-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 071925
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
325 PM EDT Tue Sep 7 2021

.SYNOPSIS...
-- Changed Discussion --
High pressure move to the east tonight. A cold front will
progress through the area by Wednesday night. A large area of
high pressure will drift eastward from the Midwest for the end
of the week and weekend.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
High pressure is located just off the east coast this afternoon.
Low pressure is located over Ontario, with a cold front curling
through the upper Great Lakes. Southerly winds are supporting a
warm afternoon, and that trend will continue into tonight. Lows
will be in the upper 50s and 60s. Diurnal cumulus will give way
to a few mid and high level clouds, with dry conditions expected
through tonight.
-- End Changed Discussion --

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
-- Changed Discussion --
Surface low pressure will remain well to the north in Canada
Wednesday while the trailing cold front approaches from the
west. This front will reach the Appalachians by early evening
and slowly progress eastward across the area Wednesday night.
The front will also provide the primary/focused forcing for
showers and thunderstorms during the evening. However, height
falls, weak perturbations aloft, and a strong warm advection
regime will support the development of isolated to scattered
showers and thunderstorms during the afternoon as well. Most
guidance focuses this activity closer to a convergence zone near
the Chesapeake Bay. Evening activity should be a little more
widespread, especially across the northern half of the CWA east
of I-81, where the best forcing and instability will overlap.
Otherwise, it will be a warm and humid day with highs in the mid
to upper 80s.

Deep layer shear will increase through the afternoon and evening
ahead of the frontal zone, while richer low level moisture will
support 1000+ J/kg of CAPE, especially east of the Blue Ridge.
Therefore, some storms could become strong to severe, especially
any linear segments with the evening activity. Damaging winds
will be the primary threat, although some hail could also occur.
A tornado also isn`t out of the question, first due to back low
level winds near the bay with any early activity, then with any
properly oriented evening linear activity as a low level jet
increases, although low level winds may be more southwesterly by
that point (limiting SRH). Time of day and less than ideal low
level thermodynamics could be limiting factors. In addition, as
the front becomes more parallel to the upper flow, forward
propagation of linear features will tend to slow down as well,
which could limit robust, forward-moving downdrafts. This fact
may also lead to some locally heavy rain totals, especially in
eastern part of the CWA, as clusters could train over areas for
a time. Hydrologically sensitive areas due to recent rainfall
will be most susceptible to any flooding, as precipitable water
values will not be excessively high.

With the frontal zone slow to progress to the east, some showers
may continue into the night, especially east of the Blue Ridge,
along with some thunderstorms for southern Maryland. Some patchy
fog could develop in the western valleys as skies clear there.

Some models develop low pressure along the front which further
delays eastern progression on Thursday. These solutions could
result in some additional showers across southern Maryland,
especially through the morning. Meanwhile, the upper level
trough axis and a secondary surface boundary will drop toward
the Appalachians, which could spark a few showers and storms
near and west of the Allegheny Front. In between, clouds should
gradually clear. Cold advection will result in highs in the mid
70s to lower 80s. Dry weather is expected by Thursday night with
cooler lows in the 50s for much of the area.
-- End Changed Discussion --

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
The large scale pattern of an upper-level ridge over the west and a
trough in the east will start to flatten over the weekend. At the
surface, high pressure will be nearby Friday before shifting
offshore over the weekend. A weak system will then move by to the
north on Sunday, but the better moisture and forcing appears to be
north of the FA so there may just be an increase in clouds or stray
shower near the PA border.

Temperatures Friday will be slightly below average. Given the dry
airmass associated with the region of Canadian high pressure and
clear skies, have gone with the colder guidance across the valley
locations Friday night. Currently have widespread 50s west of I-95
except for in the mountains where temps could fall into 40s, with
60s expected near the metros/bay. The colder elevated valleys of
Garrett, Grant, Pendleton, and Highland counties could make a run
into the 30s. Temperatures do moderate over the weekend as heights
rise as an upper level ridge builds over the southeast US.
-- End Changed Discussion --

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
Departing high pressure will result in VFR conditions and
light southerly winds through Wednesday morning. Gusts could
near 20 kt by Wednesday afternoon. An approaching cold front
will result in showers and thunderstorms, scattered in nature
during the afternoon, with one or more clusters/lines of
showers and storms possible during the evening. Some of these
storms could contain gusty winds and locally heavy rain. For
now, have only included a VCSH at BWI after 18Z, since models
indicate the area near the bay has the highest chance of
activity, although thunderstorm development is still uncertain.

Winds will shift to the northwest Wednesday evening/night with
frontal passage, with the chance for showers and intervals of
sub-VFR conditions gradually dwindling. VFR conditions expected
Thursday/Thursday night.

High pressure will allow for mostly clear skies and dry conditions
leading to VFR condtions Friday into Saturday. Light winds up to 10
knots will be out of the northwest on Friday before becoming more
southerly on Saturday as the high shifts offshore.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
South/southwest will remain light the remainder of this
afternoon. As high pressure slides off the coast and a cold
front approaches from the northwest, southerly channeling should
result in fairly widespread SCA conditions tonight into
Wednesday, although winds may subside at times during the
middle of the day, especially over the wider waters. However,
did add the remaining zones on the Potomac to the SCA, since
there is a good signal for occasional 20 kt gusts through the
day and evening. Isolated strong storms may required special
marine warnings later Wednesday into Wednesday night. The chance
for thunderstorms and SCA gusts will diminish late at night as
winds shift to the northwest behind the front.

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

Northwesterly winds could promote SCA conditions Friday morning into
the afternoon as high pressure builds over the Ohio Valley. As the
region of high pressure moves east Saturday and eventually offshore,
the winds will become lighter out of the south (below SCA
criteria).
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
-- Changed Discussion --
Southerly breezes will persist through Wednesday, allowing for
tidal anomalies to creep upward and bring a threat for minor
tidal flooding. Anomalies are expected to be highest in the
northern bay during the high tide cycles late tonight and
during the day on Wednesday. Minor flooding is possible in these
locations, though will monitor through this evening before
issuing any advisories since confidence is low. Many other
sites will achieve action stage, with DC Southwest Waterfront
possibly reaching minor flood during this time period as well.
A cold front will push through the area Wednesday and early
Thursday, allowing winds to turn offshore, reducing anomalies.
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 PM this evening to 2 AM EDT
     Thursday for ANZ531>534-537-539>543.
     Small Craft Advisory from 6 AM Wednesday to 2 AM EDT Thursday
     for ANZ530-538.
     Small Craft Advisory from 10 AM to 11 PM EDT Wednesday for
     ANZ535-536.
-- End Changed Discussion --

&&

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