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000
FXUS61 KLWX 080757
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
357 AM 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 /THROUGH TONIGHT/...
Latest surface analysis depicts a strong area of low pressure
over central Ontario with its attendant cold front draped across
the Great Lakes region down into Indiana/Ohio. The stationary
front that had been sitting along the MD/PA border has lifted
northward as a warm front this morning as high pressure moved
offshore. Water vapor satellite imagery depicts the large-
scale upper trough, whose axis runs from Ontario into Wisconsin.
Radar/IR satellite depict some ongoing convective activity out
ahead of the aforementioned cold front across the Ohio Valley,
which is starting to taper off.

With high pressure having shifted offshore and a warm front well
to our north, strong warm air advection is beginning to take
shape across the region. The first half of the day will be
rather uneventful, with only a little early morning fog in the
valleys in our far western zones perhaps. Cloud cover will
gradually increase throughout the morning as warm advection
increases.

This afternoon is where things start to get a little more
interesting. High temperatures are expected to rise into the
mid-upper 80s this afternoon, with dewpoints rising into the
mid-upper 60s and perhaps even some near 70. This will result in
a fairly wide swath of moderate CAPE values in the 1500-2000
J/kg range ahead of the approaching cold front (primarily east
of the Blue Ridge). Additionally, hi-res guidance hints at a
corridor of even higher CAPE amounts (closer to 2500 J/kg). A
subtle shortwave ahead of the main trough will move overhead
this afternoon. This combined with 10-15 kts of surface SSW flow
colliding with the bay breeze, should result in thunderstorm
development this afternoon ahead of the main line of storms,
which will come through during the evening. This afternoon round
could pose a threat for an isolated tornado or two as deep
layer shear begins to increase. The primary convective mode with
storms this afternoon will be supercellular. Tornado threat
with this round would stem from backed low level flow near the
bay breeze. Steep low- level lapse rates will yield the threat
for damaging winds as well. Additionally, while not the largest
threat, hail is still a threat with the fist round of storms.
Think it will be mostly on the smaller side given weak mid-level
lapse rates, but can`t rule out some larger hail should we get
a stronger supercell to form.

The second round of convection will come through during the
evening hours. This will be more of a linear structure
associated with the cold front. Primary threat with this round
will be damaging straight-line winds. An isolated tornado also
isn`t out of the question with round 2 either in any properly
oriented evening linear activity as a low level jet increases
ahead of the front. However, 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 to the severe threat, as the front becomes more
parallel to the upper flow, forward propagation of linear
features will tend to slow down as well. This 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.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
For Thursday, guidance has slowed down the progression of the
cold front, even developing an area of low pressure along the
front. So, have added in a slight chance of showers to linger
into the day on Thursday, perhaps even into the early afternoon,
across central VA. 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.

Mostly clear skies and high temperatures in the 70s are expected
on Friday as high pressure moves directly overhead. With clear
skies and light winds, should cool down quite nicely Friday
night, with lows expected in the 50s across much of the area.
However, areas along/east of I-95 may get closer to 60,
especially as you get closer to the waters. Then the mountains
out west could see lows into the low to mid 40s!

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Tranquil weather conditions are on tap this weekend as high pressure
overhead early Saturday begins to migrate offshore for the balance
of the weekend. Resultant weather will be dry conditions and near
normal temperatures as light winds turn out of the south. The high
will remain anchored over the Southeast and into the western
Atlantic early next week as the flow aloft remains generally zonal
in nature through Monday. A weak shortwave and slow moving front may
drop into Pennsylvania late Monday into Tuesday, which could yield
isolated showers across the northern third of the CWA, but much of
the area is expected to remain on the dry side. Temperatures will
trend warmer early next week, favoring slightly above normal levels
for this time of year, perhaps making a run at 90 degrees at the
warmer locations, while humidity levels slowly tick upwards.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions expected during the first half of the day today.
However, a couple rounds of storms are expected to develop this
afternoon into the evening. First round would primarily be
around BWI, MTN, and perhaps DCA. Second round will traverse
much of the CWA (perhaps excluding CHO). Confidence is much
lower at CHO, as most convection looks to stay north, but can`t
rule out something passing over during the evening hours. For
now, have left it out of the CHO TAF though.

After all of the thunderstorm activity ends Wednesday night, winds
will shift to the northwest in the wake of the frontal passage,
with the chance for showers dwindling. The front may not quite
clear the area into Thursday though, so the MVFR CIGs may be a
bit slower to depart than originally thought.

Could see a few showers linger Thursday afternoon around CHO as
the front slowly departs to the east, but shouldn`t cause an
issue.

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

&&

.MARINE...
As high pressure sits off the coast ahead of an approaching cold
front from the northwest, southerly channeling has resulted in
fairly widespread SCA conditions overnight, which will continue
into the morning hours. However, winds may subside at times
during the middle of the day, especially over the wider waters.
Kept the Small Craft Advisory out through the day though. Strong
to severe thunderstorms may require Special Marine Warnings
later this afternoon into the evening. The chance for
thunderstorms and SCA gusts will diminish late at night as winds
shift to the northwest behind the cold front.

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 overhead early Saturday will migrate offshore for the
balance of the weekend as light southerly winds and sub SCA
conditions prevail.

&&

.TIDES/COASTAL FLOODING...
Southerly breezes will persist today, delivering increasing 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. The highest anomalies will reside along the northern portions
of the Bay, with Coastal Flood Advisories in effect for Annapolis
and Baltimore for this mornings high tide cycle, while Havre de
Grace bears close watching. The high tide cycle this evening will
also bring near minor flooding conditions at the sites above, and
potentially DC Southwest Waterfront, while most other sites hover in
action stage. A cold front will move across the area this evening
and overnight, turning winds offshore and allowing anomalies to drop
off, decreasing the threat for tidal flooding at all sites through
the end of the work week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014.
     Coastal Flood Advisory from 6 AM to 11 AM EDT this morning for
     MDZ011.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 2 AM EDT Thursday for ANZ531>533-
     537-539>542.
     Small Craft Advisory from 6 AM this morning to 2 AM EDT
     Thursday for ANZ530-538.
     Small Craft Advisory until 5 AM EDT Thursday for ANZ534-543.
     Small Craft Advisory from 6 AM this morning to 11 PM EDT this
     evening for ANZ535.
     Small Craft Advisory until 11 PM EDT this evening for ANZ536.

&&

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