312
FXUS61 KLWX 241857
AFDLWX

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

.SYNOPSIS...
A stalled frontal boundary draped across the Ohio River Valley and
central Appalachians region will keep cloudy conditions and shower
chances in play through Wednesday. A cold front will work across the
region Thursday while Tropical Storm Helene lifts northward from the
Gulf of Mexico into the southern Appalachains and Tennessee River
Valley Friday bringing widespread rain chances to the area. High
pressure tries to build in from the north this weekend, but rain
chances remain as a cutoff low spins over the Ohio and Tennessee
River Valleys.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
A pseudo CAD wedge setup today with strong eastern Canadian
high pressure to the north wedging south and a pseudo-
stationary/warm frontal boundary draped across south-central VA
into the Shenandoah and Ohio River Valleys. Isentropic lift
associated with the boundary to the south combined with a rich-
moist boundary layer has resulted in ongoing shower activity
across the region. 12z CAM guidance continues to show an uptick
in this activity heading into the late afternoon and evening
commute as the front oscillates to the south and approaching
upper level trough/weak front push over the region. This is
evident per the latest radar trends as of 230pm with light to
moderate showers pushing from southwest to northeast across
central and northern VA into central MD. Further west into
north- central WV strong to severe thunderstorms have blossomed
due to less cloud coverage and added instability. This activity
will continue to push east toward the mountains this afternoon
before losing intensity as it crosses the mountains and works
into the stable wedge environment overhead this evening. Highest
probabilities for severe weather will be in areas along and
west of the US-220 corridor (this includes areas west of the
Allegheny Front). 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.

Besides the risk for severe weather comes the concern of localized
flash flooding due to several rounds of training showers and
embedded thunderstorms. Hi-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 inches
vat 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 upper 60s to around 70F.
Lows tonight settle into the low to mid 60s with continued rain
chances chances, low clouds, and fog.

Similar conditions are expected Wednesday as the pseudo CAD wedge is
slowly erodes. We`ll continue to monitor an approaching upper trough
that cuts off over the Mid-South/Lower MS Valley on Wednesday with a
mid-level ridge builds offshore of the Carolinas. Meanwhile, eyes
will turn toward the Gulf Coast for what potential impacts lie ahead
from now Tropical Storm Helene.

With all that said, expect another day of clouds, showers, and
perhaps a few thunderstorms Wednesday into Wednesday evening. This
is due to a wave of low pressure aloft on the eastern periphery of
the cut off low brings us scattered to possibly widespread showers
sometime Wednesday afternoon. Highest confidence for showers will be
along and west of the Blue Ridge with somewhat drier
conditions/subsidence further east. Slightly higher afternoon
temps in the low to mid 70s, and very mild Tuesday night as lows
settle in the mid to upper 60s.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
Surface high pressure to the north will continue to weaken while
shunting offshore Thursday allowing for weak southerly flow to
ensue. Mid-level ridging aloft will also start to weaken off the
VA/NC coast. Enough ridging should remain though rain and storm
chances fairly isolated to scattered across the region. Highest
confidence for precipitation Thursday morning into Thursday
afternoon will be in areas along and west of the Blue Ridge with
drier conditions further east. Cloudy skies stick around with
perhaps a few filtered breaks of sun yielding temperatures as in
the mid to upper 70s to around 80 degrees.

Thursday night, we monitor the eventual track and placement of
Tropical Storm Helene. The current track from NHC as of 2pm has
Helene making landfall as a major hurricane somewhere in the Big
Bend of Florida Thursday evening before tracking north into the
southern Appalachians and Tennessee River Valley Friday. Outer
rain bands from Helene look to move into southern portions of
west of I- 95 and south of US-50 as early as Thursday night and
continuing into Friday. The latest track keeps the heaviest rain
south and west of our region but given the width of the cone of
uncertainty forecast confidence remains low at this time.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Impacts late in the week and into the weekend will be somewhat based
on the track of Tropical Cyclone Helene. A lot of uncertainty this
far north exists with the track as well as an upper low that will
likely be nearby later this week. Heavy rain and gusty winds will be
possible heading into Friday and Saturday depending on the track
with the best chances for this to occur being across areas along and
south of I-66 and west of the Blue Ridge Mountains. Highs during the
long-term period will slowly trend cooler, with mid to upper 70s
expected Friday and upper 60s to low 70s expected by Monday
afternoon. Winds will likely be elevated out of the east Friday into
Saturday before slowly diminishing heading later in the week. A lot
of uncertainties for the long-term with a number of moving parts
that will play a pivotal role into what comes to fruition.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Sub-VFR will likely continue through Wednesday evening with a
pseudo- stationary CAD wedge overhead and front nearby. MVFR to
IFR (brief periods LIFR) ceilings will prevail through the
remainder of the day especially at terminals west of a line from
KBWI-KDCA-KRIC. Light to moderate showers will continue to work
through the corridor over the next 2-4 hours between
now-21z/5pm. Further west of KMRB and KSHD will continue to
monitor the progression of a line of strong to severe
thunderstorms. This activity should weaken as it pushes toward
KCBE and eventually KMRB and KCHO heading into the late
afternoon and evening hours between 21-01z/5-9pm. This activity
will bring additional reductions as it pushes through. IFR to
possibly LIFR CIGs are likely tonight into Wednesday morning and
perhaps early Wednesday afternoon. Slight improvement Wednesday
afternoon and evening, but still MVFR to IFR conditions, then
likely IFR Wednesday night.

Conditions slowly improve Thursday as weak southerly flow ensue
with mid-level ridging overhead. Conditions should dry out for
the corridor terminals with perhaps a few showers and
thunderstorms mainly at terminals along and west of a line from
KMRB to KCHO/Gashed. Ceilings should remain MVFR in these
locations with VFR further east.

Sub-VFR ceilings are becoming increasingly likely Friday into
Saturday given the onshore easterly flow across the area.
Precipitation and wind gusts will be highly dependent on the track
of the tropical cyclone moving north from the Gulf of Mexico late in
the week.


&&

.MARINE...
Strong east to southeast onshore flow will remain across the
waters through Wednesday. 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 gusts around 18-23 knots this evening into
tonight.

Winds will fall back below SCA levels tonight and remain at sub-SCA
levels through Wednesday morning into Wednesday afternoon.
Additional SCAs may be needed Wednesday evening into Wednesday
night. 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 until 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 until 3 AM EDT Wednesday for ANZ530>534-
     537>543.

&&

$$

SYNOPSIS...EST
NEAR TERM...EST
SHORT TERM...EST
LONG TERM...ADM
AVIATION...ADM/EST
MARINE...ADM/KRR/EST
TIDES/COASTAL FLOODING...KRR/LFR