Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 060140
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
840 PM EST Tue Dec 5 2017

.SYNOPSIS...
A cold front will move through the area tonight. Weak high
pressure returns to the area for Wednesday and Thursday. Low
pressure will pass to the east Friday and Friday night, then
another low pressure may cross the region Saturday and Saturday
night.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A cold front extends from Pennsylvania through West Virginia
this evening. A southerly flow ahead of the boundary has allowed
for mild conditions...but plenty of clouds and showers are ahead
of the boundary over our area.

The cold front will pass through from northwest to
southeast...first through the Potomac Highlands and Shenandoah
Valley by midnight...then through the Baltimore and Washington
Metro areas between 1 am and 3 am...then across southern
Maryland Rain by 5 am. A wind shift to the northwest is
expected behind the boundary and noticeably chillier conditions
are expected. Northwest winds will gust around 20 to 30 mph for
a couple hours behind the boundary.

Showers may hang around for a couple hours even after the
frontal passage due to a southwest flow aloft and shortwave
energy passing through. However...drier air will eventually win
over causing precipitation to end by daybreak for most
locations. The one exception may be across southern Maryland
where light rain may still be hanging around.

Little if any snow is expected...and the best chance will be
across the ridge tops of the Allegheny Highlands. Drier air
should take over before temps are cold enough for any snow for
most locations. Lows by morning will be in the 30s/40s.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/...
Surface ridging will build in behind the front for Wednesday,
although still expecting a lot of high clouds with strong
southwest flow aloft. Otherwise, temperatures will be more
seasonable with highs in the 40s to around 50F.

The frontal zone will settle southeast of the area along the
VA/NC coastline, and will set the stage for a couple waves of
low pressure and moisture to ride along the boundary through the
end of the week and weekend. The first will occur Wednesday
night and remain largely south/east of the area, although high
clouds will remain, and perhaps a shower can make it into our
southern waters or far southeastern Maryland. Otherwise, dry
weather Wednesday night with lows in the 30s.

Quiet weather expected to continue through Thursday with high
pressure over the region. Temperatures on the cool side, but
within 5 degrees or so of normal with highs in the 40s.

A second more prominent wave will push northeastward along the
baroclinic zone to our south/east Thursday night out ahead of an
approaching digging full latitude trough. Questions remain on
how far north/west moisture from this wave can make it, and is
discussed further in the long term section below. Either way,
Thursday night likely remains dry with lows in the 20s/30s.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Upper level trough will continue to deepen on
Friday while the frontal zone remains to our southeast.
Upper level energy ahead of this trough will result in the
development of a surface low off the North Carolina coast on
Friday into Saturday... and it will move northeast away from our
region. Likely areas to see some type of precipitation is the
southeast of our CWA.

A second system to watch for Saturday is an upper level low that
will move through our area on Saturday into Sunday. There is some
disagreement on the guidance on how much precipitation we might get
from this system but instability associated to it suggests there is
the possibility of snow squalls.

&&

.AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
A mix of VFR/MVFR is being observed this afternoon, and this
general trend will continue before rain associated with an
approaching cold front overspreads the area later this
afternoon and evening. This should result in more MVFR rather
than VFR.

Conditions should then improve after cold frontal passage late
tonight, with VFR returning by Wednesday morning and continuing
through Thursday night. Southerly winds which may gust up to
about 20 knots at times today will shift northwest and gust up
to 20 knots again after frontal passage later tonight and into
Wednesday morning.

VFR conditions expected early Friday. Sub-VFR
conditions possible Friday night into Sunday as some rain/snow showers
could move through the terminals.

&&

.MARINE...
SCA conditions expected on southerly flow out ahead of an
approaching cold front through this evening. Following frontal
passage, winds will turn west/northwest with SCA gusts likely
again. SCA continues through Wednesday although winds may lessen
during the afternoon. A secondary surge of winds may occur
Wednesday night into Thursday and a SCA may be needed again.

Winds are expected to stay below small craft advisory
threshold Friday into Saturday night. Winds may increase enough on
Sunday to need a small craft advisory.

&&

.TIDES/COASTAL FLOODING...
Anomalies have risen on southerly flow out ahead of an
approaching cold. Minor coastal flooding is forecast at
Washington this evening...but it will be close. Water levels
should decline on Wednesday with northwest winds returning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until midnight EST tonight for DCZ001.
VA...Coastal Flood Advisory until midnight EST tonight for VAZ054.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST Wednesday for ANZ530>543.

&&

$$
SYNOPSIS...MM
NEAR TERM...BJL/MM
SHORT TERM...BJL/MM
LONG TERM...IMR
AVIATION...BJL/MM/IMR
MARINE...BJL/MM/IMR
TIDES/COASTAL FLOODING...BJL/MM



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