834
FXUS61 KLWX 270119
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
919 PM EDT Thu Sep 26 2024

.SYNOPSIS...
A weak cool front approaching from the north will usher in drier
conditions to parts of the region. A few showers may develop or
persist through tonight. Hurricane Helene will make landfall along
the northeastern Gulf Coast tonight, then moves into the
southern Appalachians and Tennessee River Valley Friday into the
weekend. Widespread rainfall possible Friday, especially across
parts of Central Virginia. High pressure will build in from the
north this weekend. Rain chances remain into next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
KLWX radar loop as of 9:15PM shows isolated to scattered rain
showers moving across the area with the majority of
precipitation confined south of the area. The KIAD 00Z sounding
was noticeably drier at the surface compared to last nights
with 76 percent RH in the low levels(compared to 96 percent
yesterday). Precipitation chances will gradually increase from
southwest to northeast overnight as Hurricane Helene tracks
northward. Low clouds and drizzle/mist will continue on and off
throughout the night ahead of rain showers.

Previous Discussion Follows:
In response to upcoming rainfall tonight and through the day on
Friday, a Flood Watch remains in effect late this evening
through early Friday evening for parts of the central Shenandoah
Valley and Virginia Piedmont. Additional flood watches or
warnings may be issued during the course of this evening.

Lows tonight will drop into the middle to upper 60s.

Hurricane Helene is expected to make landfall this evening
along the northeast Gulf of Mexico, then track northward into
the Southern Appalachians tonight.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Through the day Friday into Friday night, Hurricane Helene is
expected to become absorbed into an upper low over the Lower
Mississippi Valley as it moves north then northwesterly into the
Tennessee Valley and remain over the region over the weekend.

Widespread showers, heavy at times, are expected to overspread
the region from south to north in parts of Central Virginia and
the Central Shenandoah Valley. There remains a very strong
signal for a significant Predecessor Rain Event (PRE) across the
Southern Appalachians into Friday. Rainfall expectations still
look like 1 to 3 inches.

Along with the rainfall will be the wind. Winds could gust over
40 mph at higher elevation along the central Blue Ridge and over
the highest elevations in the Appalachians.

As for Saturday, the absorbed former tropical system and the
upper level low combined over the Tennessee Valley will
gradually propagate eastward later Saturday and Saturday night.
Near normal temperatures in the 70s.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
By Sunday, the remnants of Helene will have been absorbed into a
large cutoff low over the Tennessee Valley. This low is expected to
gradually weaken and advance eastward through early next week,
currently progged to move offshore by late Tuesday. Continued
chances for showers and possible storms are expected during this
time as a result, as lingering moisture and remnant energy move
through. There is significant uncertainty for further details as the
outcome will be dependent on the interaction of the low and Helene`s
remnants and the resultant position of the low, as well as how far
south the surface ridge over the Northeast extends.

A break from the rainy conditions is possible by late Tuesday.
Guidance has trended weaker and drier with a cold front sweeping
through late Tuesday into Wednesday associated with an upper trough
over the Great Lakes. PoPs for Wednesday have dropped to slight
chance. Regardless of what unfolds, a drier and cooler air mass is
expected behind the front.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
Widespread MVFR to IFR conditions will become VFR throughout
the afternoon as the low clouds erode.

This will be a short lived reprieve as low clouds build back
in tonight. IFR to possible LIFR CIGs expected at all terminals
late tonight into Friday. The remnants of Hurricane Helene are
expected to bring widespread showers Friday.

Conditions likely remain sub-VFR into Saturday due to a stalled
frontal boundary over the region.

Shower and possible thunderstorm chances continue into early
next week as remnants of Helene make their way east.
Additionally, persistent E/NE flow is expected with surface high
pressure to the north, so sub-VFR conditions remain possible at
times, especially during any shower activity near the
terminals.

&&

.MARINE...
SCA in effect for Friday afternoon through Friday night.
Widespread showers move across the local waters Friday into
Friday night.

Easterly to northeasterly onshore flow is expected Sunday into
Monday. Winds over the waters could approach SCA criteria,
especially during the afternoons, with the tight pressure gradient
as the remnants of Helene move east from the Tennessee Valley
towards the Mid-Atlantic/Carolina coast.

&&

.TIDES/COASTAL FLOODING...
Decreasing astronomical tides will lead to slightly lower (though
still elevated) tides, with bouts of minor flooding likely around
times of high tide through Friday morning. The higher of the two
daily tides is during the overnight/early morning hours, and would
be susceptible to the most widespread minor to perhaps locally near
moderate tidal flooding (particularly at Annapolis for moderate).

By Friday, Helene will orbit around an upper low to the southwest as
high pressure holds firmly to the north. This pattern results in
enhanced onshore east to southeast flow at times through early next
week , and has the potential to result in multiple rounds of
widespread minor to moderate tidal flooding through at least
Sunday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 8 AM EDT Friday for DCZ001.
     Coastal Flood Watch from late Friday night through Saturday
     morning for DCZ001.
MD...Small Craft Advisory from 4 PM Friday to 3 AM EDT Saturday for
     MDZ008.
     Coastal Flood Advisory from 1 AM to 6 AM EDT Friday for MDZ011.
     Coastal Flood Advisory until 8 PM EDT Friday for MDZ014.
     Coastal Flood Watch from Friday evening through Saturday
     morning for MDZ014.
     Coastal Flood Advisory until 2 AM EDT Friday for MDZ016-018.
     Coastal Flood Advisory from 5 AM to 9 AM EDT Friday for MDZ508.
VA...Coastal Flood Advisory until 2 AM EDT Friday for VAZ057.
     Flood Watch from 2 AM EDT Friday through Friday evening for
     VAZ025-036>040-503-504-507-508.
     Wind Advisory from noon to 7 PM EDT Friday for VAZ503-504-507-
     508.
WV...Wind Advisory from noon to 7 PM EDT Friday for WVZ501-505-506.
MARINE...Small Craft Advisory from 4 PM Friday to 3 AM EDT Saturday for
     ANZ530>532-538>540.
     Small Craft Advisory from 1 PM Friday to 3 AM EDT Saturday for
     ANZ533>537-541>543.

&&

$$

SYNOPSIS...KLW/KRR
NEAR TERM...AVS/KLW
SHORT TERM...KLW/KRR
LONG TERM...CAS
AVIATION...KLW/CAS
MARINE...KLW/CAS
TIDES/COASTAL FLOODING...LWX