Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 231049
Area Forecast Discussion
National Weather Service Raleigh NC
650 AM EDT FRI SEP 23 2016
A surface low and trough will hold along the Carolina coast through
today, before weakening tonight. Weak high pressure will hold over
the area through Saturday, then a backdoor front will push in from
the north Saturday night and Sunday.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 645 AM Friday...
Minor update to account for low coverage of light precip confined to
the southeast CWA this morning. -GIH
Earlier discussion from 240 AM Friday: Rain chances will finally be
dwindling, but not before another round of scattered showers and few
storms through this afternoon, mainly southeast of the Triangle.
Surface low pressure persists just off the SC coast with a weakening
inverted trough extending across coastal NC, capped by a persistent
mid level shear axis over central/eastern NC. PW and low level
moisture convergence remain rather high today over the eastern and
southern portions of the forecast area, warranting a continuation of
good chance pops there, in line with consensus of guidance from
parameterized-convection models, convection-allowing models, and
ensemble output. Chances will be low (0-20%) in the NW CWA as drier
air (noted just to our NW on water vapor imagery) and warmer/more
stable mid levels ease toward the ESE as the mid level shear axis
likewise drifts to the ESE today. Will include a mention of just
isolated thunder in the SE CWA, with model forecasts of marginal
instability and poor deep layer shear. Low level thicknesses
adjusted for greater sunshine in the NW CWA and greater cloudiness
with later partial clearing in the SE supports highs in the lower-
mid 80s, cooler in south-central sections and warmer in the NW. PW
values fall from NW to SE late today into the evening, with lowering
low-level moisture transport and rising heights aloft from the west
as central-CONUS ridging starts to build in. Will trend pops down
and out for tonight, with a trend to mostly clear (NW) / partly
cloudy (SE). Lows in the mid-upper 60s. -GIH
-- End Changed Discussion --
.SHORT TERM /Saturday through Saturday night/...
As of 255 AM Friday...
Dry and very warm weather expected. Mid level ridging aloft
continues to build in from the west, while at the surface, the weak
trough drifts further offshore as weak high pressure moves in from
the west, although lee troughing does form in the afternoon over the
western Piedmont. With low PW and a fairly stable column, will keep
pops under 15%, with mostly sunny skies. Thicknesses and statistical
guidance indicate highs in the mid-upper 80s, around 6-10 degrees
above normal. The anticipated backdoor front moving toward NC from
the NNE is poised to move into the NE CWA Sat night into early Sun
morning, driven by energy rotating around a large vortex centered
near the Newfoundland/Quebec border. But this low does not appear to
dig markedly down the East Coast, hinting that the front may not
work very far into NC despite being propelled by a cool dense
surface high to our north. The ECMWF tends to hold the front just
north of the VA/NC border through daybreak Sunday, while the NAM/GFS
dip the front into northern NC. Will lean toward the ECMWF solution
and hold onto mild temps overnight, with lows Sat night in the mid-
upper 60s and a minimal drop in dewpoints over the NE. Regardless of
frontal progression, we are likely to see increasing clouds from NNE
to SSW Sat night. While a few models generate shower activity ahead
of or along the front over north central and NE NC Sat night, the
presence of features to force ascent is uncertain, and will opt for
dry weather for now and monitor trends. -GIH
.LONG TERM /Sunday through Thursday/...
As of 320 AM Friday...
Confidence in the long term forecast is still fairly low but nothing
like it was yesterday morning as model solutions are slowly starting
to come up with somewhat of a common solution. The big difference
maker, which was the blocking pattern in the GFS solution is not
showing up as much as in previous runs and has allowed high pressure
over to come southward out of Quebec on Sunday to become more
transitory and move into New England on Monday and out to sea on
Tuesday. This allows a cold front to drop into the Carolinas on
Wednesday, hang up over the area on Thursday before exiting to the
south Thursday night into Friday. During this time period the best
chances for thunderstorms would be on Tuesday and Wednesday ahead of
and with the cold front. Otherwise some slight chances for rain
throughout the period, mostly in the form of diurnal showers are
possible. Temperatures will generally stay in the upper 70s to low
80s before the cold front with some hints at some highs in the mid
70s for the end of the week.
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 650 AM Friday...
Widespread IFR to LIFR conditions will linger through 12-13z this
morning across all of central NC, a result of light winds along with
stable and very moist air in the low levels. Cigs are expected to
rise to MVFR between 14z and 16z, and then to VFR between 16z and
18z west (INT/GSO), 18z-20z central (RDU), and 19z-21z east
(RWI/FAY). Rain chances today are low west (INT/GSO) trending higher
east (RDU and especially RWI/FAY), with the highest coverage near
FAY, which could see a few hours of showers, and perhaps a storm or
two, with MVFR vsbys and cigs this afternoon. Conditions will trend
toward VFR everywhere after sunset tonight, as weak high pressure
starts to build in from the west with drier air moving in.
Looking beyond 12z Sat morning, VFR conditions will dominate into
Sat evening, with the risk of sub-VFR stratus and fog returning late
Sat night/Sun morning as a backdoor front approaches from the north.
MVFR cigs may linger through much of Sunday and Sunday night,
although confidence is not high, as there is uncertainty regarding
the southward pace of the backdoor front through NC. After mostly
dry weather Sat, the chance for sub-VFR showers and storms returns
for Sunday along the backdoor front. Shower/storm chances will
increase further late Mon into Tue as an upper level low pressure
trough approaches from the NW. There may be a risk for low level
wind shear early Tue morning. -GIH
-- End Changed Discussion --