172
FXUS62 KRAH 242012
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
412 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 and VA 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 410 PM Monday...

...A Flash Flood Watch has been issued for the northern NC Piedmont
for tonight...

...A threat of severe storms also remains, mainly across the
northern/western Piedmont this afternoon-tonight...

A warm front arcing this afternoon from the NC wrn Piedmont to the
cntl Coastal Plain will continue a slow retreat across the nrn
Piedmont, to near the VA border, through tonight. Storms that have
developed in the mountains and more recently ewd invof the warm
front over wrn NC will continue to spread ewd and tend to focus
along the retreating front. Training storms capable of severe
hazards will exist when instability will be maximized through this
evening, while a risk of flash flooding will exist, as convection
possibly trains along the front, throughout the night. 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. Unseasonably mild/humid
conditions with a redevelopment of widespread low overcast will
otherwise result, with low temperatures in the upr 60s to around 70.

&&

.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 345 PM Tuesday...

In the mid/upper-levels, an anticyclone over FL today will drift
newd to about midway between Bermuda and the South Atlantic coast,
while a cyclone settles across the mid-South. Deep sswly flow
between the two will provide the steering for Tropical Cyclone
Helene into the eastern Gulf of Mexico and also convectively-
enhanced disturbances from the cntl Gulf coast through the wrn
Carolinas.

At the surface, the quasi-stationary front now over cntl NC will
retreat nwwd into srn and cntl VA on Wed, while an upstream cold
front now moving across the TN and lwr MS Valleys will slow and
stall invof the srn Appalachians and GA. Aside from a sea breeze,
the main foci for convergence and convection will consequently
become oriented just to the north and west of cntl NC and where the
otherwise weakly-forced warm sector will exist.

While isolated showers and storms will be possible with diurnal
heating/destabilization, as early morning stratus retreats nwwd and
lifts/scatters within the warm sector and/or along the sea breeze,
the axis of convection will likely materialize through the wrn
Carolinas and especially upslope into the mountains.

temperatures should be more-uniformly warmer and in the 80s on Wed,
with continued unseasonably mild/humid conditions and lows in the mid
60s to near 70 Wed night.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
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.

&&

.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...
Flood Watch from 8 PM EDT this evening through Wednesday morning for
NCZ007>010-021>026-041.

&&

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