Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 300201
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1001 PM EDT Sat Apr 29 2017

.SYNOPSIS...
A front will sag south into the area through early
Sunday...before lifting north as a warm front late Sunday. A
cold front will then cross the mid-Atlantic Monday night. A
secondary cold front will push southeast Tuesday night. Low
pressure from the Tennessee Valley will head toward the Mid-
Atlantic Thursday and Friday.

&&

.NEAR TERM /THROUGH SUNDAY/...
Guidance has consistently overdone convection today. While there
are still suggestions of thunderstorms developing as the
backdoor front drops down overnight, have overall cut POPs
somewhat, though still maintain chance showers and slight chance
thunder for a descent portion of the region. That having been
said, if little to nothing occurs, would not be shocked, but
there remains plenty of instability out there, which if
triggered could still result in thunderstorms.

Low clouds and reduced visibility remain an issue overnight,
with low clouds more of a threat north of the front, and reduced
vis from fog an issue more likely to the south. Lows will be in
the 60s for most areas.

See climate section for record information.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
Backdoor front will likely stall out across the area on Sunday
and be slow to lift back north. At the minimum, this results in
a very tricky temperature forecast with 60s to the north and
upper 80s to the south. The boundary will snake back north over
the mountains, and this will be the most likely place for
instability to combine with terrain forcing to produce a few
thunderstorms during the afternoon. Shear will still be strong
enough that gusty winds could be a threat with stronger storms.
Absent of other forcing, these storms should end quickly with
sunset. Temperatures will once again remain in the 60s
overnight.

On Monday, a deep low pressure system will work from the Plains
to the Great Lakes, which will drag a cold front toward the
forecast area (after ensuring the warm front lifts back to the
north). Timing of the front (or prefrontal trough which could
be what actually initiates convection), appears slightly slower,
but the focus remains on the afternoon and evening, with rain
quickly departing after midnight. Shear will remain strong, but
instability should be lower, with hints of a lingering wedge
over the region. The result may be linear storms which pose a
locally gusty wind threat. Given slower timing and potential for
lingering wedge/marine influence, best chance of severe would
appear to be west of the metros. Highs likely reach the upper
70s to 80s, with lows Monday night falling into the 50s to lower
60s.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Weak high pressure will build in behind the cold front Tuesday.
A gusty westerly breeze will usher in drier air. Temperatures
will not be as warm as recent days. As the high moves east, a
secondary cold front will move across the region from the
northwest Tuesday night. This front should be a dry front. The
front could sag into the Carolinas Tuesday night before starting
to move northward as a warm front on Wednesday.

A low pressure system is expected to develop along the western
end of the warm front over the Lower Mississippi Valley
Wednesday night and early Thursday. We introduce a slight chance
of showers or a rumble of thunder Wednesday night with a higher
chance to likelihood of encountering showers and thunderstorm
Thursday through early Friday.

At the tail-end of an associated cold front with the storm
system, another storm system could develop and deepen over the
interior Southeast U.S. Friday and Friday night. The chance of
showers and thunderstorms will linger during the period.
Temperatures will also be closer to normal Friday and Friday
night.

Brief high pressure could build into the mid-Atlantic region
Saturday ahead of a cut-off low pressure system over the
Southeast U.S. Dry conditions will be follow by increasing
clouds later in the day then a chance of showers Saturday night.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
At the moment, VFR conditions exist across the area with gusty
S-SW winds as high pressure to the southeast exerts its
influence. Have removed mention of thunder from TAFs since odds
now appear significantly lower than earlier, but the chance for
a storm is not zero overnight as a backdoor cold front slides
south across the area, which could still trigger something given
the unstable air mass in place.

Winds become NE to E behind the front, and some low clouds
and/or light fog could develop. Uncertain if front reaches CHO.
While guidance indicates IFR potential, have limited to MVFR
for now.

Front likely bisects area on Sunday, with low clouds potentially
persisting into the afternoon. Thunderstorm development may be
limited to MRB vicinity. Gusty south winds develop Monday ahead
of cold front. Showers/storms will likely affect the area during
the late afternoon and evening, with the cold front moving east
overnight.

VFR conditions Tuesday through Wednesday night. Winds west-
southwest 10 to 15 knots Tuesday, west 5 to 10 knots Tuesday
night, becoming light and variable Wednesday, and northeast 5 to
10 knots Wednesday night. Conditions deteriorate on Thursday as
low pressure moves toward the area.

&&

.MARINE...
Thunderstorm potential has lowered significantly, but a stray
storm could still develop overnight as backdoor front slides
south into the unstable air mass. The front will stall out
across the waters late tonight and Sunday with east winds north
and southerly to the south. Winds may approach SCA criteria
across the far southern waters on Sunday. Think thunderstorms
chances are fairly low on Sunday. Gradient increases Sunday
night into Monday, with SCA likely all waters on Monday.
Potentially gusty showers and storms will arrive ahead of a cold
front Monday evening. SCA conditions will continue behind the
front.

Small craft advisories likely continue into Tuesday before
abating Tuesday night into Wednesday. Winds west- southwest 10
to 15 knots Tuesday, west 10 knots Tuesday night, becoming
northwest 5 to 10 knots Wednesday, and northeast 5 to 10 knots
Wednesday night.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will develop Sunday as a backdoor cold front
stalls overhead. A stronger cold front will approach from the
west Monday before passing through Monday night. Southerly winds
will strengthen ahead of the front during this time. Elevated
water levels are expected Sunday due to the onshore flow and
minor tidal flooding is possible for sensitive areas Monday into
Monday night as the southerly flow strengthens.

By the end of the week though, a coastal low is likely to cause
elevated water levels once again.

&&

.CLIMATE...
Record highs were set at DCA and IAD today, but it fell short at
BWI. Record high mins are possible at DCA, IAD and BWI for the
day if it stays warm enough through 1AM (Midnight standard
time). Records are less likely Sunday as a backdoor front drops
down. Here are the record highs/warm lows through the weekend:

Washington DC...
Saturday 29 April...91 (in 1974)/68 (in 1956)
Sunday 30 April...92 (in 1942)/67 (in 1983)

BWI Airport...
Saturday 29 April...91 (in 1974)/67 (in 1956)
Sunday 30 April...92 (in 1910)/63 (in 1983)

Dulles Airport..
Saturday 29 April...87 (in 1996)/62 (in 1996)
Sunday 30 April...86 (in 2007)/64 (in 1983)

Also of note...the warmest night time low at DCA in April is 69
degrees. That is being challenged as DCA only dropped to 70 this
morning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from midnight Sunday night to 6 AM EDT
     Monday for ANZ533-541.
     Small Craft Advisory from 6 PM Sunday to 6 AM EDT Monday for
     ANZ534-537-543.

&&

$$
SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...KLW
AVIATION...ADS/KLW
MARINE...ADS/KLW
TIDES/COASTAL FLOODING...
CLIMATE...



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