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045
FXUS62 KRAH 160112
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
911 PM EDT Sun Sep 15 2024

.SYNOPSIS...
Surface high pressure will continue to extend into the middle
Atlantic and central North Carolina from the northeast today.
Meanwhile, a non-tropical area of low pressure will strengthen along
a frontal boundary a few hundred miles off the southeastern U.S.
coastline as it drifts toward the coast of the Carolinas Monday and
Monday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 911 PM Sunday...

Moist NELY low-level flow between the wedge of high pressure
extending south down the eastern slopes of the central and southern
Appalachians and an area of low pressure off the southeast coast
will support spotty light rain showers overnight, mainly across
southeastern portions of the forecast area. While there is still
some considerable model spread in timing, steadier, more moderate
rain could spread inland into our southern Coastal Plain locations
around daybreak, with the heavier rainbands holding off until after
12z. Breezy with gusts generally 15 to 25 mph, strongest across the
SE. Lows ranging from lower/mid 60s north to upper 60s/near 70 SE.
-- End Changed Discussion --

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 335 PM Sunday...

...A low predictability but potential impactful system is forecast
to move through the Carolinas Monday into Tuesday...

There is high confidence that a low pressure system will form off
the SC/GA coast today. Regardless of any sub-tropical development,
this system has the potential to bring widespread moderate/heavy
rain, breezy to windy conditions, and threat for tornadoes to the
southern Mid-Atlantic Mon into Tues.

Uncertainty is unusually high at this time range on the eventual
track of this system, which will have a significant impact on which
areas will have the greatest flooding/tornado threat. The envelope
of solutions range from this system taking a westerly track and
moving through central SC (majority of GEFS members), to slowly
meandering off the southeast coast before lifting north through
eastern NC into central VA (most EPS/GEPS members). Hi-Res guidance
shows a "middle-of-the-road" solution where most HREF members track
the center of the low near the NC/SC border. Current forecast is
trended towards the Hi-Res solution, but expect the forecast to
change as the center of this storm develops.

The center was analyzed in the 17z ASCAT pass with a closed
circulation roughly 300 miles southeast of the Carolina coast as
well as a recon mission sampling the system as of 1930z. The
forecast confidence should rapidly increase over the next 12 to
24 hours as this center is assimilated into model guidance.

Flooding threat: A swath of 2-4 inches with locally higher amounts
around 6 inches appears most likely along and just east of the track
of the storm system as it moves through the southern Mid-Atlantic.
Isolated to scattered flash flooding will likely be the greatest
threat, especially in urban and poor drainage locations. Where this
swath occurs will be highly dependent on the eventual track which
will likely change and become more refined in the coming model runs.
Given the uncertainty and potentially impactful rain could occur
anywhere within central NC, a Flood Watch has been issued for the
whole area from 12z Mon to 12z Tues.

Wind/Severe threat: A tight pressure gradient already in place will
further tighten as the surface low tracks towards the region.
Northeast winds will increase with sustained winds 15-20 mph with
gusts between 25-40 mph as the system approaches Monday and
diurnal heating mixes down strongest winds just off the surface.
At this time, confidence is too low on frequent gusts over 40
mph to preclude issuing a Wind Advisory at this time.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 240 PM Sunday...

Given the amount of uncertainty in the forecast regarding the low
pressure system developing off the Southeast US over the next 24-72
hours, as well as the complexity in the upper level wave structure
beyond 72 hours, it is safe to say that forecast confidence overall
is rather low.

Currently the longwave pattern features a mean ridge over the
eastern US (with a broad shallow trough along the Gulf Coast) and a
trough nearing the US West Coast.  The low pressure off the
Southeast coast is forecast to move north or northwest beneath the
ridge and eventually become a weak closed low over in the vicinity
of the OH Valley, then settling down into the Southeast US while the
ridge tries to build back across its northern periphery.  While this
is occurring, a series of shortwaves in the West Coast trough
attempt to push east but may evolve into another upper low is they
are unable to progress east is a blocked flow pattern.

At the surface a weak baggy trough may linger over the Carolinas in
the wake of the aforementioned SE low before high pressure should
eventually build back down the east coast by the weekend.

The presence of the upper low will likely result in unsettled
weather through the end of the week, though with low confidence in
exactly how much coverage of daily showers there will be or how much
rain will fall (generally expect it to be on the lighter side
overall).  The trend should be drier headed into the weekend.  There
is higher confidence, however, in below normal temperatures, through
the weekend given that we will be under a baggy upper trough, as
highs are likely to remain in the 70s.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
As of 840 PM EDT Sunday...

Northeasterly winds of 10 to 15kts overnight with isolated wind
gusts of 20-30 later in the evening and overnight hours especially
in the southern portion of the FA. By morning, gusts will pick up
everywhere to 20-30kts with stronger gusts by the afternoon
especially in the south as PTC8 makes landfall in SC and continues
inland towards NC. By the late afternoon and into the evening most
of the region will see winds of 15-20 with gusts of 20-30kts. Winds
just above the surface around 2k ft, will be strengthening as the
PTC moves inland. The threat for LLWS will begin as early as 15z at
most sites but timing would depend on speed of the storm. For now
have LLWS at all TAF sites with 2kft winds ranging from 35kts in the
NW to 45-50kts at the KRDU/KRWI sites and stronger at KFAY. The
strongest LLWS is expected to occur in the late afternoon into the
evening hours. With that, isolated showers will continue through the
night, with at times bringing TAF sites down below MVFR conditions
with the heavier showers. Consistent showers are expected to being
before sunrise in the south and spreading north through the morning.
With that, ceilings and visibilities are forecast to be MVFR/IFR
with conditions degrading late afternoon some areas expecting to be
sub-IFR at times.

Outlook: There remains some uncertainty with the track and evolution
of the offshore low, but after similar rainy, low stratus, shear
conditions on Tuesday, the system should likely have pulled to our
north by Wednesday. However, additional showers and associated low-
level moisture appears to stick around through the end of the
extended outlook period.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Monday morning through Tuesday morning for
NCZ007>011-021>028-038>043-073>078-083>086-088-089.

&&

$$

SYNOPSIS...BLS
NEAR TERM...CBL
SHORT TERM...Swiggett
LONG TERM...BLS
AVIATION...CA/Luchetti