Flash Flood Warning 31, NWS Baltimore/Washington

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
312 PM EDT Tue Aug 1 2017

.SYNOPSIS...
High pressure will gradually slide offshore through the middle
of the week. A cold front is expected to cross the area late
Friday or early Saturday. High pressure will return on Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Two sources of thunderstorms at mid-afternoon: higher terrain of
the Blue Ridge and central Appalachians, and north central
MD...what originally started on the Catoctins, but now
additional isolated cells are developing in 1500+ J/kg of
SBCAPE. In addition to the locally higher CAPE in this region,
outflow and bay breeze interactions may help trigger cells too.
HRRR has been hinting at this all morning. An approaching
shortwave trough could help enhance convection coverage in the
southwest CWA despite lower CAPE values, and allow storms to
spread across the Shenandoah Valley and Blue Ridge. These storms
may persist a bit longer thanks to the trough, though it appears
that the diurnal minimum in instability will cause activity to
dissipate. While a rogue stronger pulse storm is possible as was
seen near Frederick, wind fields are very light and thus storms
should be disorganized. However, this also means locally heavy
rain could occur before storms rain themselves out.

Some patchy fog is possible tonight, especially where it rains,
though clouds associated with the trough may be a limiting
factor. Lows will be in the mid to upper 60s with lower 70s in
the urban areas.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The defined shortwave trough will be departing Wednesday morning
as the surface high moves offshore. However, there will still be
some vorticity rotating through the mean eastern CONUS trough,
and a similar instability pattern will develop today, with
highest values north and west. Thus expect storms to initiate on
the terrain again, but perhaps have slightly better coverage and
impetus to move into eastern areas during the late afternoon and
early evening, as a weak surface trough may develop. Wind fields
remain light though.

Trough digs into the upper MS Valley on Thursday which will
cause heights to rise a bit locally. Thus thunderstorm coverage
should be limited more to the mountains. Storms will once again
be diurnal in nature.

Temperatures will remain slightly above the normal Wednesday and
Thursday, with Thursday being the warmest day with southerly
flow firmly established. Low 90s for highs will be possible east
of the Blue Ridge.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
A cold front will approach the Mid-Atlantic region Friday into
Friday night. Showers and thunderstorms are possible later Friday
afternoon into the evening as the front approaches the Appalachians.

The cold front is expected to cross the Mid-Atlantic region Friday
night into Saturday. The front will likely move to the east by
Saturday afternoon. Dry and breezy conditions will follow with high
pressure building in from the northwest.

High pressure will move overhead by Sunday. Slightly below normal
temperatures are expected this weekend.

High pressure will move off the coast Sunday night and return flow
begins Monday. Unsettled weather conditions may kick off the work
week and continue into Tuesday.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Widely scattered showers and storms are developing this
afternoon, although current radar trends suggest BWI has the
best chance to be affected. A little light fog is possible
tonight, but have limited to MVFR at MRB due to possible cloud
cover.

Thunderstorm coverage may be a little better on Wednesday,
especially during the late afternoon and early evening. Coverage
should be more limited to western areas on Thursday.

Vfr conditions Friday morning. Mvfr to ifr conditions expected
Friday afternoon through Saturday with a few rounds of showers and
thunderstorms. Vfr conditions should return Saturday night. Winds
southwest 5 to 10 knots Friday and Friday night. Winds southwest
becoming northwest 5 to 10 knots Saturday and Saturday night.

&&

.MARINE...
Variable flow this afternoon becomes southerly tonight,
especially over the bay. Southerly flow then remains in place
through Thursday night. Channeling may marginally approach SCA
conditions on Wednesday evening, but the better chance looks to
be late Thursday. Isolated storms the next several days should
large be inland, but can't rule out one reaching the waters.

SCA conditions may continue into Friday with southerly flow
ahead of a cold front. Winds southwest shifting north on
Saturday with the frontal passage.

&&

.TIDES/COASTAL FLOODING...
Water level departures have eased to around a half foot. No
additional inundation anticipated through the middle of the
week. Ensemble guidance suggests the late week period will have
to be monitored as southerly flow will cause water levels to
rise.

&&

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

&&

SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...KLW
AVIATION...ADS/KLW
MARINE...ADS/KLW
TIDES/COASTAL FLOODING...ADS

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