261
FXUS62 KCHS 260219
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1019 PM EDT Wed Sep 25 2024

.SYNOPSIS...
Hurricane Helene is forecast to track northward across the Gulf
of Mexico tonight, then move inland across portions of Florida
and Georgia Thursday into Friday. Multiple impacts are expected
across our area Thursday and Thursday night. Drier weather then
arrives later Friday and persists into the weekend.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Mid evening update: Initial surge of moisture continues to push
into southern/southeast Georgia with expansion of showers/
thunderstorms across Georgia...aided by strong upper level
divergence ahead of the detached closed low over the lower Ohio
Valley. Some semblance of an outer rainband is organizing off
the Florida coast and may advance up into southeast Georgia
overnight. Finally, also have a few showers moving through the
South Carolina coastal waters toward the tri-county area. Just
cosmetic tweaks made to the forecast to cover trends.

Previous discussion...
Early evening analysis reveals a nearly cutoff upper low over
the lower Ohio River Valley and sharp troughing down into the
northern Gulf region. Surface boundary extends from the eastern
Great Lakes to the southern Gulf Coast states with much of the
southeast situated in the warm sector. Helene continues to
gather strength while moving into the far southeastern
Gulf...and will be moving into a region of enhanced upper level
divergence and increasing northerly steering flow along the
southern side of the upper trough.

Axis of deeper moisture is already beginning to surge northward
into the southeast states well ahead of Helene, with showers and
some thunderstorms working into southern and southeast Georgia.
That trend will continue through the rest of the night with a
decent coverage of precip anticipated across southeast Georgia
by early Thursday morning. A few showers will also continue to
stream off the Atlantic and into southeast South Carolina as
well. Only minor tweaks were made to the going forecast with the
early evening update.

Given precipitation and persistent cloud cover, overnight low
temperatures will remain mild, only dropping into the low 70s
inland and mid to even upper 70s along the coastline.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...
Thursday and Thursday night: Hurricane Helene will be located in the
eastern Gulf of Mexico at daybreak and is forecast to make landfall
as a Major Hurricane across the Florida Panhandle around nightfall.
In addition, the fast forward speed while Helene crosses the coast
will result in a farther inland penetration of strong winds than is
typical. We lie on the stronger east side of Helene, and as a result
we have the entire forecast area covered by a Tropical Storm
Warning. Even though Helene is far inland, it`s broad expanse will
bring multiple hazards to the immediate area. For full details
please refer to weather.gov/chs and our Hurricane Local Statement
(HLS).

Tornadoes: The risk for tropical cyclone induced tornadoes will be
present across the entire area later Thursday into Thursday night.
Tornadoes are certainly a concern at any time, but since the risk
continues after dark, this makes the situation even more
treacherous. Have a tornado plan in place in case a warning is
issued, but keep in mind that tornadoes of this type can form with
little advance warning and move very quickly (potentially in excess
of 45 to 55 mph). The tornado risk is driven by long and looping
hodographs, with sufficient buoyancy (dew points in the mid
70s) and favorable SRH (200-300 m2/s2). The SCP and STP is also
highlighting the risk, as is the updraft helicity swaths as
shown by the HREF, coinciding with the rainbands associated with
Helene.

Winds: As mentioned above, a Tropical Storm Warning is in effect for
the entire forecast and warning area. Based on the 5 pm NHC
advisory, the Earliest Reasonable Arrival Time of Tropical Storm
Forecast Winds (39+ mph) in our area is mid to late afternoon across
Georgia, and early evening across South Carolina. The Most Likely
Arrival of these winds is early evening across Georgia and
during Thursday night across South Carolina.

The potential exists for downed and/or uprooted trees, isolated to
scattered power outages due to these winds, as wind gusts could
reach as high as 40 or 50 mph when the strongest winds occur
Thursday night.

Please note that NHC cautions users to be aware that the Tropical
Storm probabilities are likely running low due to the unusual size
and fast forward motion of Helene.

Flooding Rainfall: Multiple rounds of showers/rainbands in
association with Helene will begin to impact the area Thursday
morning (starting with our Georgia counties, then overspreading the
South Carolina counties in the afternoon). Showers and a isolated
rumble of thunder will then persist through Thursday night into
early Friday morning, before much drier air overtakes the area
Friday and coverage dramatically diminishes. Rainfall totals are
mainly in the 1.5 to 3 inch range, but with locally higher amounts.
There is even a non-zero chance of 5 inches or more in a few spots.
The risk currently does not support the issuance of a Flood Watch,
with the dry soils in many places per NASA Sport guidance. That
said, area rivers and creeks will rise in response to the rainfall,
with some flooding possible, even after the rains come to an end.

Storm Surge: The latest forecast has storm surge values in the 1 to
3 foot range. As such, coastal flood products might be required at a
later time. Additional information can be found in the Tides/Coastal
Flooding section below.

Temperatures: Low confidence on how warm we get Thursday, but to
blend with the surrounding offices and to lean toward the previous
forecast, we have temperatures below normal, generally about 80-83F
degrees. Thursday night will be warm, and we could be close to
record high minimums for KCHS and KSAV.

Friday: Helene will be Tropical Storm, eventually transitioning to a
Post Tropical Cyclone as it moves from northwest Georgia early,
across Tennessee and into Kentucky late, with dry air locally
wrapping around the southern flank of the storm. This is due to a
cutoff low in the vicinity of the borders of Tennessee, Alabama, and
Mississippi. As this occurs, the main barotropic low will absorb
Helene, and that system  will rapidly become baroclinic. The
absorption of Helene will feature a potent dry conveyor belt on the
southern flank of the storm, with a weak front or trough expected to
pass through Friday morning. That feature will be more of moisture
boundary, as PWATs fall from 2.0"+ (in association with Helene) to
around 1-1.25" during the afternoon. Tropical Headlines will likely
start to come down, but even so, deep mixing and the broad
circulation around Helene will cause a breezy to perhaps windy day.
We might need to consider non-tropical wind headlines as a result,
such as a Wind Advisory for Lake Moultrie. With some sunshine
potentially returning, we should be able to warm into the mid and
upper 80s, although the previous rains could hold temperatures a bit
lower.

Saturday: A mid and upper low with absorb the remnants of Helene
across Kentucky as it meanders through the day. While there is an
increase in moisture within a deep southwest flow out of the Gulf of
Mexico, forcing is lacking, so we do anticipate a rainfree day and
much less wind for any recovery efforts that are necessary.
Sufficient insolation and a full day of drier conditions will yield
max temperatures in the mid and upper 80s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
A mid and upper low initially over the Ohio Valley will eventually
open into a wave as it reaches the Mid-Atlantic coast Tuesday. A
broad trough then takes hold over much of the eastern U.S. on
Wednesday. Since the bulk of the energy stays away from the forecast
counties much of that time, this suggests little to no chance of
convection through the period. Temperatures look to average close to
climo.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
KCHS/KJZI/KSAV: We keep conditions VFR tonight into Thursday morning
at the South Carolina terminals, and through much of the night at
KSAV, before they drop into the MVFR range as showers start
developing in advance of Helene. Prior to then there is a small
chance of a few light showers at all sites. But since any impacts
look to be negligible, we did not include in the forecasts at this
time. Showers will then become more widespread as we progress
through the day on Thursday, as Helene gains latitude and moves
toward the Florida Panhandle from the Gulf of Mexico. E and SE winds
will also slowly increase, reaching around 15 kt sustained with
gusts near 25 kt to occur. Guidance provides mixed signals regarding
how low the cloud decks will get with the steadier and heavier rains
developing. While the statistical guidance implies IFR ceilings, the
HREF is showing MVFR, as is most of the forecast sounding. For now
we`ll keep low-end MVFR ceilings around 1000-1500 feet during the
afternoon, but IFR is certainly a possibility. There is also a risk
for a tornado at any or all of the terminals due to the rainbands
associated with Helene.

Extended Aviation Outlook:
Thursday night: Flight restrictions, potentially down to IFR, will
occur at all terminals due to impacts from Hurricane Helene. Pilots
and aviation interests can anticipate restrictions in both ceilings
and visibilities due to low cloud decks and moderate or heavy rains
at times. In addition, strengthening SE winds will eventually turn
more S, as speeds increase to at least 30-40 kt with higher gusts.
Depending upon the exact strength of the surface winds, there is a
risk for low level wind shear. There is also a potential for a
tornado at any or all of the terminals.

Friday: Much drier air will overtake the area as Helene pulls
further away as she crosses the Appalachians. VFR weather will
gradually return, and although winds will be less than during
Thursday night, they will still be strong and gusty as they veer
more SW.

Saturday through Monday: VFR will prevail. Little to no chance of
any convection.

&&

.MARINE...
Tonight: Conditions across the marine waters will begin to
deteriorate through the overnight period as Hurricane Helene
approaches the southeastern Gulf Coast. High pressure across the
local waters will continue to retreat into the Atlantic as Hurricane
Helene in the Gulf of Mexico works its way northward. The pressure
gradient will become pinched through the overnight as the region
will be placed between the approaching hurricane and the high
pressure further offshore. Southeast wind gusts will approach 20 to
25 knots with seas building to 5 to 6 ft across the marine
zones.

Thursday through Friday: Dangerous marine conditions will occur as
Helene strengthens across the eastern Gulf of Mexico during the day,
then moves onshore of the Florida Panhandle and into southwest
Georgia at night. Since this is such as large storm, and the coastal
waters is on the stronger eastern side, Tropical Storm Warnings have
been issued for all marine waters. Winds as high as 45 or 50 knots
will start impacting the Georgia waters Thursday evening, then
spreading into the South Carolina waters Thursday night. Winds will
then drop below Tropical Storm Force (34+ kt) during the late
morning or afternoon on Friday. Even after that, Small Craft
Advisories will be likely. Seas will build as high as 10-15 feet
within 20 nm from shore, and up to 17 feet on the outer Georgia
waters. Seas will only slowly subside on Friday.

Mariners are also alerted to the risk for waterspouts later Thursday
and Thursday night. Some of these will be Tornadic Waterspouts,
moving much quicker and producing much stronger winds than most
waterspouts in our area.

Saturday through Monday night: Much, much better marine conditions,
with the only period of a possible headline would be for a Small
Craft Advisory early Saturday due to lingering 6 foot seas on the
Georgia waters beyond 20 nm from shore.

Rip Currents and High Surf: Conditions at the beach will be
dangerous Thursday through Friday, as winds and seas build in
response to Hurricane Helene that moves through the northeast Gulf
of Mexico and into the interior of the Southeast. A High Risk of Rip
Currents will be in effect for all beaches both days, and breakers
will reach at least 5 feet in height starting Thursday afternoon and
persisting through at least Thursday afternoon or evening. Areas of
significant beach erosion are also likely during this time.

&&

.TIDES/COASTAL FLOODING...
Strengthening onshore winds will lead to a continuation of the
elevated tides Thursday, and minor coastal flooding is likely along
the Charleston and Colleton County coasts with the afternoon high
tide. The threat for coastal flooding is then expected for the
entire coast through Thursday night and early Friday due to impacts
from Hurricane Helene. Storm surge inundation of 1 to 3 ft is
possible along the coast of southeast Georgia and Southern South
Carolina. Keep in mind that if the higher surge and/or heavy rains
occur near or during the time of the high tides, the coastal
flooding could be worse.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141.
     High Risk for Rip Currents from 8 AM EDT Thursday through Thursday
     evening for GAZ117-119-139-141.
     High Surf Advisory from 2 PM Thursday to 5 PM EDT Friday for
     GAZ117-119-139-141.
SC...Tropical Storm Warning for SCZ040-042>045-047>052.
     High Risk for Rip Currents from 8 AM EDT Thursday through Thursday
     evening for SCZ048>051.
     High Surf Advisory from 5 PM Thursday to 8 PM EDT Friday for
     SCZ048>051.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...