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000
FXUS61 KLWX 051852
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
252 PM EDT Sun Sep 5 2021

.SYNOPSIS...
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A cold front will cross the region from the northwest this
afternoon into tonight. High pressure will return later Monday
into Tuesday. Another cold front will progress through the area
during the middle of the week with high pressure building in by
late in the work week.
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&&

.NEAR TERM /THROUGH MONDAY/...
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On the large scale, broad cyclonic flow extends over the Great
Lakes region with a series of shortwaves tracking from the
Middle Mississippi Valley into the Ohio Valley. The more
prominent height falls/forcing will be displaced to the north,
while little if any instability exists in the warm sector
across the Mid-Atlantic. Steady light warm advection continues
to spawn shower activity along a mid/upper moisture axis over
the Tennessee Valley into the Central Appalachians. Lackluster
amounts are being observed with 3-hour totals in the 0.05 to
0.10 inch range. An area of disorganized showers will remain in
the picture through the evening which also supports below
average temperatures. Most locations within the thick cloud
cover are struggling to escape the low/mid 70s. Areas which are
seeing sunshine in central Virginia are seeing some low/mid 80s,
particularly around Spotsylvania, Stafford, and King George
Counties.

High-resolution models continue to show some local enhancement
in the activity overnight, generally from I-95 eastward. Most
notably, the 12Z NAM nest/HREF mean are showing such solutions
so have maintained higher chances for showers in this corridor.
All of this activity will be falling ahead of a eastward
propagating cold front. This frontal zone is forecast to exit
toward the Eastern Shore by Monday morning with winds shifting
to west-northwesterly through the day. Before this occurs, some
patchy fog may form in areas west of I-95, particularly along
the lower lying areas. Any fog should quickly burn off an hour
or so beyond sunrise. In the wake of the frontal passage, model
soundings general unidirectional west to northwesterly flow
which will raise temperatures a bit given downsloping effects.
This flow regime will combine with ample sunshine to bring highs
into the low to mid 80s. The usual cool spot is across the
mountains with mid 60s to low 70s being more likely.
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.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
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High pressure will continue to build off to the west leading to
some cooler overnight temperatures. Expect lows in the low/mid
60s inside the I-95 corridor, with 50s in most other spots.
Numerical models show progressive flow aloft which allows a
shortwave ridge to sweep through on Tuesday ahead of a strong
system spinning over the Upper Great Lakes. Warm advection in
the open warm sector will maintain mild weather with another day
in the mid 80s and abundant sunshine. The dry forecast continues
into the night with southerly winds keeping temperatures on the
warm side. Compared to early September norms, lows will be
around 4 to 8 degrees above average.
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&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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Models are in agreement with an upper level trough moving over the
Great Lakes on Wednesday. At the surface, a region of low pressure
will form over Canada, with a cold front extending south through the
local region. Showers and thunderstorms are possible along the
front, however the spatial coverage remains in question. Guidance
has trended slower with the frontal passage, meaning temperatures on
Wednesday may be a few degrees higher than previously forecast.
Wednesday could be the only day over the next week with a chance of
high temperatures near 90 degrees, mainly along and east of I-95.
Given the slower frontal passage, CAPE will increase to 500-1000
J/kg. The modest CAPE and ample shear (40-45 knots), could support
organized storms along the front. The biggest uncertainty remains if
storms will actually form this far south due to the better dynamics
being offset towards New England. There is the potential for at
least an isolated strong to severe wind gust. Given the progressive
nature of the already moisture lacking front, do think the flash
flood threat is low/minimal.

The winds will turn northwesterly behind the front and a region of
Canadian high pressure will move in through the end of the week.
This will bring high temperaturesback to the upper 70s to near 80
for most (60s in the mountains). Temperatures moderate a few degrees
Saturday as heights rise.

Attention will need to be paid to two features in the tropics this
week and how they evolve (Hurricane Larry off in the Atlantic and an
area being monitored for potential development in the Gulf of
Mexico). Both of these feature are currently not anticipated to
cause direct impacts to our region through the long term.
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&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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Disorganized showers continue to move from west to east this
afternoon. VFR conditions are being maintained, but a brief
reduction to sub-VFR is possible as the day presses on within
any more intense shower. The lack of instability maintains very
low chances for any thunder. Showers linger into the overnight,
particularly across terminals along and east of I-95. Some
patchy nighttime fog may develop around KCHO and KMRB.

Southerly winds will shift to west-northwesterly on Monday with
VFR conditions anticipated through Tuesday.

The winds will be southerly ahead of the cold front Wednesday. There
could be a period of sub VFR conditions Wednesday in any showers or
thunderstorms.VFR conditions return Thursday with light northwest
winds behind the front.
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&&

.MARINE...
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Expect southerly channeling of the winds across lower sections
of the Chesapeake Bay and tidal Potomac. Small Craft Advisories
are in effect through tonight for this pattern. Conditions
should remain below SCA levels in the west-northwesterly flow
behind the front on Monday. The resurgence of southerly warm
advection on Tuesday will raise marine winds to near SCA levels
again by the evening into the overnight.

There could be a period of SCA winds Wednesday into early Thursday
along and behind the cold front. SCA conditions are not likely to
end the week as high pressure builds.
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&&

.TIDES/COASTAL FLOODING...
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Southerly flow will allow anomalies to remain high today, with
highest ones near heavy fresh water inflows such as the tidal
Potomac as well as the middle Chesapeake Bay. As winds turn
westerly behind a weak cold front later tomorrow, anomalies
likely drop a bit, and this will also be assisted by a slow
diminishment of freshwater inflow.
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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 6 AM EDT Monday for ANZ534-543.
     Small Craft Advisory until 6 PM EDT this evening for ANZ537.
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&&

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