-- Discussion containing changed information from previous version are highlighted. --
436
FXUS62 KRAH 230540
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
140 AM EDT Mon Sep 23 2024

.SYNOPSIS...
A back-door cold front will move through central NC through this
evening, then stall and waver over SC and southwestern NC Monday
through Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 945 PM Sunday...

Behind the passage of an initial cold front and north to northeast
wind shift, a continued moist but cooler airmass has been deposited
over much of central NC. A secondary and drier cold front is seen in
regional observations and satellite imagery spilling southwest over
southeast VA and northeast NC. In between these two fronts over
central NC, the cooler and still moist airmass will promote the
development of a widespread blanket of stratus over most of the
area. Slowest to cloud over looks to be the southern Piedmont which
may be the favored area of some initial fog < 1 mile to develop.

Some moisture pooling in the upslope region, and gradual moist
isentropic ascent over the northwest Piedmont may allow for some
light drizzle to develop early Mon morning, but flow through the
saturated layer will be weak so confidence this will result in any
measurable precipitation is low given inconsistencies within high-
res guidance. Lows will gradually drop into the mid/upper 60s (W) to
low 60s (NE).

&&

.SHORT TERM /MONDAY AND MONDAY NIGHT/...
As of 415 PM Sunday...

A ridge accompanying a sub-tropical, 500 mb anticyclone along the
Gulf coast will progress across the Southeast through 12Z Tue, with
associated anticyclonic flow across cntl NC. Meanwhile, convectively-
perturbed, wswly to swly flow will be directed from the lwr MS
Valley to the OH Valley and cntl Appalachians, downstream of a
synoptic shortwave trough that will deamplify across the cntl Plains
and mid MS Valley.

At the surface, the backdoor cold front now progressing through cntl
NC will stall over w-cntl SC early Mon, then retreat nwd into the
srn NC Piedmont Mon afternoon-night. High pressure centered over
Atlantic Canada will otherwise extend swwd across the Northeast and
middle Atlantic, including ern and e-cntl NC.

Widespread low overcast will likely have developed over most of cntl
NC by daybreak Mon. This post-frontal, low overcast may be
accompanied by patchy light rain and drizzle over the nw Piedmont
through the morning, in the upslope regime there where point
forecast soundings depict saturation in the lowest 1000-2000 ft. The
early day clouds will lift and at least partially clear, especially
away from the nw Piedmont, with afternoon warming into the mid/upr
70s north to lwr 80s south. Low-level moisture pooling along and on
the immediate cool side of the surface front will favor weak to
moderate diurnal destabilization, strongest across the srn Piedmont
and Sandhills. There remains a signal in model guidance for an area
of showers and storms to move into the srn and wrn NC Piedmont
during the late afternoon-evening, with a risk of a strong storm or
two over the srn Piedmont, where both instability and
unidirectional, wnwly flow are forecast to maximize. A chance of
showers, and generally weaker storms, should then spread ewd through
the night, as PWs increase to between 1.75-2" and support continued
weak instability. Widespread low overcast should otherwise redevelop
in an unseasonably moist/humid regime, with low temperatures in the
60s.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
As of 248 PM Sunday...

The frontal boundary from Mon is likely to linger across the western
and southern Piedmont early Tue, before slowly shifting northward on
Wed. Aloft, models are showing an impulse of energy tied to a
shortwave in the OH valley tracking across the region in the SW flow
aloft. Morning stratus across the region is likely to also set up a
differential heating boundary as it slowly erodes over the southern
Piedmont and Sandhills, with highs mid 70s NE to mid 80s SW. While
SPC does not have us outlooked, models are showing MLCAPE between
1000-1500 J/kg with deep-layer shear of 30-35 kts, focused over the
southern Piedmont. This could pose a non-zero severe threat. As a
result, guidance is fairly high on shower/storm chances. Morning
showers are possible in the NE, but aftn/eve convection is favored
to develop first in the west/southern Piedmont and track ENE in the
evening to overnight.

By Wed, the surface boundary is largely favored to lift into VA as a
warm front. A cold front will be weakening to our west in the OH
Valley. Highest storm chances Wed will be over the NW in closer
proximity to the northward moving front and mid-level energy,
focused in the afternoon and evening with diurnal heating. Highs
should trend warmer than average in the upper 70s N to mid 80s S.

Thu to Sun: The overall synoptic pattern and potential impacts from
what develops in the Gulf remains uncertain. While the latest
deterministic GFS, ECMWF, and CMC show decent agreement that the
potential tropical system forecast to develop in the Gulf (80-
percent now from NHC) will get pull northward by troughing over the
mid to lower MS valley sometime late Thu or Fri over the SE US,
their respective ensembles still show a large spread in possible
scenarios. As a result, confidence on expected impacts from heavy
rainfall or gusty winds late this week to weekend, remain low. Where
the tropical system tracks ultimately will depend on the location of
the trough in the midsection of the country, as well as the ridge
off the SE coast, and whether any blocking sets up over the Great
Lakes or northern Great Plains. The inland landfall could be as far
west as the coast of AL or as far east as NE florida, per the ECENS
00z low tracks. For now, we have highest PoPs late Thu into Sat of
30-40 percent, decreasing to 20 percent by late in the weekend Sun.
This is likely to change, however, as models come into better
agreement in the coming days. Consequently, temperatures are also
uncertain, but for now are favored to be near or slightly below
average in the mid/upper 70s to around 80 Fri-Sun.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --
As of 140 AM Monday...

NELY low-level flow behind back the back-door cold front passage has
ushered in IFR to MVFR ceilings into the area. Ceilings should lower
further to LIFR through 12z, especially at KINT and KGSO, where the
low-level saturated layer will bank up against the eastern slopes.
Additional, patchy fog may promote reduce visibilities around
daybreak, with models currently showing the best fog signal over the
northern coastal plain, INVOF KRWI.

At KINT, KGSO, and KRDU:  The sub-VFR ceilings will be slow to lift
throughout the day, eventually lifting to MVFR during the afternoon.
Ceilings could temporary improve to VFR during the late
afternoon/early evening, but should be short-lived before ceilings
lower again during the evening and overnight hours

At KRWI and KFAY: Drier air across eastern NC should allow flight
conditions to improve much quicker, Ceilings at KRWI are expected to
lift to VFR by mid/late morning, while KFAY could be delayed
until the afternoon.

Precipitation wise: Scattered rain showers and storms are expected
to spread west to east across the area during the evening and
overnight hours, with the best rain chances expected at KINT and
KGSO.

Outlook: Expect daily scattered showers and storms through much of
the work week. A tropical system could bring impacts to our area by
Friday, but confidence is currently low. Additionally, sub-VFR
restrictions in fog/stratus will be possible each morning.
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...MWS
NEAR TERM...Swiggett
SHORT TERM...MWS
LONG TERM...Kren
AVIATION...CBL