461
FXUS62 KRAH 270812
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
412 AM EDT Fri Sep 27 2024

.SYNOPSIS...
What is now Hurricane Helene, centered near the Florida Georgia
border, will weaken gradually as it races northward through
Georgia tonight through early Friday morning. The storm will
then slow down and weaken further to a depression as it moves
northwestward over Tennessee into southwest Kentucky through
Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 400 AM Friday...

Tropical Cyclone Helene made landfall at 1110 PM last night as a
category 4 hurricane with max sustained winds of 140 mph near the
Florida Big Bend. The rapid northerly movement of this strong storm
of 25 to 30 mph continues as it plows into through the
Southeast. HREF ensemble mean MSLP moves Helene into northern
GA by 12z this morning where the official forecast shows Helene
still as a warm-core tropical storm. Helene will undergo extra-
tropical transition as it becomes absorbed into an occluded low
over the lower Ohio Valley through the evening hours. Hi-Res
guidance has come into good agreement that a feeder band of
convection within a low-level convergence axis will begin to
lift north into our area between 600 and 800 AM and progress
east-northeast across central NC through the early afternoon.
This feeder band will bring multiple hazards to central NC today
and each are discussed below with conditions rapidly improving
in its wake by mid/late afternoon.

Wind: The strength TC Helene was able to reach, rapid inland
movement, and expanding wind field as it begins to transition
into an extra-tropical cyclone will combine to bring strong to
locally damaging winds to portions of central NC. Wind gusts
over northern GA and SC early this morning, well in advance of
the core of of Helene, range from roughly 25-40 kts (30 to 45
mph). These winds will spread north as Helene approaches and
into most of central NC, strongest in the western Piedmont. A
blend of latest hi-res guidance suggest that sporadic gusts of
55 mph will be possible over the western Piedmont shortly after
sunrise into the mid morning hours before gradually weakening
into the afternoon.

Tornado: An incredibly strong kinematic environment will continue to
spread into central NC through the early morning into the afternoon
hours. Large looping hodographs, especially in the low-levels,
will result in 250 to over 400 m2/s2 of 0-1 km SRH in advance of
and within the convective band as it moves through central NC.
Tornado potential will be maximized from early this morning over
the southern Piedmont, Sandhills, and into the southern/central
Coastal Plain and spreading north into the early afternoon
hours. The greatest uncertainty will be the degree of
instability can be achieved. MLCAPE of 500 J/kg of instability
will overspread most of the area and will likely support
rotating updrafts that will be capable of producing short-lived
tornadoes anywhere in central NC. Greater instability closer to
1000 J/kg of MLCAPE is expected over the Sandhills into the
southern/central Coastal Plain in advance of the convective line
and may result in a greater/stronger tornado threat as updrafts
are able to further deepen and may result in a more classic
supercellular structures.

Flooding: PWAT values will continue to climb into the 2 to 2.5 inch
range as Helene approaches which will likely break daily records at
GSO/MHX with the 12z RAOBs. This anomalous moisture will fuel heavy
rain and torrential downpours within the axis of convection as
it pivots across the area. Quick storm motions will thankfully
keep totals more manageable for most locations (0.5 to 1.5),
but swaths of 2-3 inches will be possible anywhere across
central NC and may lead to flash flooding especially in urban
and poor drainage locations.

&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
As of 400 AM Friday...

A vertically stacked low pressure system will be gradually filling
over the lower Ohio Valley by 12z Saturday. The axis of rich dew
points in the low/mid 70s will be shifting out over the Atlantic
with a streamer of dry mid-level air overhead. After some early
morning fog and low overcast, generally clearing skies and fair
weather diurnal cumulus clouds bubbling during the afternoon. A
disturbance rotating around the base of the closed low to our west,
may bring a passing shower to the western Piedmont during the
evening into the overnight hours, but overall, expect a dry forecast
for a majority of the day. Temperatures will peak in the low/mid
80s during the afternoon with overnight low settling in the 60s.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
As of 325 AM Friday...

A chance of showers and a few storms will persist Sun into Tue,
mainly each afternoon/evening and primarily across the N. Dry
weather will follow for mid week.

The upper low which will have absorbed Helene is expected to drift
slowly ENE over the Ohio Valley Sun/Mon, moving from NW KY to N WV.
This will keep our area within SW flow aloft and in the warm sector
at the surface, between the surface low well to our WNW and high
pressure centered off the Southeast coast. A diffuse frontal zone
extending E of the KY low across VA combined with upslope-directed
low level flow suggest the better chances for sufficient focus and
lift will be across the N including the Triad/Triangle regions Sun
and Mon. A surface low will develop in W NC early Mon and move into
central NC Mon night, bringing a surface cold front with it. As this
low is expected to deepen a bit as it pushes east off the SE VA/NE
NC coast by late Tue, we should see a shift of better rain chances
into NE sections Tue. The upper low over N WV will finally start to
shear out and weaken into an open wave before shifting E and
offshore Tue night, propelled by a strong northern stream wave
pushing through S Canada and the N CONUS, reaching Quebec and the
Northeast states Wed and pushing off the Northeast coast Thu. This
will put our area into a drier NW flow Wed, prompting a trend to a
dry forecast, then a secondary cold front associated with the strong
northern stream wave will move through Wed night, drawing a cooler
air mass into NC by Thu. Models are in fairly good agreement on this
scenario, although the Canadian has a deeper low over N Ontario mid
week and as such is cooler in our area by Thu. Will have temps 3-8
degrees above normal Sun/Mon, 1-6 degrees above normal Tue/Wed,
then within a degree or two of normal Thu, although if the
Canadian is correct, it could be even cooler Wed and esp Thu.
-GIH

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
As of 110 AM Friday...

Adverse aviation conditions are expected over much of central NC,
primarily all but the far NE, from early this morning through
this afternoon. The worst conditions, which will be in the S and
W Piedmont, will include low cigs, periodic strong wind gusts,
and a few hours of low level wind shear. The center of what is
now Hurricane Helene is located over extreme S GA and will track
quickly north through GA through daybreak. While the current
rainfall over central NC is mostly light, cigs are largely IFR
(with some MVFR far NE), and these conditions should hold
through this morning, with gradual improvement to MVFR then VFR
S to N through the day. This lull in rain will last until around
10z, when a strong tropical rainband will begin to lift NE
through central NC, lasting through about 20z. As this rainband
passes through central NC terminals SW to NE, at any given
location, a few hours of torrential rains producing MVFR-IFR
vsbys are likely, along with enhanced wind gusts and a potential
for isolated tornadoes. In addition, a period of low level wind
shear is expected, primarily during 10z-16z this morning,
however strong surface gusts and very strong and shifting winds
with height will dominate through this entire TAF valid period
at all sites, dangerous aviation conditions for smaller aircraft
in particular. Once the intense tropical rainband lifts N and
NE of the area, after about 21z, VFR conditions are expected
through the end of the TAF valid period. Surface winds will be
mainly from E and ESE, shifting to be from the SE and SSE this
morning, and finally from the S this afternoon, gradually
diminishing starting mid to late afternoon.

Looking beyond 06z Sat, sub-VFR cigs may return esp across the NE
(RDU/RWI) after 08z early Sat morning, lasting until improving to
VFR by mid morning. Otherwise, VFR conditions will dominate through
Tue, with possible exceptions late Sun (rain chances mainly
INT/GSO), Mon afternoon (rain chances areawide), and early Tue
morning (chance of sub-VFR fog). -GIH

&&

.HYDROLOGY...
As of 305 PM Thursday...

Will issue a Flood Watch for the Piedmont and Sandhills region of
central NC for tonight through 6pm Friday.

Multiple rounds and bands of rain and thunderstorms will spread
north across central NC through Friday. Given recent wet
conditions, streamflows across Central North Carolina are
significantly above normal per 714 day USGS stream flows. NASA
SPoRT soil moisture percentiles are above the 90th percentile in the
010 cm and 040 cm layer across the southern coastal plain. In
addition, 1-3 hr hour Flash Flood Guidance is essentially below 2
inches across most of the Piedmont.

While the heaviest rain associated with Helene will be focused
across the foothills and mountains, a general 1-3 inches with
embedded heavier rain rate will cause a risk of flash flooding
through late Friday afternoon. This will be especially true where
creeks and streams are already swollen, and there are even some
areas with ongoing road closures owing to rainfall over the past 2
to 3 days. Lower rainfall totals and a reduced threat of locally
heavy rain across the northern and central Coastal Plain results in
a reduced flood threat and have omitted those locations from the
watch.

Main stem river flooding is not expected to be as significant of an
issue based on the 1-3 inch rain forecast, although heavy rates in
the vicinity of the Rocky River near Norwood, which drain from west
to east and may see heavier rain upstream, and also the routing of
significantly heavier rainfall in the upper reaches of the Yadkin
River may cause some flooding in the Yadkin basin. The Neuse and Tar
are also abnormally high with the Neuse at Smithfield currently in
minor flood.

&&

.CLIMATE...
Record High Minimum Temperatures:

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

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for NCZ007-021>025-038>042-
073>077-083>086-088-089.
Wind Advisory until 6 PM EDT this evening for NCZ007-021>025-
038>042-073>077-083>086-088-089.

&&

$$

SYNOPSIS...Hartfield
NEAR TERM...Swiggett
SHORT TERM...Swiggett
LONG TERM...Hartfield
AVIATION...Hartfield
HYDROLOGY...Blaes
CLIMATE...RAH