-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 081921
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
321 PM EDT Wed Sep 8 2021

.SYNOPSIS...
A strong cold front will approach from the west today before
pushing through the area tonight into Thursday. A large area of
high pressure will then move overhead Friday into the weekend
before pushing offshore early next week.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Stacked low is located well to the north in Canada while a cold
front trails through upstate NY and the Ohio and Tennessee
Valleys. An MCV is moving through southeast VA this afternoon,
and will likely represent the first "round" of thunderstorms
which will brush by the Northern Neck and Southern Maryland.

Guidance has been quite variable with where and when the next
storms will develop, although so far it appears the stable
layers in the 12Z IAD sounding are holding strong. The terrain
and/or a weak pressure trough could serve as an initiation
point later this afternoon. Additional convection could form or
strengthen as the actual cold front presses eastward this
evening. For this afternoon, moderate instability and DCAPE are
in place near and east of the Blue Ridge, though shear is still
lacking. Thus expect activity to be somewhat pulsey and
disorganized. Increasing shear will overspread the region this
evening, so storms may be able to organize into a line or
clusters. Damaging winds will be the main threat, although some
hail is possible. There may be a brief window for a tornado or
two this evening as a low level jet spreads into the region.
This jet and slow frontal motion may also result in some
training of storms which could lead to some localized flooding.

While the biggest severe weather threat will likely be this
evening, showers and some thunderstorms will likely continue
along the frontal zone through the night as it progresses
eastward, some anafrontal in nature. Some locally heavy rain
could continue during this time.

Some patchy fog could occur, especially in western valleys late
tonight due to saturated low levels, although continued cloud
cover will likely limit its extent. Lows will be in the 60s for
most locations.
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&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/...
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Still some uncertainty with how quickly the front slides to the
southeast on Thursday, possibly being held up by low pressure
developing along it. PoPs have trended up over the southeastern
half of the area to cover this. Little to no thunder is expected
since the surface boundary should be to the southeast. Extensive
cloud cover will persist until the afternoon, especially east
of the Blue Ridge. The upper trough axis and a secondary
boundary will approach from the northwest late in the day. The
highest chance for additional showers and perhaps a thunderstorm
will be across the Allegheny Front. Downsloping should limit
coverage to the east, but did add isolated showers to the
forecast since the trough axis will strong. High temperatures
will be limited to the mid 70s to lower 80s.

Surface high pressure will build in from the west Thursday night
before becoming positioned overhead by Friday night. Dry and
pleasant weather is expected along with below normal
temperatures. Friday night will likely be coolest, with
widespread 50s and some 40s in outlying valleys.
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&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
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A pretty benign pattern will remain over our region Saturday through
Monday as a near zonal upper level flow remains to our north while
Bermuda High pressure builds over the Mid-Atlantic and SE US.
A warming trend will being on Saturday as winds become more south to
southwesterly leading to increased advection of warm and moist air
into the region. Mostly dry conditions expected at this time through
Monday with afternoon temps trending up into the mid to upper 80s by
early next week.

An upper trough axis is forecast to form over central Canada and
drop down into the eastern half of Canada Tuesday into Wednesday.
Shortwaves moving through the flow ahead of the trough axis may
bring the next chance for precipitation on Tuesday. Afternoon temps
will be remain above normal in the mid to upper 80s.
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&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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Gusty southerly winds will continue through this evening as a
cold front approaches. A couple rounds of storms are expected
to develop late this afternoon into the evening ahead of the
front. However, as timing of storms has be generally delayed at
the TAF sites, specifically identifying when and where storms
will occur has become challenging, other than identifying the
pre-frontal and frontal regimes. Have timed these two rounds
into the TAFs, but there are still wide ranges of model
solutions, so wouldn`t necessarily take those windows
verbatim...though the late evening round is higher confidence.
Gusty winds and heavy rain will be possible with the storms.

The front will gradually cross late this evening with a wind
shift to the northwest and potential for MVFR ceilings.
Additional showers could occur for several hours behind the
front late tonight, but coverage is uncertain. Some fog could
develop at MRB and CHO if skies clear, potentially IFR. Showers
and low ceilings should depart Thursday morning, although
clouds in general will be slower to exit. An isolated shower
could pop up in the late afternoon in association with a trough
axis, but coverage and impacts should remain sparse.

High pressure will dominate over the terminals Friday into the weekend,
delivering VFR conditions and light southerly breezes at around
10 knots or less.
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&&

.MARINE...
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Sporadic southerly gusts to around 20 kt continue this
afternoon. Winds may increase slightly this evening as a cold
front approaches. Strong to severe thunderstorms may require
Special Marine Warnings this afternoon into the evening (and
potentially a bit longer into the night). Winds will shift to
the northwest tonight, although in this case, it should bring an
end to advisory conditions.

While winds remain light most of Thursday, a secondary front will
move through Thursday evening. SCA conditions will be possible
late Thursday into Friday, especially over the bay where
northerly winds channel.

High pressure will remain in place over our marine water with subSCA
conditions expected this weekend.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
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Southerly breezes continue to support above normal tidal
anomalies and the threat for minor tidal flooding to a number of
sites along the tidal Potomac and western shores of the
Chesapeake Bay. Locations in the northern parts of the bay, as
well as DC SW Waterfront, have the greatest chances of seeing
minor flooding. Advisories have been issued for the evening high
tide in these locations. Even though a cold front will cross
tonight, it is uncertain how quickly the anomalies will abate
since winds won`t be strong, so additional threshold flooding
could occur early in the morning. A secondary surge of northwest
winds should help drive anomalies down further by Thursday
night.
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&&

.LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory from 8 PM this evening to midnight EDT
     tonight for DCZ001.
MD...Coastal Flood Advisory until 10 PM EDT this evening for MDZ014.
     Coastal Flood Advisory from 7 PM to 11 PM EDT this evening for
     MDZ011.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 2 AM EDT Thursday for ANZ530>533-
     537>542.
     Small Craft Advisory until 5 AM EDT Thursday for ANZ534-543.
     Small Craft Advisory until 11 PM EDT this evening for ANZ535-
     536.
-- End Changed Discussion --

&&

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