Monday...this will be the higher of the two
astronomical norms.

The onshore flow will increase and turn to the southeast for
Monday through Monday night. Minor flooding is possible during
this time...especially during the high tide cycle Monday night
which will be the higher of the two astronomical norms.


Unusually warm weather has persisted over the area for
quite some time. Autumn-like temperatures are finally making an
appearance over the area, and DCA may fall below 60 degrees
early Sunday morning for the first time since June 9th (total of
107 days through September 23rd). The record most consecutive
days at or above 60 degrees in the Washington DC area is 112 set
in 2012.


MARINE...Small Craft Advisory until 10 AM EDT this morning for


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