Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 251458

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1058 AM EDT Sun Sep 25 2016

A frontal boundary will remain stalled to our south today while
high pressure builds to our north. The high will move 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 today across these areas. Extent of cloud cover not
expected to change a whole lot today, with some expansion being
observed as of late morning, and some shrinking of the coverage
still looking likely late this afternoon. 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 especially 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. 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 and
occasional IFR ceilings through about 18z or so, before lifting
to VFR. Can also be a few periods of patchy drizzle during that
time period along with reduced visibility to MVFR. 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.


Sub-SCA conditions will continue for the rest of today and tonight.
Southerly winds will then be on the increase Monday and especially
Monday night ahead of an approaching frontal system so have issued
SCA for all waters. Think there will be notable fluctuations and
uncertain just yet where exactly they will be at various times,
but think SCA is probable at some point all waters. 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.
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 has finally fallen below 60
degrees this morning for the first time since June 9th (total of
107 days). The record most consecutive days at or above 60
degrees in the Washington DC area is 112 set in 2012.


MARINE...Small Craft Advisory from noon Monday to 6 AM EDT Tuesday for


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