786
FXUS61 KLWX 241445
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1045 AM EDT Tue Sep 24 2024

.SYNOPSIS...
A front remains stalled near the area, as strong high pressure
across eastern Canada builds southward along the Appalachians.
Cloudy conditions, with passing clouds and periods of light rain
continue through mid week. A strong front and area of low pressure
over the Great Lakes and Upper Midwest will approach through the
middle of the week. High pressure builds to the north late in the
week, while a tropical area of low pressure approaches from the
south.

&&

.NEAR TERM /THROUGH TONIGHT/...
Damp and dreary conditions look to continue for a majority of the
region over the next 24 to 36 hours. It`s a CAD wedge setup today
with strong eastern Canadian high pressure to the north wedging
south and a pseudo-stationary/warm frontal boundary that is draped
across south-central VA into the Shenandoah and Ohio River Valleys.
Weak isentropic lift associated with the boundary to the south
combined with a rich-moist boundary layer has resulted in ongoing
shower/drizzle activity across the region. 12z CAM guidance
continues to show an uptick in this activity heading into the
afternoon and evening as the front waffles to the south and
approaching upper level trough/weak backdoor front push over the
region.

As a result, expected cloudy conditions to continue along with
periods of light to moderate rainfall into the front half of the
afternoon. With upper level divergence increasing aloft and the
upper level trough/weak front swinging overhead later this afternoon
and evening there is the potential for a few pockets of heavier rain
and perhaps a few thunderstorms. The highest potential for
convection remains along and west of the Allegheny Front and shown
across the southern Blue Ridge where the wedge has less of an
influence. SPC has maintained a Marginal Risk (level 1 of 5) for
severe weather right along the Allegheny Front and into Highland
County. This accounts for scattered thunderstorms moving into the
mountains from the west, a couple of which could be strong and
capable of producing damaging wind gusts and hail.

High res models also continue to be in good agreement for some
possible heavier precipitation in the south-central Shenandoah
Valley into Central VA this evening into tonight. PWATS per the 12z
IAD sounding sit at 1.75 inches with 1.60 inch values at RNK.
Rainfall amounts of 1-2" are forecast, with higher amounts up to 3"
possible. Given the ongoing drought, there does not appear to be a
signal for widespread flooding given the stable wedge leading to
more stratiform vs. convective precipitation.  However, some urban
or poor drainage areas could see some minor flooding if heavy rain
occurs there.

Very little temperature change expected today due to the clouds and
showers. Highs will struggle to reach the mid to upper 60s & low
70s. Lows tonight settle into the low to mid 60s with continued
rain chances chances, low clouds, and fog.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As we continue into the middle of the week the mid/upper-level
synoptic pattern is likely to amplify across our region. To our
west, an approaching upper trough cuts off over the Mid-South to
Lower MS Valley on Wednesday, at the same time a mid-level ridge
builds offshore of the Carolinas. The parent upper trough skirts by
just north of the area. Overall we`ll be sandwiched in between all
three features, though the building ridge looks to play the biggest
role. At the surface, Canadian high pressure starts to move
offshore of southern New England, though southeast winds remain in
place over our area - keeping the pseudo-CAD in place.

A wave of low pressure aloft on the eastern periphery of the cut off
low brings us scattered to possibly widespread showers sometime
Wednesday. Confidence for that is higher west of the Blue Ridge,
with areas to the east possibly seeing somewhat drier conditions
than Tuesday. Slightly higher afternoon temps in the low to mid 70s,
and very mild Tuesday night as lows settle in the mid to upper 60s.

For Thursday, the surface high weakens and pulls away from the area,
allowing weak southerly flow to develop. The mid-level ridge
weakens, but remains in place giving us a day of mostly dry
conditions. Cloudy skies stick around as temps warm up a bit more to
the upper 70s to around 80F.

Thursday night we monitor increasing tropical moisture into the area
as current Potential Tropical Cyclone 9 makes landfall along the
northeastern Gulf of Mexico. Rain chances look to increase late
Thursday night for areas west of I-95 and south of US-50. There is
low confidence in the forecast at this point, and will be highly
dependent on where the tropical low tracks as it moves northward
into the Southern Appalachians and/or TN Valley.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
A large cutoff upper low looks to be positioned somewhere over the
Deep South, with a tropical cyclone orbiting around it to our south.
The exact evolution of the upper low, tropical system, and another
trough to the north are all in flux and thus still uncertain. A
strong surface high to the north may tend to keep the deepest
tropical moisture and strongest winds to the south, though at least
some rain potential is evident Friday into the weekend. Uncertainty
increases with exactly how the potential tropical system and upper
low interact, and how the conglomerate ejects late in the weekend
into early next week. At some point, there may be a potential for
heavy rain and perhaps some gusty winds depending on the structure
of the low as it passes, and how closely it passes. This setup with
persistent onshore flow could result in substantial coastal flooding
(see Tides/Coastal Flooding section below).

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
Sub-VFR conditions are expected to continue across the area through
Wednesday night due to a stalled frontal boundary, and strong high
pressure building in from the north. MVFR to IFR ceilings prevail
through the day, with the lowest ceilings at IAD, MRB, and CHO. A
large area of showers (some thunderstorms possible) will cross the
area this afternoon, bringing reduced visibility. IFR to possibly
LIFR CIGs are likely tonight into Wednesday morning. Slight
improvement Wednesday, but still MVFR to IFR conditions, then likely
IFR Wednesday night.

Some improvement is likely on Thursday due to high pressure aloft
briefly building over the region. Conditions should dry out, and
while cloudy skies are expected, ceilings could reach VFR during the
afternoon.

Sub-VFR conditions are likely Friday into Saturday as onshore flow
continues between high pressure to the north and low pressure to the
south. The extent of rain or any gusty winds remains uncertain and
depends on the exact track of low pressure moving up from the
south.

&&

.MARINE...
Strong Canadian high pressure building south into the area today is
going to cause an increase in east to southeast winds across the
waters. A Small Craft Advisory goes into effect late this afternoon
for the Chesapeake Bay and surrounding rivers/tributaries, and the
Lower Tidal Potomac. Expect winds to gust around 20-22 knots this
evening into tonight.

Winds are forecast to fall below SCA conditions overnight, and
remain that way through Wednesday morning. Another period of
enhanced winds is possible Wednesday afternoon to evening, which
could necessitate additional SCAs.

Conditions improve on Thursday in south winds around 10 knots across
all the waters.

Onshore flow will likely strengthen Friday and perhaps into Saturday
depending on the evolution of low pressure approaching from the
south. SCAs are likely at times.

&&

.TIDES/COASTAL FLOODING...
Continued onshore flow will result in multiple rounds of coastal
flooding over the next several days. Astronomical tides decrease
through the rest of the week, and flow may become weak enough to
allow water levels to fall slightly Thursday. But, increased onshore
flow Friday into perhaps Saturday could result in more significant
tidal flooding depending on the evolution of low pressure that is
expected to approach from the south.

Although spread in the possible outcomes is large, moderate tidal
flooding is a possibility Friday into Saturday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Warning until 4 AM EDT Wednesday for DCZ001.
MD...Small Craft Advisory from 4 PM this afternoon to 3 AM EDT
     Wednesday for MDZ008.
     Coastal Flood Warning until 4 AM EDT Wednesday for MDZ014.
     Coastal Flood Advisory until 6 AM EDT Wednesday for MDZ011.
     Coastal Flood Advisory until noon EDT Wednesday for MDZ016-018.
     Coastal Flood Warning until 10 PM EDT this evening for MDZ017.
     Coastal Flood Advisory until 10 AM EDT Wednesday for MDZ508.
VA...Coastal Flood Warning until 4 AM EDT Wednesday for VAZ054.
     Coastal Flood Advisory until noon EDT Wednesday for VAZ057.
WV...None.
MARINE...Small Craft Advisory from 4 PM this afternoon to 3 AM EDT
     Wednesday for ANZ530>534-537>543.

&&

$$

SYNOPSIS...KRR
NEAR TERM...EST
SHORT TERM...KRR
LONG TERM...DHOF
AVIATION...DHOF/KRR
MARINE...DHOF/KRR
TIDES/COASTAL FLOODING...LWX