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000
FXUS61 KLWX 080117
AFDLWX

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

.SYNOPSIS...
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High pressure moves 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
into the weekend.
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&&

.NEAR TERM /THROUGH WEDNESDAY/...
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High pressure is located just off the East Coast this evening. Low
pressure is located over southern Ontario, with a cold front curling
through the upper Great Lakes back across the Missouri Valley and
into the Oklahoma panhandle. Southerly winds continue within the
warm advection regime, with this trend continuing into the
overnight. Lows will be in the upper 50s and 60s. A few mid and high
level clouds can be expected along with dry conditions through
tonight.
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&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
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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.
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&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
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.

&&

.AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
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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. Have added the mention of showers in the
TAFs, but uncertainty looms around thunderstorm development which
led to its omission at this time.

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.
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&&

.MARINE...
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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. However, winds may subside at times during the
middle of the day, especially over the wider waters. 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).
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&&

.TIDES/COASTAL FLOODING...
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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.
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&&

.LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 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.
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&&

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