771
FXUS62 KRAH 261157
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
757 AM EDT Thu Sep 26 2024

.SYNOPSIS...
Tropical Cyclone Helene will move across the eastern Gulf of Mexico
today and cross the Florida Big Bend coast tonight. After
landfall, Helene is expected to turn northward across Georgia
tonight, then northwestward while slowing over the Tennessee Valley
on Friday and Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 445 AM Thursday...

It will be cloudy and warm, with scattered showers/storms today.
Widespread rain, embedded convection, and strengthening ely winds
will then spread across cntl NC from the southwest late tonight-Fri
morning.

In the mid/upper-levels, a cyclone initially over wrn KY will move
in a cyclonic arc into the TN Valley through 12Z Fri. Strong sly
steering flow between that cyclone and a sub-tropical/lower mid-
latitude ridge that will drift across the wrn Atlantic toward
Bermuda will draw TC Helene generally nwd and into the srn
Appalachians. A series of mesoscale vorticity maxima will precede
Helene and track through the wrn half of NC. The most notable of
these MCVs was evident in regional radar data over nern GA this
morning, with a series of embedded ones extending sewd along a
convective band across the Savannah Basin. These features should
track across the Piedmont of NC this morning and provide the initial
focus for convection today.

Moist sely flow in a continued warm and unseasonably humid warm
sector will otherwise remain over cntl NC, while a quasi-stationary
front and principle focus for widespread showers/storms will remain
over the Foothills and ern slopes of the Appalachians. Widespread,
multi-layered (and lowering mid/high-level) overcast will keep
temperatures a category or two less warm than those of Wed, however.

Strong height falls aloft and sely, warm and moist advection
preceding Helene will spread nwd into cntl NC late tonight-early Fri
morning. While there remains timing differences in how quickly that
regime arrives, based on the ultimate track/timing of Helene, a mix
of stratiform rain and embedded convection should consequently
become widespread across at least the srn/wrn Piedmont and Sandhills
through Fri morning. Although HREF guidance is not particularly
strong in its signal for simulated updraft helicity, intense wind
fields, including up to several hundred m2/s2 effective SRH, and 250-
750 J/kg of boundary-layer based instability, will support a risk of
mini-supercells and TC tornadoes with the aforementioned embedded
convection - most likely across the srn Piedmont, Sandhills, and srn
Coastal Plain based on the official NHC forecast track and timing of
Helene. Rainfall amounts will generally range from a quarter to half
inch over northeast zones to three quarters of an inch to an inch
over the far srn/wrn Piedmont. Strengthening ely winds may gust
between 25-35 kts by daybreak over the srn/wrn Piedmont, in the
tightest MSLP gradient as Helene approaches, ranging to just 15-20
kts over the nrn-cntl Coastal Plain. Near record high minimum low
temperatures will again be in the low-mid 70s.

&&

.SHORT TERM /FRIDAY AND FRIDAY NIGHT/...
As of 400 AM Thursday...

...Strong winds, brief heavy rain, and increasing tornado threat
will develop Friday into the late afternoon hours as Helene
approaches...

The track of Tropical Cyclone Helene remains relatively unchanged
and is forecast to be rapidly moving into the southern Appalachians
Fri morning then transitioning into an extratropical cyclone as it
settles into the lower Ohio Valley Fri evening. A feeder band
extending ESE from the center of Helene is forecast to be pivoting
across the Carolinas and into central NC sometime Fri morning. There
are some timing differences with the 00z Hi-Res guidance on when
this band will lift into the area (the HRRR and Fv3 are the fastest,
while the ARW and NSSL are slowest, while the NAMNest is somewhere
in between). The timing of this band will be important and likely
bring the heaviest rain and greatest tornado threat with the later
arrival resulting in an increased tornado threat as diurnal heating
results in greater surface instability within the strong kinematic
field.

The tornado threat will quickly lift north through the area during
the morning into the early afternoon as an area of very dry mid-
level air spreads over central NC in its wake. Point soundings
suggest some lingering saturation below 800mb will still be present
with dew points in the low/mid 70s contributing to 500-1000 J/kg
SBCAPE. Dry mid-level air should limit overall storm depth given the
marginal instability, but a non-zero tornado threat may continue
into the early afternoon if low-topped supercells can tap into
lingering 100-200 0-1km SRH.

Given the expected strength of Helene and its fast forward motion,
30-40 mph wind gusts will extend well away from its center. Latest
00z Hi-Res guidance suggests these strong winds will be possible,
mainly west of the I-95 corridor and, with current forecast timing,
maximized around or shortly after sunrise with a gradual weakening
as the main feeder band lifts north into VA. If the slower solutions
trend true, then frequent gusts of 35-45 mph will be possible into
the early afternoon and a Wind Advisory may be needed. Given
saturated soils from prior rainfall over the western Piedmont and
along and north of the I-40 corridor in the past 7 days, these winds
may cause sporadic to scattered power outages.

Heavy rain to torrential downpours within the feeder band should be
expected as it lifts through central NC. However, given its overall
quick lift north and it being the lone concern for heavy rain,
forecast rainfall totals for Fri only range from 0.5 to around 1
inch. 00z HREF guidance suggests within this widespread band of
rain, streaks of up to 2 inches will be possible, and would bring a
flash flood risk to mainly urban areas, but also portions of the NC
Piedmont where prior rainfall has reduced FFG to < 2 inches in 3hrs.

&&.

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 400 AM Thursday...

A stacked low will be situated over the lower Ohio Valley after the
remnants of Helene get absorbed into the closed low by Sat morning.
Most of the day should remain dry as a steady streamer of very dry
mid-level air remains over central NC. The core of the closed low
and associated moisture trapped in its center should slide east
towards the southern Appalachians and may support some showers
overnight into early Sun morning. Instability looks to be minimal
given the unfavorable diurnal timing so thunder is not expected.

Forecast uncertainty begins to arise Sun through Tues as EPS and
GEFS differ on the evolution of the mid-level low and its associated
cloud cover and shower/storm potential. The GEFS shows a more
eastward progression over the Mid-Atlantic (greater cloud cover and
shower/storm chances) while the EPS is slower and lifts the moisture
through the Ohio Valley into northern Mid-Atlantic (less cloud cover
and less shower/storm chances for central NC). Temperatures will
trend towards near normal during this time (mid 70s to low 80s)

Despite the uncertainties in the early week, ensembles are in
relatively good agreement on an area of surface high pressure
spreading across the central CONUS and into southern/central
Appalachians Wed night into Thurs morning. Deep layer northwest to
north winds, rising surface pressure and PWAT falling to at-or-below
1 inch would favor a drier period heading into midweek. Highs will
remain near normal with overnight lows well above normal as dew
points linger in the 60s.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 755 AM Thursday...

Mid-level disturbances and an axis of tropical moisture well in
advance of Helene will result in waves of showers and storms over
cntl NC today, along with IFR-MVFR ceilings mainly over the
Piedmont. As Helene nears the srn Appalachians late tonight-early
Friday, widespread rain and embedded thunderstorms will overspread
cntl NC, as will IFR-MVFR restrictions; and strong and gusty surface
winds; and a risk of low level wind shear.

Outlook: Conditions will improve to VFR from S to N later Fri,
lasting through Sat. A chance of showers/storms will return esp to
the NW Sun into Mon.

&&

.CLIMATE...

Record High Minimum Temperatures:

September 26:
KGSO: 69/2005
KRDU: 73/1993
KFAY: 73/1930

September 27:
KGSO: 69/2011
KRDU: 73/2011
KFAY: 73/2017

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...Swiggett
LONG TERM...Swiggett
AVIATION...MWS/Hartfield
CLIMATE...