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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
221 PM EDT Tuesday Aug 4 2015

Synopsis...
a surface trough will continue to gradually shift southeast
through the area today. High pressure will build into the area
later tonight through Wednesday. The boundary will return north as
a warm front and stall out over the area for Thursday and Friday.
Low pressure will ride along the boundary...passing through the
region Thursday night and Friday. High pressure will gradually
build in behind the low next weekend.

&&

Near term /through Wednesday/...
an upper low is located near Hudson Bay while the jet stream
rounds the trough across the Great Lakes into the middle- Atlantic
this afternoon. A surface low is on the east side of Hudson Bay
while a weak cold front...more representative of a surface trough
is draped across the northeast and along the I-95 corridor this
afternoon. A coastal low off Myrtle Beach will move NE and out to
sea.

The surface trough has divided the middle-Atlantic with dewpoints dropping
across the Potomac Highlands and temperatures in the middle 80s and warm and
humid conditions east of the Blue Ridge with temperatures in the upper 80s
along I-95 and dewpoints in the upper 60s to low 70s in the southern
Maryland. A cumulus field has formed across much of the region as mixing has
occurred. The cumulus has a forward tilt to them showing shear in the middle-
levels. Showers and thunderstorms are expected to form near the
surface trough axis and east with coverage and intensity increasing
near and east of I-95 where temperatures and dewpoints are high and
instability is greater than 1500 j/kg. Effective shear ranges from
30-40kts across the region. A stable layer is apparent on rap
forecast soundings for S Maryland leading to little cumulus field across area of
concern...Piedmont and S Maryland. This should be overtaken this
afternoon. The convection from the coastal low near the Myrtle
Beach has produced northward moving outflow boundaries that may
reach the mid-Atlantic.

The best chance for strong to severe storms appears to be I-95 and
east including southern Maryland...Tidewater region and surrounding
waters. Severe threats include damage winds...large hail and isolated
tornadoes.

The surface trough moves off the coast this evening and high pressure
moves into the region. Forecast min temperatures range from the low 60s in The
Highlands to the low 70s in coastal areas including the metros.

The gradient between high pressure moving in from the west and the
surface trough near the East Coast will cause northwest winds 10-15mph with
gusts up to 25 miles per hour.

&&

Short term /Wednesday night/...
high pressure continues to move into the region Wednesday night.
Clouds will begin to increase from the SW into Thursday morning as
the next system approaches.

A strong shortwave trough aloft will induce a wave of low
pressure along a stalled frontal boundary over the Ohio Valley
Thursday...then track basically directly over the County Warning Area Thursday
night into Friday. Deep-layer southerly flow across the Gulf Coast
into the Tennessee/Ohio valleys will infuse plenty of moisture
into the system as it approaches the area...with precipitable
water values of 1.75 to 2.00 inches near and south of the low
track. Expecting widespread showers with a few embedded convective
elements and possibly some isolated thunder given potential modest
cape/strong forcing. Heavy downpours possible...with a soaking
rain expected overall. 12z NAM offers worst case with 3 to 5
inches of rain but think this is overdone given progressive nature
of the system.

Low pressure pushes east into the western Atlantic later Friday with
showers coming to an end from west to east. With abundant clouds and
high humidity...expect nighttime temperatures to be around
normal...but daytime highs to be a few degrees on the cool side of
normal.

&&

Long term /Thursday through Tuesday/...
high pressure/mid- to upper-level ridging will move across over
the weekend. Dry weather and seasonably mild weather is expected
during this time.

As ridging begins re-building over the south-central Continental U.S. By early
next week...heights will begin to fall over the Great Lakes and
northeast. As this happens...another front will approach the area
and increase the chance for showers and thunderstorms next Monday
and Tuesday.

&&

Aviation /18z Tuesday through Sunday/...
rain showers/thunderstorms in the vicinity possible at cho-iad through the afternoon and continues
for dca-BWI-mtn through 00z. Some ts may produce gusty winds and
small hail. At this time coverage seems low to put thunderstorms and rain in
taf...may have to amend this afternoon and early afternoon.

VFR conditions expected tonight and Wednesday.

Widespread sub-VFR expected by Thursday evening...lasting into
Friday am in -ra. Wind variable at or below 10 knots as low pressure passes overhead.
VFR likely returns by Sat am...W/ winds becoming northwesterly around 10 knots.

&&

Marine...
SW-west winds expected on the waters this afternoon. Showers and
thunderstorms expected on the waters this afternoon and early this
evening. Some thunderstorms may produce gusty winds and
waterspouts and smw may be warrented. The best chance for strong
to severe storms is along the tidal Potomac and souther of Baltimore
Harbor this afternoon and early evening.

High pressure will move eastward but a gradient will persist
across the waters through Wednesday. A Small Craft Advisory is in effect for the
waters Wednesday afternoon.

Low pressure will ride along a stalled front and cross the
waters Thursday night into Friday. Depending on the exact track of
the low...a Small Craft Advisory could be required for parts of the
waters during this time.

&&

Climate...
Reagan National Airport /dca/ hit at least 90 degrees today for
the 12th consecutive day. This is the longest such stretch since
July of 2010...and the 15th longest stretch on record at the site.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...Small Craft Advisory from noon to 8 PM EDT Wednesday for
anz530>532-538>540.
Small Craft Advisory until midnight EDT tonight for anz534-537-
543.

&&

$$
Synopsis...has
near term...has
short term...has/dfh
long term...dfh
aviation...has/dfh
marine...has/dfh
climate...dfh

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