Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 230743
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
343 AM EDT Sat Jul 23 2016

.SYNOPSIS...
A weak cold front will disipate north of the area today. High
pressure will then be in control of the area`s weather Sunday
before another front enters the area Monday into Tuesday. This
front may stall across the region during the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Synoptic features across the eastern portion of the United States
early this morning are quite weak. The most noticeable item is a
trough axis across New England, which extends southward along the
Mid Atlantic coast. There is also a dewpoint gradient across
northern New England and the upper Great Lakes.  Believe that these
two features will play a role in today`s forecast.

Very warm and humid (dewpoints in the lower 70s) conditions reside
across the Mid Atlantic this morning. Daytime heating upon this
airmass, with 850 mb temperatures around 22c, should support
temperatures reaching well into the 90s. But, surface flow on the
back side of the trough suggests there could be west or even
northwest winds. While that does support higher temperatures, it
also would dry dewpoints out to the mid-upper 60s. That makes it
less certain whether strict Heat Advisory criteria would be
achieved. From the mean have sided toward the higher temp guidance,
but not toward the edge of that guidance. Also have dried out
dewpoints, but not as much as truly possible. This yields a Heat
Index that`s teetering on criteria. Have opted to lean to the side
of public safety and issue the Advisroy for the metro corridor.

In terms of thunderstorm chances, the dewpoint boundary, which will
be sinking south this afternoon and evening, may provide a zone of
weak forcing. But the westerly flow likely will act to surpress any
activity. While plenty of instability abounds, can`t come up with
any other reason why storms would organize and utilize that CAPE.
Have therefore back off on PoPs, keeping an isolated risk of storms
from roughly mid afternoon through evening.

Benign weather will be in place overnight, with perhaps patchy fog
in climo-favored locations. That will be especially true in the
higher dewpoint air if any storms do cross those areas.
&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
High pressure will return for Sunday, with ridging in place aloft
too. Temperatures at 850 mb will still be with a degree or two of
22C, but dewpoints should be in or near the mid 60s, which means
that the heat index should remain under 100F for most. Temperatures
may be a degree or two cooler as well since downsloping is lacking,
further supporting the feeling that an Advisory won`t be warranted.
If any thunderstorms develop, they would be initiated by terrain
circulations, and have relegated low end PoPs to the mountains.

Monday looks to be the most active day of the three. First of all,
return flow stands a chance at pumping 70F dewpoint air north across
the forecast area once again. The airmass could certainly support
temperatures that rival Saturday, so headlines will once again be
possible.  In terms of storms, shortwave energy tracking across the
northeastern states will drop a front southward, which would provide
forcing. While instability across the forecast area once again
remains ample, the better shear will be found north of us. Thus its
not certain if something more organized can develop, and if it
does would it survive the trip. Regardless, believe the setup
would support at least scattered convection. Given heat of the
airmass, storms could become strong assuming they can penetrate
the warm mid-levels. PoPs have been lowered from previous forecast
given these uncertainties, but still have 30-40% PoPs from midday
through the evening.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
High pressure will continue to move into the Mid-West Tuesday while
a cold front will be located near the Mid-Atlantic region. Multiple
waves of low pressure are expected to move along the frontal
boundary and could potentially lead to an active weather pattern for
the region. Showers and thunderstorms will be possible Tue-Fri with
the best chance in the afternoon and evening. Warm and humid
conditions are expected to continue through the end of the work
week.


&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions will largely dominate through the valid TAF period.
The only exception may be localized fog at climo-favored
terminals... MRB/CHO. Even there am carrying only a couple hours of
MVFR. The only other risk today will be thunderstorms, which should
be isolated at best. This risk much too low to carry in a TAF at
this time.

A similar setup carries through Sunday...VFR with a low end risk of
patchy fog. Thunderstorm chances even less than today.

Out of the next three days, the risk of storms highest on Monday.
Coverage would most likely still be scattered. Gusty winds/brief
restrictions a possibility.

SHRA/VCTS possible Tue-Wed.

&&

.MARINE...
Still have a few 20 kt wind gusts lingering around Tangier Sound.
Anticipate a timely expiration of the Small Craft Advisory for the
lower Maryland waters.

Flow this weekend will generally be light...10 kt or less. An
isolated thunderstorm would be possible this afternoon-evening.

Gradient winds will likely increase Monday ahead of a cold front. Do
not have high confidence on reaching Small Craft conditions, but its
within the realm of possibilities. Will leave the wording in the
synopsis at this time.  Thunderstorm chances increase Monday as
well, which may be the greater risk of locally high winds.

A cold front may linger near the waters Tue-Wed and showers and
thunderstorms are possible.

&&

.CLIMATE...
Record highs and warm lows for the upcoming weekend...

Saturday...
DCA...102 in 2011...84 in 2011
BWI...102 in 2011...79 in 1978
IAD... 99 in 2011...76 in 2011

Sunday...
DCA...101 in 2010...84 in 2011
BWI...101 in 2010...82 in 2010
IAD... 99 in 2010...78 in 2010

Monday...
DCA...100 in 1930...79 in 1965
BWI...100 in 2010...77 in 1887
IAD... 98 in 2010...76 in 1965

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Heat Advisory from noon today to 8 PM EDT this evening for
     DCZ001.
MD...Heat Advisory from noon today to 8 PM EDT this evening for
     MDZ011-013-014-016-503>506-508.
VA...Heat Advisory from noon today to 8 PM EDT this evening for
     VAZ050>057-502-506.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT early this morning for
     ANZ534-537-543.

&&

$$
SYNOPSIS...HTS
NEAR TERM...HTS
SHORT TERM...HTS
LONG TERM...HAS
AVIATION...HTS/HAS
MARINE...HTS/HAS
CLIMATE...HTS



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