Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 241402

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1002 AM EDT Sun Jul 24 2016

High pressure will be in control of the area`s weather today
before another front enters the area Monday into Tuesday. This
front may stall across the region during the middle of next week.


Quiet morning underway as the mid/upper level ridge nudges in from
the west. The dew point gradient is near the Washington metro area
as expected, with mid 60s to the north and around 70 in the south.
A brief survey of obs suggests temperatures today are on a
2-degree slower pace than yesterday with similarly sunny
conditions, which adds confidence to the slightly lower high
temperature forecast. It will still be a hot day though with a
heat index around 100, higher toward central VA. 12Z IAD sounding
also shows potential CAPE is less today, and there will be few
forcing features outside of the terrain.

Previous discussion:

There are some suggestions that shortwave energy will be riding
atop the ridge, and will be entering the upper Ohio Valley by mid
afternoon. This will need to be watched. Inherently, there is
plenty of CAPE, and mid level lapse rates in the mountains are
favorable to support convection. Add in terrain circulations, and
there could be thunderstorms developing in the Potomac Highlands
by mid afternoon. The concern is that the vort energy will feed
off the instability, and organize a cluster of storms. A majority
of the ensemble members of the NSSL-WRF suggest there will be
something on our northwest doorstep by the end of the day. Have
bumped up PoPs in western Maryland/adjacent West Virginia by early
evening, and will be tracking this energy across the northern
forecast area (mainly north of Winchester-Washington DC) through
the evening and first part of the overnight. There are still
placement/track/timing/intensity uncertainties, so don`t want to
make too much of a commitment, but do believe its warranted to
make some sort of a mention.


After the lead shortwave exits early Monday morning, warmer air
will once again advect into the area. Suggestions are that
humidity would have increased overnight, leaving dewpoints in the
lower 70s in place. Temperature forecasts will be at least as
warm if not warmer than Saturday. That means records may be
threatened. Therefore, the heat/humidity combo will once again
push heat index values above advisory criteria.

A cold front will plow into this airmass near the end of the day.
Again, there are some timing/strength differences, and the front
itself likely will be weakening as it interacts with the ridge.
Although shear may be lacking, there will be plenty of energy to
diffuse, and any storms that are able to develop will become
strong quickly. Believe the presence of a boundary will be
sufficient for scattered thunderstorms to develop along it, with
the possibility for storms ahead of this boundary due to subtle
height falls atop orographic forcing. That means the thunderstorms
may develop by noon in the Potomac Highlands, with a boundary
sinking across the forecast area through midnight. It remains to
be seen if the boundary actually makes it through the area, or
weakens and stalls nearby during the overnight hours. Regardless,
anticipate some sort of diurnal weakening, but an isolated storm
would still be possible pre-dawn Tuesday.

With that stagnant boundary potentially in the area on Tuesday
and more shortwave energy liable to pass through, believe there`s
a decent potential for another round of thunderstorms. Strength,
timing, and coverage are all uncertain...but believe there is
enough potential to carry chance PoPs, especially south of the
Potomac River and across southern Maryland. Have some sort of
diurnal enhancement in grids right now, but ultimately the
presence and strength of vort energy will dictate when and where
thunderstorms will develop. And since there won`t be a significant
chance in airmass, temperatures once again will be higher than
normal, up to the mid 90s east of the Blue Ridge.


An upper level ridge off the southeast coast will likely be
suppressed to the south Wednesday and zonal flow will continue
across the Mid-Atlantic for much of the week. A frontal boundary
will be near the region while high pressure builds into the Ohio
valley and northeast. The boundary will stay near the region for
most of the week and showers and thunderstorms are possible each
day. A shortwave trough is also possible through the end of the
work week and may increase coverage of thunderstorms. Uncertaintyis
high at this time. The heat continues through the week with above
normal temperatures expected.


VFR conditions should prevail through much of the day. Thunderstorms
will struggle to develop. However, there may be a little forcing
late in the day through the evening and into the early overnight
hours. There are quite a few uncertainties still, so there is low
confidence at a direct strike. However, MRB/MTN and perhaps BWI
have the highest potential for restrictions from storms tonight.
DCA/IAD would be on the fringe.

Fog may be a possibility early Monday. There should be enough low
level moisture, and if there are any nighttime thunderstorms it
will aide in fog development. After daybreak, any restrictions
will erode. It will be a hot day, with an approaching cold front,
which will assist in another round of thunderstorms, primarily
(but not exclusively) during the afternoon and evening hours.
Flight impacts possible, but coverage again a concern.

The boundary will stall near the terminals Tuesday-Wednesday.
Additional thunderstorms a possibility.


Light NE/E flow will become southerly by this evening. Primary
marine hazard through Tuesday will come from thunderstorms. The
most likely opportunity will be tonight for the northern waters,
Monday afternoon and evening for all waters (although the threat
will linger into the overnight for the lower Potomac and mid Bay)
and again on Tuesday for the lower Potomac and mid Bay. Any of
these storms may contain lightning and locally strong gusty winds.

A frontal boundary may linger near the waters Wed and showers and
thunderstorms are possible.


No records were broken or tied on Saturday. Here are the record
highs and warm lows for the next three days. Monday`s numbers
appear to be the most vulnerable.

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

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

DCA...103 in 1930...79 in 1987
BWI...101 in 1940...80 in 1940
IAD....98 in 2012...76 in 2005




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