Area Forecast Discussion
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FXUS62 KRAH 301811
Area Forecast Discussion
National Weather Service Raleigh NC
210 PM EDT FRI SEP 30 2016
A weak wave of low pressure and associated frontal zone will drift
east across our region through early Saturday, then offshore through
the rest of the weekend, while a persistent upper level low over the
Tennessee Valley lifts across the Lower Great Lakes.
.NEAR TERM /TODAY AND TONIGHT/...
As of 1000 AM Friday...
Storm chances were increased and expanded a couple of hours ago to
account for broken convective band curving from SW VA to Raleigh to
Clinton to Wilmington, with elements tracking roughly northward.
The latest surface analysis shows the surface low over the NC
Foothills with a cold/occluded front extending down through western
and southern SC and a warm/stationary front extending eastward
across northern NC. Already a few strong to severe storms have
formed in this band within a somewhat favorable environment of good
deep layer shear (45-55 kts), strong mid level jet streak (55-60
kts), and marginal instability (~1000 J/kg of MUCAPE), although
dynamic forcing for ascent is largely to our west, closer to the
influence of the vortex centered over the KY/IN border. Will
continue to monitor the severe threat over the next several hours as
the mid level speed max tracks up the east side of the vortex, with
the surface cold/occluded front easing slowly toward the ENE. Will
keep good chance to likely pops in the Triangle to the north and
east, with lower slight-chance to low chance pops over the SW CWA.
Highs still expected to be in the upper 70s to lower 80s. -GIH
Earlier discussion from 200 am Friday: Strong low-level warm
advection in advance of a cold front extending southward along the I-
77 corridor and upper level forcing in the form of a jet
entrance/exit and small amplitude waves/weak DPVA/ rotating around
the eastern periphery of a stalled upper level low in Kentucky have
allowed deep convection to develop within the dry mid/upper level
airmass in place over central/western NC since 00Z. Nocturnal
destabilization assoc/w low-level warm advection will yield 1000-
2000 J/Kg of MUCAPE through sunrise. Low-level FGEN and upper level
forcing on the eastern periphery of the stalled upper low may
further enhance thermodynamics via the release of potential
instability, in the presence of vertical wind shear sufficient for
supercells (SCP values 2-6). Any isolated convection in central NC
this morning should remain elevated in nature, with parcels
launching from ~925 mb (atop a ~2500 ft stable layer). With the
above in mind, isolated convection will remain possible overnight,
primarily in the NW Piedmont where low-level forcing will be
juxtaposed with marginal upper level forcing on the eastern
periphery of the stalled upper low. If layer-lifting is sufficient
for deep convection to survive entrainment of dry air aloft in the
NW Piedmont, updrafts could acquire mid-level rotation and exhibit
deviant motion. Given the elevated nature of any convection and the
lack of such characteristics thus far, severe weather appears
unlikely. Should a robust elevated updraft acquire rotation, small
to marginally severe hail could not be ruled out.
Instability will decrease from west-east today as a drier low-level
airmass advects into portions of the area (western Piedmont for
sure) attendant a slow moving cold front and temps aloft (H5) begin
to warm. The front is expected to further slow down this afternoon,
stalling in a NW-SE orientation along/near a line from INT-TTA-CTZ.
As a result, expect dry conditions in the SW Piedmont with isold/sct
convection possible elsewhere and the relative best chances from in
the NE Coastal Plain. Expect highs ranging from near 80F in the N/NW
Piedmont to ~85F in the Sandhills and SE Coastal Plain. Lows tonight
ranging from the mid 50s west to mid 60s east.
If convection can survive entrainment of dry air in the mid/upper
levels this afternoon, instability/shear would support supercellular
organization along with a potential for damaging winds/large hail.
Though diurnal destabilization will be greatest across portions of
the the Sandhills/SE Coastal Plain, the potential for an isolated
severe storm may be confined to the N/NW Piedmont in closer vicinity
to the stalled upper low where upper level forcing /layer-lifting/
(and release of potential instability) is more likely. -Vincent
.SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
As of 200 AM Friday...
Dry/stable conditions will prevail sat/sat night as a dry low-level
airmass advects into the region beneath an increasingly warm/dry mid-
level airmass as the upper level low retrogrades northward away from
the region (toward the Great Lakes). Expect highs in the upper 70s
to lower 80s and lows mid/upper 50s to lower 60s. -Vincent
.LONG TERM /SATURDAY THROUGH THURSDAY/...
As of 350 PM Thursday...
he upper low over the Great Lakes will finally open up and weaken as
it finally ejects eastward off the New England Coast late Monday and
into Tuesday. Warm dry air aloft will cap Central NC off from any
convection Sunday and Monday with daytime highs continuing to run a
good 3 to 5 degrees above normal. Highs in the upper 70s north to
lower 80s south. Lows 60 to 65.
In the wake of the low lingering off the New England Coast, Canadian
high pressure will wedge south down the Atlantic Seaboard, bringing
cooler, more seasonable temperatures Tuesday through Thursday.
The NHC forecasts has Matthew moving northward through the Bahamas
on Tuesday, with large model spread thereafter in both the track
and speed of Matthew for the latter half of next week. Refer to the
National Hurricane Center for the latest information on TC Matthew.
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
-- Changed Discussion --As of 210 PM Friday...
MVFR cigs persist at RDU/RWI/FAY with a continued threat for
scattered showers and isolated storms, while conditions have
improved to VFR at INT/GSO as a slow moving cold front has shifted
just east of these locations, with drier air moving in. The dominant
MVFR conditions at RDU/RWI/FAY will improve to VFR from late
afternoon through early to mid evening as dry air gradually works in
from the WSW, with a corresponding end to the shower/storm chances.
There is a small chance of MVFR or IFR vsbys in fog at RWI/RDU early
Sat morning but chances appear too remote at this time to include in
the terminal forecasts. VFR conditions are expected at all sites
late tonight through Sat as high pressure builds in from the WSW.
Surface winds from the SE at 6-12 kts will become light/variable for
much of tonight before becoming southwesterly Sat behind the front.
Looking beyond 18z Sat: VFR conditions are likely to dominate
through Wed, although a period of sub-VFR fog/stratus is possible
areawide early Wed morning. -GIH
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