206
FXUS62 KRAH 181742
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
142 PM EDT Wed Sep 18 2024

.SYNOPSIS...
A large upper level low over the western Carolinas will slowly drift
east then northeast, tracking along the Mid Atlantic and Northeast
coast through Friday. A trailing upper level trough will persist
just off the East Coast through the weekend, as an upper level high
pressure ridge builds over the Gulf States. This ridge will shift
east into the Southeast states early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 930 AM Wednesday...

Only minor tweaks to the forecast were needed with the mid-morning
update. High temperatures where raised a degree or two across the
Sandhills, southern/central Coastal Plain and into the Triangle
given the quickly decaying low cloud layer and increasing sunshine.
Additionally, water vapor imagery shows the core of the remnant mid-
level vort max spinning over central SC is contributing to a
streamer of very light showers that may leak up into the southern
Piedmont and towards the Triad through this morning, but a
weakening/drying trend is expected with northward extent as they
become more displaced from the forcing mechanism.

Previous forecast discussion as of 400 AM Wednesday...

Water vapor imagery shows a decaying mid/upper low (the remnants of
the coastal low that affected us with heavy rain on Monday) swirling
over the western Carolinas. This low will slowly move east then
northeast today and tonight, opening up into a trough as it does so,
with the trough axis centered over central NC by early Thursday
morning. The surface reflection is very weak at this point, but an
occluded front still extends to its NE across northern NC. Anomalous
low-level moisture along and north of this front is currently
resulting in areas of fog, locally dense, across the northern
Piedmont and northern Coastal Plain. Some clearing has occurred
across the south, but guidance shows this may result in patchy fog
there as well early this morning.

Any fog will quickly dissipate after sunrise. As the mid/upper low
moves overhead, it will bring mostly cloudy skies and provide weak
forcing for ascent. When combined with increased moisture (PW values
of 1.5 to 2 inches) and weak convergence along the occluded front,
there will likely be some scattered showers around. Forcing looks
too weak for anything above chance POPs at this time. LPMM on the
HREF indicates some very isolated amounts of 1+ inches will be
possible, but it looks too localized for any widespread flooding
concerns. The best precipitation coverage will likely follow the
front, initially in the north and west before shifting south and
east in the evening. Instability of 500-1500 J/kg may also result in
isolated storms, with the best chance in the SE where more clearing
will take place. However, deep-layer flow will be quite minimal so
any storms should be short-lived, and not expecting anything severe.
A cool wedge of high pressure building down from the Northeast US
will keep temperatures from getting above the mid-70s in the far
north, while the SE should get into the lower-80s with more sun.

Showers will dissipate on Wednesday night with loss of heating, but
there will still be plenty of low-level moisture around. So
widespread low stratus will redevelop, with maybe some patchy fog as
well. This will keep low temperatures mild, in the mid-to-upper-60s
(5-10 degrees above normal).

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
As of 400 AM Wednesday...

Mid/upper troughing will extend down the Eastern Seaboard on
Thursday. Meanwhile a surface low will linger off the northern Mid-
Atlantic coast. This will bring one last day of upper forcing for
ascent and above-normal PW values. But with even weaker forcing
compared to today, shower coverage should be even less and more
focused in the east where the trough axis will be. There could be
enough instability for a few storms, but again with such weak flow
not expecting anything severe. Temperatures should be a bit cooler
than today across the SE as there will be northerly flow around the
aforementioned coastal low. Forecast highs are mid-70s to 80 with
forecast lows Thursday night in the upper-50s to mid-60s.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 230 AM Wednesday...

This looks to be a largely dry period, with slightly below normal
temps favored most days.

The mid-upper level low over the Mid Atlantic coast early Fri will
continue a drift to the NNE over the coastal Northeast into the
weekend, as a trailing mid-upper trough along the East Coast shifts
very slowly offshore. This will keep us in a dry NW steering flow
through Sun as an anticyclone over SE TX builds slowly eastward into
the Mid South and across the Gulf States. At the surface,
corresponding low pressure over the far NW Atlantic off the
Northeast and New England coast will periodically reload but
maintain a slow E drift, as its trailing backdoor fronts extend SW
then W through the Carolinas and Mid Atlantic region. Deterministic
models vary with backdoor front placement and movement; the GFS and
Canadian, for example, show a weak front to our S and W early Fri,
before washing it out by Sat and allowing temps to moderate, only to
have them knocked back down below normal by a second backdoor front
passage Sun night and a cool wedging high nosing down through
central NC through early next week. The ECMWF takes this second
backdoor front southward through NC earlier (Sat night). Weak low
level mass convergence along any backdoor front is possible, and
despite generally low PW and a lack of moisture availability from
850 mb up through the mid levels, such convergence could prompt a
few shallow showers. But overall, there`s a lack of opportunity for
deep moisture return through at least Mon, so will carry nothing
more than isolated pops at most, an outcome in line with the latest
LREF and other ensemble suites. By Tue, there are indications that
the mid level ridge over the Gulf States will shift E into the
Southeast, nudged by deep mid-upper troughing over central Canada
through the US Plains states, and this onset of WSW flow aloft could
support rising chances for convection over the NC mountains into the
foothills and perhaps our far NW Piedmont. This timing could shift,
however, so will keep Tue pops aob climo for now. Will keep temps
near to slightly below normal through Sun, before lowering them a
few more degrees for Mon/Tue. -GIH

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
As of 140 PM Wednesday...

Isolated to scattered airmass showers and widely scattered storms
are possible in the vicinity of all TAF sites through the afternoon
hours, lowest confidence at FAY. A northerly wind shift over
southeast VA will slowly sag south this afternoon before collapsing
through central NC this evening into the overnight hours. Along and
behind the wind shift, cigs will quickly fall to IFR and remain
there through the early morning hours Thurs. Landing minimums will
be most probable at INT/GSO early Thurs morning, but confidence
remains low at this time. A slightly higher chance for a shower will
also accompany this wind shift as weak instability and minimal
inhibition will continue through midnight. Cigs are expected to
gradually rise to MVFR by early afternoon.

Outlook: An unsettled period will continue through Thursday with
areas of overnight and early morning stratus and a risk of some
afternoon showers and possibly a storm. Aviation conditions will
generally improve for the weekend.

&&

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

&&

$$

SYNOPSIS...Hartfield
NEAR TERM...Swiggett/Danco
SHORT TERM...Danco
LONG TERM...Hartfield
AVIATION...Swiggett/Luchetti