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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 081441
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1041 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 eastern Ontario with its attendant cold front draped
across the eastern Great Lakes into the Ohio Valley. Forecast
for potential severe thunderstorms later this afternoon and
evening is generally on track, although there is still some
question how much pre-frontal activity can occur this afternoon,
and if it does, how strong it will be. 12Z IAD sounding shows a
lot of dry air aloft couple with some mid level stable layers.
Most models show some semblance of these features lasting into
early evening, so updrafts could be thwarted. The evening round
with potential linear segments close to the front will also have
severe potential.

Slightly modified previous discussion (with latest thinking)
follows...

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 mid to late 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 multicells
and perhaps supercells. 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 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 fact may lead to
some locally heavy rain totals, especially in eastern parts 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 /15Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions expected through early or mid afternoon at a
minimum with southerly wind gusts beginning to increase.
A couple rounds of storms are expected to develop late 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. 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 evening round is higher
confidence. Gusty winds and heavy rain will be possible with the
storms.

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...
Southerly flow continues today. Winds are at a bit of a lull at
mid morning, but should increase some this afternoon. Therefore,
kept the Small Craft Advisory out through the day. 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...
-- Changed Discussion --
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. Locations in the northern parts of the bay, as well as DC
SW Waterfront, have the greatest chances of seeing minor
flooding. However, most are threshold type events, so some
uncertainty looms. Advisories will be issued based on the latest
trends. 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.
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Coastal Flood Advisory until 2 PM EDT this afternoon for
     MDZ508.
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.

&&

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