Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200046

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
846 PM EDT Sat May 19 2018

A warm front over southern Virginia will slowly lift northward
tonight. A cold front approaching from the Ohio Valley will
cross the Mid-Atlantic from west to east Sunday, then stall to
the south Sunday night into Monday. This front will return
northward as a warm front late Monday into Tuesday. Another cold
front will approach from the Great Lakes Wednesday, with high
pressure following later in the week.


845 PM: Dense Fog Advisory issued for Baltimore/Harford
Counties. Light winds and humid low-level airmass have resulted
in areas of dense fog, with reports of visibility near zero at
times (from trained spotters). There aren`t many ASOS/AWOS
observations in the advisory, but traffic cameras in the area
show dense fog as well. The fog should begin to lift by midnight
as southerly flow increase to 5 to 10 mph.

Previous AFD:
Flood Watches have been dropped. There are still ongoing Flood
Warnings along main stem rivers (including much of the Potomac
River), and given the amount of rain and runoff we have
received, these warnings will likely last for at least the next
day or two in some places.

Patchy fog and drizzle with low clouds are expected as the
airmass remains soupy and stagnant. Low temperatures won`t budge
much from where they are now. A few scattered showers
(measurable rain) are possible as well, with an isolated
thunderstorm possible over the highlands this evening.


Should see some breaks in the clouds Sunday with as winds turn
westerly. Scattered afternoon/early evening showers and
t-storms are anticipated as cdfnt from the OH valley crosses the
area. Southern MD will be again the most vulnerable area for
flash flooding due to saturated soils and is the area where
models show the best moisture convergence. Faster cell motion
and mid- level winds nearly perpendicular to sfc front should
keep cells moving and minimize flash flooding threat.

The front will be returning north again on Monday with
additional showers expected especially west of the Blue Ridge.


A chance of showers and thunderstorms will continue into Tuesday,
with a front draped across our southern zones. Temperatures will be
in the upper 70`s to low 80`s across the region, depending on the
exact location of the front. A chance of showers and thunderstorms
again on Wednesday, as a stronger low pressure system passes by to
our north. A little more agreement in the latest guidance as far as
frontal passage timing. The front should be through the area by
Wednesday night. Temperatures will remain in the low 80`s. Nor a lot
of precipitation expected during these two days, with maybe a
quarter of an inch of additional precipitation. However, higher
amounts are possible in areas that experience thunderstorms.

High pressure builds into the area Thursday, and quickly slides off
the east coast by Friday. Expect dry conditions during this period,
along with seasonable temperatures.


LIFR/IFR conditions tonight improving rapidly to VFR after 14Z
Sun as winds turn from the west. A chance of t-storm sites Sun
aft mainly IAD/DCA ahead of an approaching cold front. Sub-VFR
possible in patchy fog Sun night, then in increasing showers
Mon-Mon night as aforementioned boundary returns north as a warm

Mainly VFR conditions forecast Tuesday and Wednesday, with
threat of sub-VFR conditions being a passing shower over the
terminals. Winds will be light and favor a southwesterly
trajectory on Tuesday, turning Westerly on Wednesday.


SCA conditions gradually expand the waters tonight, then
subside by late Sunday after the passage of a cold front. Winds
should be light as the front stalls to the south Sunday night,
then slowly returns north as a warm front late Monday through
early Tuesday. SCA possible late Tuesday into Wednesday as
another cold front approaches from the Great Lakes.


Water levels are running around 1 to 2 ft above astronomical
predictions on the Bay and lower tidal Potomac. Minor coastal
flooding is expected at Annapolis and Straits Point late tonight
and Sunday, and is possible elsewhere.

For the upper tidal Potomac around Washington DC and Alexandria,
fresh water piling down from the Potomac River will cause
anomalies to continue increasing tonight. This will affect
Georgetown more than southwest DC or Alexandria.
-For Alexandria: An advisory is in effect through 4 PM Sunday.

-For Washington DC: Georgetown will be the driver due to the
 freshwater contribution, where moderate flooding is forecast
 for five cycles (with levels not dropping below minor through
 Sunday night). At the southwest waterfront, best confidence for
 moderate flooding will be near high tide tonight through
 Sunday; subsequent cycles may only be minor flooding. A Coastal
 Flood Warning is in effect for Washington DC through Sunday
 and it may need to be extended through Monday. A Coastal Flood
 Advisory is in effect for Alexandria through 4PM Sunday, and it
 may need to be extended through Monday as well.


DC...Coastal Flood Warning until 10 PM EDT Sunday for DCZ001.
MD...Dense Fog Advisory until midnight EDT tonight for MDZ006-011-
     Coastal Flood Advisory until noon EDT Sunday for MDZ017.
     Coastal Flood Advisory until 3 PM EDT Sunday for MDZ014.
VA...Coastal Flood Advisory until 4 PM EDT Sunday for VAZ054.
MARINE...Small Craft Advisory until 6 PM EDT Sunday for ANZ530-531-535-
     Small Craft Advisory until 6 PM EDT Sunday for ANZ532>534-536-


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