348
FXUS62 KCAE 242348
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
748 PM EDT Tue Sep 24 2024

.SYNOPSIS...
High pressure over the SE US will keep warm conditions over the
Southeast through mid week. Multiple shortwaves crossing the
region will bring chances for precipitation through midweek.
Confidence is increasing that impacts due to the tropical system
in the Gulf will be felt in the forecast area Thursday into
Friday, but uncertainty remains in the exact track and impacts
of the system.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Key Message(s):

- Isolated shower possible overnight

Convection will largely subside over the area this evening,
however a lingering shower will still be possible overnight as
shortwave energy lifts over the ridge centered to our southeast.
The highest rain chances will be across the northern Midlands,
further from the ridge and where moisture is deeper. Low
stratus will likely redevelop, as it has the past few nights,
and push southward towards morning. Overnight lows should be
limited by clouds and very moist air mass with lows expected to
be in the upper 60s to around 70 degrees.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Key Message(s):

- Quiet weather expected on Wednesday
- TS Helene begins to influence our weather on Thursday
- Heavy rainfall, tornadoes, and gusty winds are all expected
  threats from Helene

Complex and impactful period of weather is on tap with the synoptic
pattern setting the stage for Tropical Storm Helene to enter the
stage and take our full attention. We`ll increasingly see height
falls across the area, with the deep upper level trough currently
across the midwest digging into the central MS Valley by midday on
Wednesday. A strong upper level jet streak will increasingly take on
an anti-cyclonic curvature, placing the southeastern US in the
favorable right-entrance region. By this point, interactions with
the large convective envelope of Helene appear likely, resulting in
widespread cloud cover developing across much of the southeast and
numerous showers and storms developing to our west as moisture
increases ahead of an approaching front. We will likely see limited
impacts on Wed/Wed Night as we will be too far east of the
front/parent trough and too far north of Helene to see substantial
impacts. Look for highs in the mid to upper 80s, potentially
modulated by how much high cloud cover develops. Lows overnight will
likely be warm in the low to mid 70s as that cloud cover increases
in coverage.

Significant impacts are expected locally from Tropical Storm Helene,
beginning as early as midday Thursday and lasting through Friday
morning. Forecast guidance is in good agreement on Helene entering
the Gulf of Mexico on Wednesday and beginning to push north-
northeastward on Wednesday night and Thursday. This is in response
to a cutoff mid/upper level low that is forecast to dig southward
into the Ozarks on Thursday. The combination of favorable synoptic
conditions, as well as a very warm Gulf of Mexico, is forecast to
allow Helene to quickly intensify into a Hurricane during the day on
Thursday, accelerating to the north as it does so. Locally, rainfall
is likely to begin across the western Midlands and CSRA by midday
Thursday. The right-entrance region of the aforementioned upper
level jet streak is forecast to be in place across the area, as well
strong 850 hPa warm advection pushing towards the area in advance of
Helene. Temperatures are likely to be held down in general by the
amount of clouds but if widespread rain does materialize on
Thursday, temps in the western CWA may not make it out of the mid
70s.

The most significant impacts to the region are likely to be felt
between 9p Thursday and 9a Friday. By this point, Helene is forecast
to make landfall in Florida sometime between 6p and 9p on Thursday,
rapidly pushing northward as the upper-level low to our west draws
it into its synoptic sphere. This is a very unique setup in that all
guidance - deterministic models and their ensembles and ensemble
members - show very little weakening of the central pressure as
Helene rapidly moves into central GA on Thursday night. As a result
of this, the core is expected to remain in tact as it approaches the
forecast area, resulting in widespread, heavy rainfall and a flash
flooding threat along the path of the center. Guidance suggests that
3-6" of rain is possible with this activity, with isolated higher
amounts possible. The heaviest rain will likely fall along and just
east and west of the track so any deviations will have an impact on
how much rainfall one area sees.

Given the expectation of lesser than normal weakening, the wind
field looks to remain in place as well. The GFS/ECM/Can all show 65-
75 knot winds in place at 850 hPa as the center passes to our west.
This is a two fold problem: one, gradient winds could be gusting in
excess of 40 mph on Thursday night and early Friday. NBM & LREF
probabilities for this are >70% for the 24h period ending 7a
Saturday, so confidence is high in this possibility. The second
threat that this strong wind field yields is a threat for nocturnal
tornadoes. Climatologically, this type of low track is very
favorable for tornadoes in our forecast area with tropical systems.
This setup is unique in that there will be a robust wind field &
synoptic forcing to help augment a lack of SBCAPE, of which the
deterministic models are actually yielding sufficient low-level
values of 250-500+ j/kg across parts of the area on Thursday night.
This looks to be a better tornado threat than we have seen with
recent tropical systems, so please have multiple ways to get
warnings on Thursday night. Ultimately, this looks like quite the
punch from a tropical system across our FA, with heavy rainfall,
tornadoes, and gusty winds all possible on Thursday night and Friday
morning. Please stay abreast of the latest forecasts!

Friday should see things die down a bit, especially as we get into
the afternoon hours. Helene will push off to the north through the
day, with dry air wrapping around the back side. Strong low-level
winds may continue to mix to the surface in the wake of Helene but
these will wane during the day.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Key Message(s):

- Helene becomes absorbed by upper level low on Friday night and
  Saturday
- Cutoff low hangs around through early next week

This period is taking lower priority due to our expected impacts
from Tropical Storm Helene. Look for persistent chances for clouds
and showers/storms in the afternoon as the aforementioned upper
level low will slowly meander eastward through the rest of the
period. Highs and lows will be seasonal for this period.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions for most of tonight with a period of MVFR/IFR
restrictions possible around sunrise Wednesday morning.

VFR conditions are expected to continue this evening as any
remaining showers decay over the next few hours. Later tonight,
a return of the low MVFR/IFR stratus is forecast to redevelop
and push southward into the terminals between 10-12z, with those
conditions then lasting through 14-15z once again. Winds mainly
southerly between 5-10 kts this evening becoming light and
variable overnight. Windspeeds pick back up after sunrise out of
the south or southeast with some gusts of 15 to 20 kts,
especially Wednesday afternoon.


EXTENDED AVIATION OUTLOOK...Some restrictions in stratus
possible along with strong winds becoming possible Thursday
through Saturday associated with Tropical Storm Helene moving
west of the forecast area.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
SC...None.
GA...None.

&&

$$