Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 010138
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
938 PM EDT Sun Apr 30 2017

.SYNOPSIS...
A front which is draped across the area will lift north as a
warm front tonight. A strong cold front will cross the region
late Monday and Monday night. High pressure will bring cooler
and drier weather during the middle of the week. Low pressure
will move into the area late in the week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Currently, the nearly stationary backdoor front has retreated
slightly, stretching from near Camp David southeast towards
College Park and on to Chesapeake Beach. North of the front,
temperatures are generally in the mid to upper 60s, with mid to
upper 70s commons south of it. Scattered thunderstorms which
developed primarily over the higher terrain, but were able to
take advantage of the high shear, have now dissipated with the
loss of insolation, and no further storms are expected tonight.
As southerly winds increase ahead of the approaching cold front,
the backdoor front is expected to very slowly retreat northward
overnight. Despite the front`s passage, the remainder of the
night should be dry, but lower clouds may once again develop
toward dawn. Can`t rule out patchy fog in areas that remain
clearer. It will be very mild once again with lows in the 60s
for most locations.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/...
On Monday, a well defined cold front will be approaching the
area. There remains some differences in solutions regarding the
evolution of convection. Some convection looks tied to a
prefrontal trough, with additional convection along the actual
front. While coverage/timing is a little uncertain, the wind
fields are not. There will be plenty of wind throughout the
column, and it will be a breezy day even outside of the storms.
We likely remained capped most of the day, as well as mostly
cloudy. Thus instability is likely to remain on the lower end of
the scale, but nevertheless enough to support convection which
could pull down the strong winds. SPC has maintained the Slight
Risk across parts of the area. Temperatures will reach the 70s
to mid 80s.

Convection will be exiting the area during the evening hours,
with most areas likely drying out after midnight. Cooler
(compared to the past few days) and drier air move in behind the
front, with gusty winds likely continuing into Tuesday (gusts
25-35 mph). Vort max will skim northwestern areas during the
day, with perhaps a shower or sprinkle in the Potomac Highlands.
The low will pull farther away to the northeast Tuesday night,
likely allowing winds to relax.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
High pressure builds over our region on Wednesday with W to NW
winds over our region. A storm system will develop across the
Mississippi valley Wednesday night into Thursday. An increase in
clouds and precipitation could begin overnight Wednesday and
stretch into Friday night as this system moves NE. Thunderstorms
and heavy rain possible... with guidance suggesting PW values
above one inch... and even higher for some areas.

The warm front associated to this system will push north
through our area Thursday night with southerly flow advecting
moisture over our area into Friday night. The upper level and
surface low will push NE near or over our CWA sometime Friday or
Saturday. Conditions start to dry out Friday night into
Saturday with some showers into Sunday as upper level trough
axis moves across.

High temperatures will be mainly in the 60s and 70s with...
some 50s at higher elevations.

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
Precipitation isn`t expected tonight, but low clouds are likely
to redevelop late, with MVFR cigs possible. These clouds may
stick around much of Monday morning.

A strong cold front will push into the area during the late
afternoon to evening, likely to be accompanied by at least a
broken line of thunderstorms. These storms could contain strong
wind gusts. Even outside thunderstorms, winds could gust to 25
kt. Front pushes east late Monday night, ending precipitation
chances, but gusty WSW winds of 20-30 kt will continue through
Tuesday.

VFR expected for Wednesday over the terminals... then an
extended period of sub-VFR conditions possible starting
Wednesday night and into Friday as a storm system approaches.
Gusts as high as 20 kt possible.

&&

.MARINE...
Gradient flow (southerly) will increase overnight into Monday as
the backdoor front lifts back to the north ahead of a stronger
cold front approaching from the west. A Small Craft Advisory
is in effect for the southern waters overnight, and then for
all waters on Monday. The front itself will impact the marine
area Monday night. Thunderstorms with gusty winds will likely
affect the waters ahead of the front.

Behind the front flow turns westerly. Mixed profiles appear
deep, and the SCA continues into Tuesday. Winds gradually
diminish Tuesday night, but SCA may need to be continued on
portions of the Bay.

Dry conditions expected for Wednesday over the waters... then
an extended period of rain could start Wednesday night and into
Friday as a storm system approaches. Wind gusts will be near the
small craft advisory threshold... therefore expect advisories
for parts of this period.

&&

.TIDES/COASTAL FLOODING...
Water levels are running very close to astronomical normals
this evening. Tidal anomalies may increase through tonight due
to an onshore flow...but water levels should remain below minor
flooding thresholds since the flow will be light.

Southerly flow will strengthen ahead of a cold front Monday
into Monday night. Tidal anomalies will continue to increase
during this time and minor tidal flooding is possible for
sensitive areas. However...confidence is low since the higher of
the two astronomical high tides will occur Monday morning into
early Monday afternoon. This will not line up with the highest
anomalies which are most likely late Monday afternoon into
Monday night...during the lower of the two astronomical high
tides.

Some guidance suggest elevated levels could linger into
Tuesday, as winds will turn gusty but will be from the
west...and thus won`t prompt a blowout.

Low pressure will come up the coast Thursday, which likely will
increase water levels from wherever they stand up to that
point.

&&

.CLIMATE...
Record highs were set at DCA and IAD yesterday, and record warm
lows were set at all three airports (DCA, IAD, and BWI). The
record warm low at DCA was also a record for the month of April.

Records highs are less likely today as a backdoor front drops
down. However, the record warm lows are a possibility. Here are
the record highs/warm lows for Sunday:

Washington DC... High 92 (in 1942); Warm low 67 (in 1983).
BWI Airport... High 92 (in 1910); Warm low 63 (in 1983).
Dulles Airport.. High 86 (in 2007); Warm low 64 (in 1983).

April 2017 is likely going to go down as the warmest on record
at DCA and IAD, and one of the three warmest at BWI. The
current records are:
51.9 degrees in 1981 at DCA/Washington DC
46.4 degrees in 1994 at IAD/Dulles VA
50.7 degrees in 1941 at BWI/Baltimore MD

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 AM Monday to 6 AM EDT Tuesday for
     ANZ530>532-535-536-538>540-542.
     Small Craft Advisory until 6 AM EDT Tuesday for ANZ533-534-537-
     541-543.
     Small Craft Advisory until midnight EDT tonight for ANZ535-536.

&&

$$
SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...IMR
AVIATION...ADS/IMR
MARINE...ADS/IMR
TIDES/COASTAL FLOODING...BJL
CLIMATE...HTS/RCM


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