Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 261904
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
304 PM EDT Wed Jul 26 2017

.SYNOPSIS...
High pressure will move offshore tonight. An upper level
disturbance will move across the region late Thursday into
Thursday night. A stronger upper level low pressure system will
approach the region on Friday before slowly moving across the
area Friday night and then passing to the east on Saturday. An
associated surface low pressure center will cross the area late
Friday and Friday night...with it`s cold front passing off the
east coast early Saturday. High pressure will build in late
Saturday and continue through early next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
High pressure will move off the southern New England coast this
afternoon while sfc winds become southerly across the Mid-Atlantic
region. Steep low-level lapse rates have led to a widespread cu
field across the region today. A micro-climate set up along the
Chesapeake Bay this morning, with water temperatures in the low to
mid 80s. Showers were regenerating along the western shore of the
Bay. Places very close to the Patapsco mainly south of Baltimore
City likely saw over an inch of rainfall since early this morning.

High pressure has led to subsidence elsewhere and besides a stray
shower I-95 and east, dry conditions should persist through the
afternoon. Temps will be below normal again today, reaching the
around 80 deg.

High pressure moves off the coast this evening and southerly flow
will increase in the mid-levels across the Mid-Atlantic region. A
cold front will push southward across the Mid-West tonight while
multiple disturbances move eastward, approaching the Mid-Atlantic
region. Convection will likely be diving down across the Mississippi
Valley overnight. Mostly dry conditions are expected across the
region tonight. A few NCAR ensemble members show iso-sct showers
forming late tonight into Thu morning due to the WAA. Kept forecast
dry for now as confidence is low at this time.

Clouds and possibly some showers will be around Thursday morning as
moisture continues to move northeast into the Mid-Atlantic region.
As the mid-west disturbances approach from the Ohio Valley Thursday
morning, the wind field is expected to strengthen as do the lapse
rates. There is a model spread for the amt of available energy by
Thursday afternoon but 1000 J/Kg or more is expected by Thu
afternoon. DSoutherly surface flow will continue during the day with
about 30 kt of 0-6km shear. Isolated strong to severe storms are
expected Thurs afternoon into the evening.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
The showers and thunderstorms will likely move to the south and a
brief break in activity is likely Thu night. The cold front will be
approaching the Appalachians Thu night while a closed low passes
across the Great Lakes. Cyclogenesis will be occurring across the
Mid-Atlantic region Friday and depending on the placement of the sfc
low and front hazardous weather is possible. The main threat at this
time is heavy rain due to the increased PWAT air and overrunning Fri
morning into the afternoon. Thunderstorms are possible mainly in the
warm sector and across the southern CWA. Deepening of the sfc low
will continue off the Mid-Atlantic coast resulting in northerly flow
on the backside of the low Fri night. The wind field will strengthen
aloft as the upper level low approaches the Mid-Atlantic region.
Bands of heavy rain may continue Friday night into Saturday.

There will likely be multiple rounds of heavy rain Friday morning
into Saturday. Uncertainty exists in the location of the heaviest
bands of rainfall but all interested parties across the Mid-Atlantic
region should stay tuned to the rainfall threat during this
time.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Still some uncertainty in the forecast for Saturday. Some
guidance holds back the upper level trough longer during the day
while the new 12Z GFS/06Z GEFS push moisture out quicker.
Fairly confident though that lingering impacts from the
departing surface low will continue into Saturday morning and
maybe into the afternoon east of the Blue Ridge. Clouds and
showers are a good bet all areas Saturday morning. Then by
Saturday afternoon drying will begin in earnest across western
areas with clearing skies during the afternoon while in the
east...clouds linger longer into afternoon with even some spotty
showers activity.

By Saturday night...most areas will be drying as a large area of
high pressure over the Great Lakes builds into the region.

Sunday through Tuesday will be influenced by a large area of
high pressure centered over the eastern U.S., and overhead on
Tuesday. Sunday will be a great day featuring dry weather with
sunny skies and low humidities and well below normal
tempeartures for late July. Monday`s weather is similar to
Sunday with just slightly warmer temps but dry. And by Tuesday,
there`ll be a bit more humidity as winds get more of an
east/southeast component and pull moisture in from the Bay and
western Atlantic. Temperatures on Tue near seasonable normals.

For Wednesday, center of the ridge of high pressure will slide
east off the coast and allow more a more humid and warmer flow
to return across the region. With the return of more semi-
tropcical moisture on Wednesday...a return to a threat of
diurnally forced convection is likely


&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
VFR conditions expected the rest of the afternoon. Low-level
moisture will cause cigs to drop again tonight and into Thu
morning. At this time, MVFR cigs are possible at MRB/BWI/MTN/IAD
with lower confidence at DCA/CHO.

By Thursday, south/southwest flow returns along with increasing
chances for showers/thunderstorms late in the day. Showers and
possible thunderstorms become more widespread on Friday with periods
of sub-VFR conditions possible.

Possible MVFR/IFR conditions possible Saturday morning in
showers and low ceilings. Conditions will gradually improve
during the afternoon and evening hours on Saturday. High
pressure will build overhead for Sunday and continue into early
next week with VFR conditions expected.

&&

.MARINE...
Southerly winds will continue on the waters through Friday.
Showers and thunderstorms are possible on the waters Thursday
afternoon and again Friday-Saturday morning. Heavy rain will
likely cause reduced visibilities. Low pressure will impact the
waters Friday and SCA will be possible.

Low pressure will slowly move away from the waters Saturday
morning and high pressure will build overhead for Sunday and
Monday. A Small Craft Advisory will likely be needed for the
waters Saturday through Saturday night...and may be needed
Sunday. A Gale Warning may even be needed for portions of the
waters Saturday into Saturday evening.

&&

.TIDES/COASTAL FLOODING...
An onshore flow will develop today and persist through Thursday.
Elevated water levels are expected during this time...but since
the flow is light minor flooding is not expected at this time.

Potent low pressure will approach the waters Thursday night
before slowly passing through later Friday into Saturday.
Increasing southerly surface winds ahead of the low may enhance
the threat for minor flooding Thursday night through
Friday...and possibly into Friday night. Confidence is low due
to low certainty on the exact track of the low.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...SMZ
NEAR TERM...HSK
SHORT TERM...HSK
LONG TERM...SMZ
AVIATION...HSK/SMZ
MARINE...HSK/SMZ
TIDES/COASTAL FLOODING...LWX



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