Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210726

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
326 AM EDT Thu Jul 21 2016

High pressure centered over the Mid Atlantic this morning will
drift offshore today. A system will push north of the region on
Friday, followed by a front moving into the area on Saturday.
High pressure will be in control of the area`s weather Sunday
before another front approaches early next week.


High pressure will be in control today as it slides from a
position overhead this morning to offshore of the Atlantic
coastline by later this afternoon. This will provide for mostly
sunny skies, aside from some fair weather afternoon cumulus. Can`t
rule out a stray shower over the higher terrain but will go with a
dry forecast for now. As high moves eastward, this will allow for
a light southerly flow to return. Will be looking at highs in the
upper 80s to around 90F today. Dew points however will still be on
the "drier" side, with readings from the upper 50s to mid 60s.
Therefore heat indices will remain in the 90-95F range.


Calm and tranquil night expected tonight with mostly clear skies
and mild temperatures. May be some patchy fog around again. Lows
generally in the 60s to around 70F, locally 75F in the urban
centers and along the Chesapeake.

The weather then heats up Friday and into the weekend. Sprawling
upper ridge will remain centered over the Central Plains with the
Mid-Atlantic/Northeast overspread with northwest flow aloft. This
pattern is typically conducive for thunderstorm complexes to
develop along the northern periphery of the ridge and track east-
southeastward in the northwest flow aloft. However the location
and track of these systems is uncertain this far in advance.

Model guidance hinting at one moving through the
Great Lakes and towards the region Friday morning, however,
current indications are that it will weaken and not impact the
region on Friday. In addition there will be a frontal system
pushing through the Northeastern US on Friday. This will be the
trigger for additional scattered shower/thunderstorm development
Friday afternoon. Most of the activity should remain north of our
area towards the better forcing. However as the front sags
southward, any ongoing convection may work its way into the region
late Friday/Friday night. Otherwise, Friday`s temperatures will be
hot with highs in the low to mid 90s. With dew points in the
mid/upper 60s, heat index values will likely approach 100F.

The frontal boundary, which at this point will be more of a
moisture/dew point boundary, will push southward on Saturday and
cross the region. This will become the focus for the possibility
of some additional isolated-scattered shower/thunderstorm
activity. Temperatures a few degrees hotter again with highs mid
to upper 90s. Dew points will be sensitive to the location of the
potential boundary, with mid 60s on the northern side to low 70s
on the southern side. This will ultimately drive heat index
values, but 100-105F looking most likely right now and a Heat
Advisory may be needed.

Any thunderstorm activity should wane Saturday night. Temperatures
will remain warm with lows in the 70s.


Signs continue to support excessive heat continuing through
Sunday into Monday. Strong 500 mb ridge will dominate most of the
continental USA, with the westerlies trapped across Canada.
Temperatures at 850 mb solidly at 20C, perhaps up to 22C.
Questions remain whether we will hit 100F (temperature). But with
dewpoints at (or above) 70F, heat headlines appear quite likely.
Although there will be plenty of instability, initiation will be
challenged due to warmth and suppression aloft. Thunder chances
will be low, but non- zero.

Low pressure traversing the border will pass north of the Great
Lakes Monday. This will have two effects: heights will lower,
thereby offering a more favorable environment for thunderstorms to
develop; and an attendant cold front will help provide a focusing
mechanism for these storms. Consequently, PoPs will be higher Monday

The pattern for Tuesday and Wednesday still looks hot and summery,
with the impact of the cold front limited.


Predominantly VFR expected through at least Saturday night. May be
some patchy fog early this morning and then again tonight, but
not currently expected to be widespread enough to warrant mention
in the TAF. Could also be some isolated to scattered
showers/thunderstorms around both Friday and Saturday. Light and
variable winds this morning turn light southerly this afternoon,
southwest Friday, and then westerly Saturday.

Primarily VFR Sunday. May have some early morning fog, with a low
chance at late day thunderstorms.

Thunderstorm chances improve Monday, but coverage still appears to
be scattered.


Sub-SCA conditions expected today, tonight, and Friday. Light and
variable winds this morning turn southerly later today and
tonight, but gusts should remain below 18 knots. Winds turn
southwest Friday, and a SCA may be needed for Friday night as
southwest winds increase. Winds then turn west on Saturday.

High pressure remains in control Sunday, with southerly channeling
possible overnight leading to elevated winds near SCA criteria.
Gradient winds will increase Monday ahead of cold front.




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