-- Discussion containing changed information from previous version are highlighted. --
975
FXUS62 KRAH 241833
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
230 PM EDT Tue Sep 24 2024

.SYNOPSIS...
Mid-level disturbances, in moist, southwesterly flow aloft, will
interact with a stalled surface front that will weaken over the
Carolinas through mid-week. Tropical Cyclone Helene is forecast to
make landfall in the Florida Big Bend late Thursday, then move
quickly north, while weakening, through the southern Appalachians
and OH Valley Friday and Friday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1150 AM Monday...

...Threat of Severe Storms and Flash Flooding mainly across the
northern/western Piedmont this afternoon-tonight...

The overnight-early morning round of (elevated) convection, forced
along and north of a slowly nwd retreating surface warm front over
the nrn NC Piedmont, has weakened in the past few hours while moving
away from the front and into more-stable air over nern NC/srn-cntl
VA.

This front, analyzed at 1530Z from near EWN to RDU to BCB, will
likely remain nearly stationary through tonight, where it will
provide a renewed focus for training storms capable of severe
hazards with diurnal heating this afternoon and evening and flash
flooding throughout the night. WPC has coordinated a focused,
categorical upgrade to a slight risk of excessive rain along the
VA/NC border with the upcoming DY1 update; and we`ll assess the need
for a similarly focused Flash Flood Watch with the afternoon
forecast package. Additionally, the redevelopment of a modest low
level jet this evening will result in an increase in low-level shear
(~100-150 m2/s2 effective SRH) focused along and on the immediate
cool side of the front, such that an isolated tornado will be
possible and accompany the previously noted risk of damaging wind
and hail.


.PREV DISCUSSION... /ISSUED 335 AM EDT Tue Sep 24 2024/

Very moist air will remain in place through tonight as a strongly
amplifying north stream trough digs south through the central Plains
today and into the middle MS Valley tonight. In response to this
digging upstream trough,  the upper ridge centered over the SE US
will move off the Atlantic coast. Meanwhile, at the surface, the
stalled front over SW portions of the Carolinas, though weakening,
will remain in place.

Multiple rounds of showers and storms are expected through tonight,
especially across the NW Piedmont. The first round of scattered
showers and storms is currently ongoing as weak disturbances eject
east across the area. Convective coverage is forecast to
expand/blossom in coverage, mainly across the northern Piedmont and
northern coastal plain, as the storms move east through late
morning/midday.

Round two is expected during the afternoon, with tempered daytime
heating/insolation within the moist 2-2.2" PWATS fueling moderate
buoyancy and the development of scattered showers and storms. The
greatest coverage is expected INVOF the weakening frontal zone
extending over the western Piedmont. Deep layer shear of 30-40 kts
will be sufficient to support a severe cluster or two, with damaging
winds and hail the primary threats.

Finally, the approach of the upper trough dynamics and continued
disturbances moving through the southern Appalachians will support
the re-development of showers and storms through the overnight
hours, with the bulk of the convection expected over the western
Piedmont. The multiple rounds of showers and storms will also bring
a threat of flash flooding this afternoon and into tonight. The
greatest flooding threat also appears to be across the western
Piedmont, where HREF PPM of exceeding 1"/3 hrs are maximized at 50-
70% and probability of exceeding 3"/6 hr are maximized at 40-50%.

Under extensive lows clouds, highs in the lower/mid 70s north to
lower/mid 80s south. Lows 65-70.

&&

.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 335 AM Tuesday...

An upper low at the base of the upper trough extending from the
Great Lakes to the Lower MS Valley will become cut-off Wednesday.
Central NC will lie sandwiched between this  wave trough and the
upper anticyclone off the SE US coast. We will actually see a brief
reprieve from the moist airmass as drier air off the SE coast
spreads west into the area. Meanwhile, the deeper plume of moisture
will get funneled up the spine of the Appalachians as eventual
Tropical Cyclone Helene gains strength as it moves north into the
GOM. At the surface, there are still signs of the stalled front
across southwest Piedmont.

After a late morning/mid day lull in precip/rain, isolated to
scattered storms will develop during the afternoon, with the best
coverage expected across the western Piedmont.

We`ll still have widespread cloudiness, with morning stratus lifting
throughout the day. Slightly warmer with highs ranging from near 80
north t o mid 80s south. Lows 65-70.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --
As of 230 PM Tuesday...

The 11am track of Tropical Storm (eventually Hurricane) Helene calls
for a landfall along the Big Bend region of Florida Thursday
evening, moving up through northern Georgia Friday morning, and
becoming a post-tropical cyclone over Indiana by Saturday morning.
The cone for Helene remains to the west of our forecast area,
although the cone specifically forecasts the center of the storm.
Helene is expected to be a larger than typical storm and impacts
will be felt well outside of the cone.

The impacts from Helene should start making their way into the
forecast area Thursday, with the greatest rainfall intensity and
wind speeds occurring Thursday night and Friday. It`s too early to
look at specifics, but multiple inches of rain are likely across
western counties, with lesser amounts expected to the east. It also
appears that the wind and wind gusts will remain below tropical
storm strength of 40 mph, with the higher values remaining to the
west.

The forecast should begin to dry out by Friday night as the
circulation from Helene moves over the Midwest. However, as Helene
becomes post-tropical and develops warm/cold fronts, scattered
thunderstorms should remain in the forecast through early next week.

Normal late-September temperatures are around 80/60 degrees, and
Saturday is expected to be the warmest day with slightly above
normal highs. Lows will be above normal throughout the forecast.
-- End Changed Discussion --

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
As of 159 PM Tuesday...

Anomalous moisture embedded within swly flow will continue to
generate MVFR/IFR stratus along a stalled boundary over the northern
Piedmont.  This will effectively keep KINT/KGSO and perhaps KRDU sub-
VFR through this evening.  KFAY has lifted to VFR the past hour or
so and KRWI may follow suit here in the next few hours.  Regardless,
all sites will sock back down to IFR/LIFR later tonight through
overnight period.  The cloud deck will slowly erode from south to
north through Wednesday afternoon (KINT/KGSO may very likely remain
sub-VFR however through the end of the 24 hour TAF period).

In addition to stratus restrictions, mountain showers and storms
will likely move sw to ne through the overnight period. Some of
these storms may trickle across the Piedmont and possibly impact
KINT/KGSO and even KRDU.  Additional showers and storms may be
possible Wednesday afternoon, primarily near KINT/KGSO.

Outlook: Under a persistent anomalous moisture regime, late
night/early morning sub-VFR ceilings and visibilities will be
possible each day in the outlook period. Additional showers and
storms will also be possible each day. While there is still some
uncertainty wrt to the evolution of Tropical Storm Helene, it does
appear that heavy rain and perhaps gusty winds will be possible late
Thursday night through early Saturday morning (highest impacts as of
now would be likely at KINT/KGSO).

&&

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

&&

$$
SYNOPSIS...MWS/CBL
NEAR TERM...MWS/CBL
SHORT TERM...CBL
LONG TERM...Green
AVIATION...Luchetti