Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221001

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
601 AM EDT Tue Aug 22 2017

High pressure will redevelop over the mid-Atlantic region today.
A strong cold front will drop southeastward across the Lower Great
Lakes today, before pushing across the mid-Atlantic tonight into
Wednesday morning. The front could stall just offshore the Delmarva
Peninsula Wednesday afternoon through Wednesday night. Reinforcing
upper level energy will help to push the front farther offshore and
allow for high pressure to build in from the northwest Thursday into


Radar as of 3am is much quieter to nil than 12 hours ago across the
region in part due to radiational cooling at night and yesterday`s
disturbance moving well to the east. We are still dealing with
the warmth and humidity through sunrise. Some patchy fog could form
in spots through sunrise, mainly in parts of the Virginia Piedmont
and Shenandoah Valley.

The heat and humidity will carrying on well into this afternoon.
High temperatures are expected to reach the middle 90s in most
places east of the Blue Ridge Mountains and away from the coastal
areas of the Chesapeake Bay. Dewpoint temperatures in the lower to
middle 70s today indicates rich tropical moisture across the region
and when combined with the heat and plenty of sunshine, we will see
heat index values near 100 to lower 100s in a large area from
just east of US 15 to just inland of the Chesapeake Bay region.
Use caution with outdoor activities and drink plenty of fluids
to aid in conquering this heat.

Along with the heat and humidity comes increasing clouds from
northwest to southeast later this afternoon through tonight as a
strong cold front pushes across the eastern Ohio Valley and
into the mid-Atlantic region. Although we have a slight chance
of showers and a thunderstorm along the Mason- Dixon region and
northeast Maryland early this afternoon, we anticipate the
showers and thunderstorms to increase in coverage and intensity
from western Maryland toward northern Virginia and the I-95
corridor late this afternoon and through the evening. There is a
slight risk for severe thunderstorms in western Maryland and
parts of the eastern West Virginia Panhandle. A marginal risk
for strong to severe thunderstorms extends from the mountains of
Highland County in western Virginia to the metropolitan areas
of the District of Columbia and Baltimore. The main threats with
the strongest thunderstorms will be damaging winds and large
hail and would be more likely to accompany such thunderstorms
between 3pm and 8pm this afternoon into this evening.

As we get into this evening and lose the daytime heating, the
damaging wind threat should diminish in the thunderstorms. The
biggest uncertainty later today and tonight is the timing of
frontal passage, along with the coverage of the thunderstorm
activity tonight as many of the models are hinting that a wide
line of severe thunderstorms spread out and dissipate as it
crosses the I-81 corridor.


For the most part, Wednesday begins with a cold frontal passage
across our region. As we get into the midday hours, the front could
show signs of stalling just to our southeast as clouds and widely
scattered showers linger along the front and just to the north. With
added daytime heating and some backing of winds with height with
frontal passage, additional showers and a few thunderstorms could
form and be scattered throughout the eastern Piedmont of Virginia,
southern Maryland and a large portion of the Chesapeake Bay. At this
time, there is a marginal risk of severe thunderstorms across lower
Southern Maryland and south into the Tidewater of Virginia Wednesday
afternoon and evening. High temperatures, along and north of the
front, will be cooler with highs in the lower to middle 80s.
Humidity will be lower as well with high pressure to the northwest
building southward and ushering in lower dewpoint temperatures.
Throughout the overnight of Wednesday, the front should move far
enough southeast to allow for conditions to dry out with the
building high pressure.

High pressure will continue to build into the region Thursday and
Thursday night. Some upper level energy will try to scoot over the
ridge of high pressure but should be unsuccessful of producing any
precipitation Thursday into Thursday night.


The weekend forecast still looks quiet as high pressure passes
north of the forecast area. One minor change is that guidance
suggesting there may be a few terrain circulations, for a stray
shower or thunderstorm, coincident with daytime heating in the
mountains. Otherwise, sunshine should be plentiful and
temperatures should be slightly below normal...with dewpoints in
the 50s.


VFR conditions expected at all terminals through Thursday night.
Showers and any severe thunderstorms could reduce conditions briefly
to MVFR at the MRB and perhaps IAD terminals later this afternoon
into this evening. Also, showers or a thunderstorm could bring down
conditions to MVFR near the CHO and DCA terminals Wednesday. Winds
southwest 10 knots today, shifting west-northwest 5 to 10 knots
tonight with frontal passage. Winds northwest around 10 knots
Wednesday and Wednesday night. Winds generally northwest 5 to 10
knots Thursday and Thursday night.

VFR conditions anticipated Fri-Sat under high pressure.


Wind gusts still on the cusp of Small Craft Advusory criteria
in the mid bay/lower Potomac at this time. Have extended
Advisory into mid morning. Small craft advisories are also in
effect from Noon today through 11pm tonight. No marine hazards
expected Wednesday through Thursday night. North/northwest winds
will be in the 5-10 kt range Friday- Saturday under high
pressure. A few gusts in the mid- teens possible.


Water levels remain around 1/2 to 2/3rds of a foot above
astronomical normals early this morning. Overall, do not
anticipate many inundation problems. At the current pace,
Annapolis will just touch minor criteria; Advisory was
previously issued and will let that hold.

The departure at Southwest DC waterfront, which was under a
quarter foot, has returned to 0.6 ft above normal. If that
remains consistent, minor flooding would result. Have opted to
issue Advisory for DC.

Northwest will will arrive by Wednesday morning. We shall see if
that will be soon enough to preclude another borderline
situation at Annapolis tomorrow.


DC...Coastal Flood Advisory until 11 AM EDT this morning for DCZ001.
MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014.
MARINE...Small Craft Advisory until 10 AM EDT this morning for
     Small Craft Advisory from noon today to 11 PM EDT this evening
     for ANZ530>543.


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