-- Discussion containing changed information from previous version are highlighted. --
630
FXUS61 KLWX 201822
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
222 PM EDT Fri Sep 20 2024

.SYNOPSIS...
Low pressure will continue to drift off the Mid-Atlantic coast
through tonight with high pressure building north of the
region. A frontal system will move into the area Saturday
afternoon before stalling through Sunday. A strong front and
area of low pressure over the Great Lakes and Upper Midwest will
approach the area during the early or middle part of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Low pressure remains southeast of Cape Cod this morning with
high pressure building into the area from the north. This will
limit shower chances today, with only Slight Chance POPs in the
Alleghenies where there will be some weak convergence.

Tonight remains dry. The best signal for fog is across southern
parts of the forecast area. Some guidance is also indicating
potential for low clouds to develop east of the Blue Ridge. Lows
will be in the upper 50s to mid 60s.
-- End Changed Discussion --

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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Come Saturday, a remnant area of low pressure will remain off
the coast of Cape Cod with high pressure over the Northeast.
The main weather Saturday now appears to be associated with the
front of a more defined low pressure near Lake Erie and a
shortwave amidst the northwest flow aloft. The area will reside
in the warm sector and bring highs in the lower to mid 80s and
dew points in the lower to mid 60s. This will aid in developing
some instability, so showers and thunderstorms should develop
with the approaching forcing. Most models suggest decent
coverage (40-60 percent) starting midday west of the Blue Ridge
and then spreading east during the afternoon and evening. While
instability will be somewhat modest (mean MLCAPE around 1000
J/kg), shear of 30-40 kt is forecast by some guidance. This
could result in some organized storms that could produce hail
and localized wind damage. Right now the greatest chance for
severe weather appears to be west of I-95, where storms are most
likely during peak heating (along with slightly higher
instability). SPC has a MRGL (Level 1 of 5) risk for SVR weather
in this area. While storms should weaken some as they push
east, there does appear to be enough forcing for showers and
isolated storms to last well into the night.

At some point, the frontal system gets influenced by the high
pressure to the northeast. This boundary now appears to stall
across the area Sunday, with cloudy, cool, and drizzly
conditions to its northeast, and more sun and temperatures
closer to 80 to it southwest. The wedge of high pressure pushes
further south Sunday night, so cloud cover may overspread most
of the area. Some drizzle is possible, and a few showers may
approach the Appalachians ahead of the next low pressure system
in the Great Lakes.
-- End Changed Discussion --

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
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Deep layered ridge will begin to flatten Monday as a shortwave-
perturbation moves across the eastern Great Lks. Expect showers and
thunderstorms both Monday and Tuesday. Global models seemed to have
flip flop from 24 hrs ago, particularly the 12Z GFS showing upper
ridge rebuilding again over the East Coast and holding strong
through the end of next week. Deep cutoff low that models had
dropping into the Midwest and northern Mid-Atlantic now stays over
the Upper Midwest into southern Canada. The 500 mb spaghetti plot
seems to show a lot of spread in the upper level pattern during the
middle and second half of next week indicating high level of
uncertainty and volatility.
-- End Changed Discussion --

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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For the remainder of the day, expect VFR conditions and light
NE winds becoming SE by tonight. There`s a little more
uncertainty in fog development for tonight, but would most
likely be at MRB at CHO again. Greater confidence near CHO. However,
some guidance indicates low clouds could form east of the Blue
Ridge.

A front will approach Saturday with showers and thunderstorms
possible during the afternoon and evening. Best chance for
showers and thunderstorms is from CHO to IAD to MRB, with lesser
confidence further east. These showers/thunderstorms will have a
NNW to SSE trajectory tomorrow. This boundary will stall near
the area through Sunday. Sub-VFR ceilings and occasional light
rain or drizzle may continue at times Sunday and Sunday night,
although confidence is low.

Showers and thunderstorms are likely Mon and Tue with brief flight
restrictions possible.
-- End Changed Discussion --

&&

.MARINE...
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Upper level low pressure will remain off the coast through the
weekend as high pressure builds to the north. Light northerly
winds turn toward the east and southeast through tonight and
Saturday. A frontal system will approach Saturday, with
thunderstorms potentially reaching the waters during the late
afternoon or evening. Better agreement in the vicinity of the
Potomac with lesser confidence near the Bay. The front will
stall near the area Sunday before high pressure surges from the
northeast Sunday night and turns winds to the northeast. There
is potential for marginal SCA conditions at times late Saturday
through Sunday night, although in general computer guidance has
backed off on stronger winds.

SCA conditions are possible Wednesday and Thursday.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
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Anomalies are above one foot across much of the waters this
afternoon, which has been resulting in minor flooding in
several locations. Water levels are forecast to rise further
into the weekend as offshore low pressure traps water in the
Chesapeake. Combined with high astronomical tide levels,
additional, and more widespread, coastal flooding can be
expected. A Coastal Flood Watch remains in effect for Anne
Arundel County Saturday evening into Sunday. However, there
could be additional bouts of moderate flooding thereafter, and
DC SW Waterfront may also near moderate flood stage.
Unfortunately this looks like a long duration coastal flood
event as an onshore wind component persists well into next week.
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 8 PM EDT Sunday for DCZ001.
MD...Coastal Flood Advisory until 8 PM EDT Sunday for MDZ011-017-
     018-508.
     Coastal Flood Advisory until 6 PM EDT Saturday for MDZ014.
     Coastal Flood Watch from Saturday evening through Sunday
     evening for MDZ014.
     Coastal Flood Advisory until 10 PM EDT Sunday for MDZ016.
VA...Coastal Flood Advisory until 8 PM EDT Sunday for VAZ054.
     Coastal Flood Advisory until 10 PM EDT Sunday for VAZ057.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...CPB
NEAR TERM...CPB
SHORT TERM...CPB
LONG TERM...LFR
AVIATION...LFR/CPB
MARINE...LFR/CPB
TIDES/COASTAL FLOODING...LWX