-- Discussion containing changed information from previous version are highlighted. --
659
FXUS62 KRAH 232320
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
725 PM EDT Mon Sep 23 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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 355 PM Monday...

A sub-tropical, 500 mb anticyclone along the Gulf coast will
continue to drift ewd and across nrn FL tonight, while its
accompanying ridge will progress across the South Atlantic states.
Convectively-perturbed, wswly to swly flow will be directed from the
lwr MS Valley to the OH Valley and cntl Appalachians, between that
ridge and a couple of shortwave troughs that will pivot across the
Upr Midwest and mid MS Valley. Cntl NC will consequently remain in
anticyclonic flow and a subsident regime through the mid/upr-levels
through early tonight, after which time the convectively-perturbed
flow will spread ewd and across the wrn and w-cntl Carolinas. Weak
low-level jet development, with ~20-25 kt sswly flow likely at 925
mb, will likely result across the Carolinas overnight. That flow
will be directed nearly perpendicular to a surface front, now
accompanying a weak frontal low over cntl SC, which may retreat
slightly nwd across nrn SC and into perhaps the far srn NC Piedmont
through this evening. WAA related to the LLJ will favor both a
gradual increase in MUCAPE, elevated atop the stable boundary and
stratus layer that has remained in place over the NC Piedmont today,
and also isentropic upglide that will increase the risk of elevated
showers and isolated storms over cntl NC (particularly areas from I-
95 wwd) overnight-Tue morning. Widespread low overcast should
otherwise redevelop in the unseasonably moist/humid regime
characterized by PWs of 1.75-2", with low temperatures in the 60s.

&&

.SHORT TERM /TUESDAY AND TUESDAY NIGHT/...
As of 245 PM Monday...

The stalled boundary is forecast to be draped NW to SE from the
southern/western Piedmont, arcing into the southern Sandhills to
start the period. With time, the boundary will lift north as a warm
front during the afternoon and evening, perhaps reaching southern VA
come Wed morning. Aloft, shortwave troughing will be present across
the Mid-Mississippi Valley, while ridging will be over the southwest
Atlantic. The resultant flow will be southwesterly, with a few
disturbances tracking through in the morning, aftn/eve, and
overnight period.

Morning isolated to scattered showers or storms will be possible
across eastern sections of central NC as a disturbance tracks across
the area. The highest PoPs are favored over the Triangle,
east/northeast Piedmont, and into the Coastal Plain. Outside of
these showers, widespread low stratus and patchy fog will be present
north of the aforementioned boundary. As the front starts to lift
north, however, we should see erosion of the stratus from SW to NE,
slowest in the north and northwest. As a result, highs should range
below normal in the mid 70s in the far north, to above normal in the
mid 80s in the S and SW.

Another round of scattered showers and storms will develop in the
late afternoon/evening, continuing into the overnight hours as a
secondary disturbance tracks through. The front and differential
heating will aid forcing for ascent, along with 2000-2500 J/kg of
MLCAPE. A Marginal Risk of severe storms is still in place across
the western Piedmont, owing to deep-layer shear of 35-40 kts and
DCAPE of 800-1000 J/kg, supportive of a damaging wind threat.
Isolated large hail cannot be ruled out as well, given nearly
uniform hodographs in place. CAMs vary on where storms initiate, but
a general consensus appears across the far western Piedmont, before
advancing east-northeast in the evening to overnight. Highest PoPs
are across the NW and Triad and lowest in the SE.

As we go into the late-evening hours, storms appear to become at
least partially elevated, and PW`s upwards of 2 inches (140-percent
of normal) with SW WAA aloft will support a flash flood threat in
training storms atop the lingering boundary. The 12z HREF is showing
a signal for 2-3+ inch rainfall totals in two areas. One area
stretches along and just SW of the Triad into VA, while a second
area stretches along US-1 into the Triangle. Given these two
regions, WPC has introduced a Marginal Risk for excessive rainfall
over much of central NC to cover the uncertainty. Convective
coverage should start to favor northern areas overnight into Wed
morning as the boundary slowly shifts north. Overnight lows will be
in the mid to upper 60s.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
As of 335 PM Monday...

Wednesday and Wednesday night: Aloft, a low will cutoff and meander
over the ARKLATEX region as the northern stream trough continues
eastward through the Great Lakes and Northeast. Meanwhile, high
pressure will ridge northwestward through the Carolinas/lower mid-
Atlantic. At the surface, high pressure over eastern Canada will
ridge sswwd along the Northeast and mid-Atlantic coasts. Meanwhile,
what is currently Potential Tropical Cyclone Nine will lift
northward into the eastern Gulf of Mexico, then is expected to
deepen and move quickly nwd through the eastern Gulf toward the Big
Bend area of FL. An inverted trough will extend nwd from the Gulf to
along the srn/ctl Appalachians, west of the aforementioned ridge, as
another trough extends ssewd through the OH Valley/nrn Appalachians
from a low moving ewd through the nrn Great Lakes. The confluent
sly/sely flow will advect warm, moist air into the area. There will
likely be plentiful cloud cover, which may limit daytime heating and
thus potential instability. The 12Z NAM forecast soundings show
MUCAPE around 1000 J/Kg, effective shear around 30 kts and PWATs in
the 1.5-1.8 inch range across central NC during the aft/eve. Given
the warm, moist, potentially unstable environment, expect any
ongoing showers to continue into the day Wed and cannot rule out
some storms as well. Highs should range from around 80 degrees NW to
mid 80s south with lows mainly in the mid 60s to around 70 degrees.

Thursday and Friday: A Rex Block will set up over the eastern US,
with the low over the ARKLATEX and the low over southeast Canada,
bisected/blocked by a ridge extending nwwd from a high off the
Southeast US coast and another high over the Desert Southwest
ridging newd into the Great Lakes. The other feature at play is
Potential Tropical Cyclone Nine, which is forecast to move inland
near the Big Bend area of FL on Thu, lift nwd through the Southeast
US Thu night, then get wrapped up into the low over the ARKLATEX
Fri. It is yet to be determined if the Block will be strong enough
to keep PTC9 with the low or whether it will shear out over the nrn
mid-Atlantic Fri night. There is still some uncertainty with the
expected track of PTC9 as it moves nwd through the Southeast US and
where it goes thereafter. Latest operational model guidance suggests
the center may stay west of central NC (NC mtns), putting central NC
in the NE quadrant of the system on Fri. Heavy rainfall/flooding,
strong, gusty winds, and tornadoes will all be possible from Thu
night through Fri night. Latest rainfall forecast from the WPC has 2-
2.5 inches along the Yadkin River to half an inch/inch in the east.
With the overcast, rainy conditions Thu and Fri, highs should be
near to slightly above normal, mid/upper 70s to low/mid 80s, while
lows will be well above normal, in the mid/upper 60s to low 70s.

Saturday onward: The mid/upper level pattern may become more of an
Omega block than a Rex block over the weekend as the low to the
northeast shifts sewd over the nrn Atlantic, pushing the high/sub-
tropical ridge wwd, while the other low sits over the ARKLATEX
region. At the surface, as the remnants of PTC9 tries to continue
nwd along the Appalachians Fri night/Sat, high pressure over
southeast Canada will shift swwd through the Northeast and mid-
Atlantic coasts and overtake the system by Sat night/Sun. The medium-
range guidance diverges quite a bit over the weekend and it is
unclear whether high pressure will dominate or whether a low will
lift through and along the Southeast US coast late Sun/Mon. If, when
and how much precipitation there will be will depend on how that all
plays out, so will hold off on those specifics for now. As for
temperatures, generally expect temperatures to gradually decrease
from Sat to Mon.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
-- Changed Discussion --
As of 250 PM Monday...

IFR-MVFR ceilings are likely to redevelop in a persistently moist
regime in place overnight-Tue morning. The low overcast should then
slowly lift with diurnal heating once again during the late morning
through afternoon Tue. Additionally, there will be a chance of
showers, mainly west and south overnight. There will be a good
chance of showers/thunderstorms with diurnal heating on Tue, as a
series of disturbances aloft overspread and interact with a frontal
zone that will waver over the region.

Outlook: A persistently, unseasonably moist air mass in place over
cntl NC will favor repeated overnight-early day low ceilings and
visibility restrictions and also an above average chance of showers
and storms this week. The chance of both may be exacerbated by the
influence of Potential Tropical Cyclone Nine Thu-Sat.
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...Kren
LONG TERM...KC
AVIATION...Badgett/MWS