Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 290121
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
921 PM EDT Tue Mar 28 2017

.SYNOPSIS...
High pressure will build over the region through the midweek.
Another disturbance will approach the area late Thursday into
early Saturday.

&&

.NEAR TERM /UNTIL 7 AM WEDNESDAY MORNING/...
The upper low/trough axis has pushed east, and is presently
moving off the Delaware coast. Drier air has begun to infiltrate
the area, which has all but eliminated the thunderstorm
potential. However, there are still a few showers roaming about,
including a cell headed toward DC. Overall trend is down, which
is already reflected in the database.

In spite of the punch of drier air seen on water vapor, the
low-levels remain quite moist. It may take a while (as in the
pre-dawn hours) for dewpoints to truly drop. That suggests that
there may be a low cloud or fog issue to contend with overnight.
Mitigating factors tonight include north flow and residual
cloudcover. Have included a few hours of patchy fog after
midnight, but have it sinking south (into central Virginia) by
daybreak.

&&

.SHORT TERM /7 AM WEDNESDAY MORNING THROUGH FRIDAY/...
High pressure builds in from the north on Wednesday with a
breezy north wind. Temperatures will not be as warm and the
humidity will be noticeably lower, but still on the nice side
for late March with 60s common. Winds will relax Wednesday night
as the pressure ridge pushes directly overhead, and with dry air
in place, we may radiate down towards freezing or even a bit
below in the colder spots.

By Thursday, clouds will start to increase as warm advection
begins ahead of the next approaching low pressure. The clouds
should limit the insolation and temps will get stuck in the
50s. Some rain may move into western areas late in the day, but
think for most areas, it will wait until after dark Thursday
night. Low pressure will slide east toward us through the day
Friday, with periods of rain looking likely. There is a good
southerly fetch with this system overruning the cool wedge at
the surface, so potential for some decent rain. Right now, it
appears the wedge will mostly hold with this system, so highs
Friday look to remain in the 50s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The chance for rain showers linger Saturday before drier and
cooler air can filter into the region Saturday night when high
pressure works its way into the mid-Atlantic.

High pressure will settle into the region Sunday through early
Monday. Dry and seasonable temperatures expected throughout the
period.

The next storm system will move into the region midday Monday
through Tuesday, bringing the chance for rain showers to the
region once again.

&&

.AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
VFR flight conditions mainly prevail this evening outside of any
showers. However, these are on the decline, aside from one
passing south of DCA through 02z. There is no additional shower
threat included in the TAFs.

Drier air will slowly infiltrate from the north overnight.
However, there will still be a lot of low level moisture,
suggesting there may be a low cloud or fog threat. Mitigating
factors tonight include north flow and residual cloud cover.
Guidance suggesting that ceilings overnight will be more toward
low-end MVFR, and reflected that change in the 00Z TAFs. Have
also included a few hours of patchy fog after midnight, with a
trend highlighting the northern terminals (MRB/BWI) first, and
then sliding south toward CHO. Did not go lower than MVFR in the
visibility forecast either. If the forecast goes astray, the
most likely culprit will be a delay in the arrival of dry air,
which would mean that conditions could hit IFR pre-dawn.

Whatever does develop tonight will erode in the morning as
north/northwest winds pick up. Some gusts could reach 20 kt. VFR
conditions will continue until Thursday night, when the next
system will bring more rain and potential for IFR cigs and vis.

Mvfr conditions Saturday. Winds northwest 10 knots Saturday.
Vfr conditions evolving Saturday night. Winds northwest 5 knots
Saturday night.

&&

.MARINE...
One last shower is passing south of Washington DC at this time.
After that, conditions will generally be benign through the
night. Guidance does not look that impressive with respect to
good mixing/channeling tonight, and have pulled the Small Craft
Advisory for the overnight hours.

However, winds will pick up with sunrise tomorrow. Northerly
winds are expected to gust to 20 kt. Have left the Small Craft
Advisory in place for the daytime hours on Wednesday. Winds
should diminish Wednesday night and remain sub SCA during the
day Thursday with high pressure passing the area.

Potential for Small Craft Advisory conditions increases Thursday
night and Friday with increasing southeasterly winds ahead of an
approaching system. Winds will then turn north/northwesterly on
Saturday behind the system, with continued potential for SCA
conditions.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 AM to 6 PM EDT Wednesday for
     ANZ530>543.

&&

$$
SYNOPSIS...RCM/HTS
NEAR TERM...HTS
SHORT TERM...RCM
LONG TERM...KLW
AVIATION...HTS/RCM/KLW
MARINE...HTS/RCM/KLW



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