868
FXUS62 KRAH 250516
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
115 AM EDT Wed 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 1055 PM Tuesday...

A stationary front separating dew points in the upper-60s from dew
points in the lower-to-mid-70s currently stretches from between KTDF
and KRDU SE to between KRWI and KGSB. The SBCAPE gradient on SPC
mesoanalysis also depicts the front well, as far NE zones are very
stable while south and west of the front has SBCAPE as high as 1500
J/kg. The air mass is very moist as well, as there is deep S/SW flow
aloft between offshore ridging and a mid-level low centered over
northern Missouri. 925-850 mb moisture transport is maximized
directly over central NC, and PW values are in the 1.7 to 2 inch
range.

Moisture interacting with the aforementioned stalled front has been
bringing lines and clusters of heavy showers and storms training
across parts of the northern and western Piedmont this evening. One
line is near the VA border that will be moving ESE into Person,
Granville, Vance and Warren counties over the next few hours. A
cluster of showers and storms to the SW of the Triangle will be
moving NE into those urban areas shortly, and additional rounds of
heavy rain currently over SW NC and NW SC will move ENE across the
northern Piedmont overnight. Latest HRRR runs indicate the greatest
threat of heavy rain will be from KRDU to the north and west, which
is where a Slight Risk (level 2 of 4) of excessive rainfall is in
effect. It won`t take much rain to get flash flooding, as some
localized spots have already received as much as 1 to 3 inches this
evening. Additional localized amounts of 1 to 3 inches will be
possible where training occurs, and very localized amounts exceeding
this can`t even be ruled out. Probabilities of heavy rain begin to
decrease after 06z as instability wanes, but some convection is
likely to persist in this corridor through 12z, and with already
saturated ground, a threat of flash flooding will continue. For
these reasons, a Flood Watch is in effect for our tier of counties
bordering VA (plus Durham, Wake and Franklin counties) from now
until 8 AM tomorrow morning.

Farther south and east away from the front, a mostly dry night is
expected, but an isolated shower can`t be ruled out. Widespread
cloud cover will keep low temperatures extremely mild across central
NC, in the upper-60s to lower-70s.

&&

.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 /05Z WEDNESDAY THROUGH SUNDAY/...
As of 115 AM Wednesday...

Heavy rain showers and thunderstorms will continue to move E-NE
through the area and will impact the northern terminals through 10z,
before lifting off to the north. Outside of convection, IFR to LIFR
stratus restrictions are expected through this morning, followed by
gradual lifting to VFR by 18z as drier air temporary spreads into
the area. Isolated to widely scattered showers and storms could
develop during the afternoon. However due to expected limited
coverage, will not include any mention at this time.

Widespread IFR to LIFR restrictions are expected to re-develop
area-wide tonight, with the lowest restrictions expected at
KINT and KGSO and KRDU.


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 Friday evening/night, with greatest impacts
expected 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...Danco
SHORT TERM...MWS
LONG TERM...Green
AVIATION...Badgett/Luchetti