-- Discussion containing changed information from previous version are highlighted. --
823
FXUS61 KLWX 261334
AFDLWX

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

.SYNOPSIS...
High pressure will move offshore through tonight. An upper-
level disturbance will pass through late Thursday into Thursday
night. Potent low pressure will develop overhead Friday before
slowly moving to the east Friday night through Saturday. The
cold front associated with this low will pass through during
this time. High pressure will return for early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Interesting set-up this morning with scattered to numerous
showers along portions of the western shore of the Chesapeake
Bay including the Patapsco River and Lower Tidal Potomac River.
Showers have been nearly stationary this morning resulting in a
narrow corridor of wet conditions. KLWX is estimating over an
inch of rainfall just southeast of Baltimore City. Showers are
expected to persist through the morning hours before the sfc
flow turns to the SE. Low stratus also expected across portions
of northeast/central MD this morning.

Further to the west...valley fog will continue to dissipate
this morning and mostly sunny to partly cloudy conditions
expected today.

Previous discussion...

Surface high pressure centered over the northeastern US this
morning will shift offshore through the day today. A push of
easterly flow carrying marine-influenced low level air looks to
make it to about the I-95 corridor this morning before flow
turns more southeasterly and mixing increases during the
afternoon hours. Thus there is the potential for some low
stratus and even a few rain showers from about I-95 eastward,
especially near the Chesapeake Bay where warmer water
temperatures have increased low level lapse rates. Elsewhere,
some slightly higher-based strato-cu will be around along with
some patchy fog.

By the afternoon, any low stratus will be gone, having mixed out
into a broken cumulus field. High temperatures from 80-85F.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Light southerly flow will continue tonight as high pressure
elongates offshore before exiting and a system begins to
approach from the west. There will be potential for some low
clouds and patchy fog once again. Lows in the 60s to around 70F.

Active period of weather is then expected for the end of the
week, although timing and details are still uncertain. Main
shortwave trough will be entering the northern Plains Thursday
morning with a leading shortwave and surface low pressure/front
ejecting from the Mississippi Valley at the same time. This
leading wave/front will move into the Ohio Valley during the day
Thursday with warmth and humidity returning out ahead of it.
Highs on Thursday will rise back up into the upper 80s to around
90F with dew points rebounding to near 70F. While mid-level
lapse rates will be quite poor, 500-1000 J/KG of MLCAPE is still
likely to develop, and with 0-6KM shear of 30-40 knots moving
over top, scattered showers/thunderstorms still look to develop,
especially west, with a severe threat. SPC has maintained the
slight risk of severe thunderstorms. Precipitable water values
will also surge to near or above 2 inches during this timeframe,
so heavy rain and flood risk will also need to be monitored.

For Thursday night, this leading wave/frontal boundary will push
near the area and stall before the primary stronger shortwave
approaches on Friday. This will induce cyclogenesis along the
boundary on Friday with the surface low tracking near the region
later Friday and Friday night. Depending on low track/placement,
an additional, perhaps greater, threat of heavy rain, flash
flooding, and severe thunderstorms exists for Friday. It should
be noted that significant model differences continue to exist
and confidence in any particular outcome at this time is
relatively low.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Low pressure will slowly move off to the east Saturday and the
cold front associated with the low will drop well to our south.
The timing of this system remains uncertain...but given slower
trend in guidance over the last 24-hours and the fact that the
upper-level low will be closed off...the latest forecast leans
closer to the slower solutions. Therefore...unsettled conditions
are likely Saturday morning before gradually improving during
the afternoon and evening hours from west to east.

Northerly flow behind the cold front will usher in noticeably
cool conditions for this time of year Saturday. Max temps may
hold in the 70s for most locations.

High pressure will return for Sunday...bringing more cool
conditions for this time of year along with low humidity thanks
to a northwest flow and upper-level trough remaining overhead.
However...more sunshine should allow for max temps to be warmer
compared to Saturday.

The upper-level trough will weaken some for Monday and
Tuesday...but weak high pressure will likely remain nearby.
Seasonably warm conditions are expected during this time...and
most of the time should be dry. Will have to watch because high
pressure will be strengthening over the Atlantic during this
time. This has the potential to usher in subtropical moisture
which would enhance chances for precipitation. However...as of
now it does appear more likely that weak high pressure nearby
should keep this to our south and east.

&&

.AVIATION /13Z WEDNESDAY THROUGH SUNDAY/...
Potential for morning MVFR stratus exists, mainly BWI/MTN/MRB along
with some patchy MVFR/IFR fog/br at MRB. This will improve by the
afternoon with a broken strato-cumulus deck area-wide with cloud
bases 3500-5000 feet. Light east/northeasterly flow this morning
will then turn southeasterly by the afternoon. For tonight, there
will be a returning chance for patchy fog and low stratus, again
mainly MRB/BWI/MTN.

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.

Subvfr conditions are possible Saturday morning along with
showers. However...conditions should gradually improve during
the afternoon and evening hours. High pressure will build
overhead for Sunday.

&&

.MARINE...
-- Changed Discussion --
Update...Coverage of showers have expanded across the western
shore of the Chesapeake Bay and Lower Tidal Potomac River this
morning. Brief reductions in visibilities are possible.


Main marine-related weather concern this morning will be the
potential for an isolated waterspout. A weak low level boundary
associated with a westward push of marine-influenced air will be
near the waters this morning. Low level lapse rates have
steepened due to the warmer water temperatures and despite weak
shear profiles, isolated showers this morning will have the
potential to produce a waterspout.

Low pressure will gradually move away from the waters Saturday
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.
-- End Changed Discussion --

&&

.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...BJL
NEAR TERM...MM/HSK
SHORT TERM...MM
LONG TERM...BJL
AVIATION...BJL/MM
MARINE...BJL/MM/HSK
TIDES/COASTAL FLOODING...LWX