Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 280044
Area Forecast Discussion
National Weather Service Raleigh NC
832 PM EDT TUE SEP 27 2016
A quasi-stationary frontal zone will meander over the area
through late in the work week, as a deep upper level low settles
over the Tennessee Valley.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 845 PM Tuesday
While the threat for isolated strong/severe storms has decreased
with loss of heating, scattered showers and thunderstorms with
locally heavy downpours will remain a decent probability across
roughly the west half of central NC through midnight, with coverage
expected to gradually decrease afterwards.
A very moist air mass for this time of year (PW values 120-150
percent of normal) coupled with lift supplied by an upper jet
crossing the northern mid atlantic and weak low level convergence
along a slow moving cold front will maintain the threat for
scattered showers/storms through the rest of the evening. this
forcing is maximized west of highway 1 through 06Z, then projected
to weaken through 12Z Wednesday. Will need to monitor for training
showers/storms as high PW values indicate the atmosphere supportive
if efficient rain producing storms, which may lead to localized
Across the coastal plain, forcing not as robust though atmosphere
just as moist. While showers/storms will not be as numerous compared
to the Piedmont, locally heavy rainfall will still occur with the
Overnight temps in the 65-70 degree range expected.
-- End Changed Discussion --
.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
As of 330 PM Tuesday...
Depending on the amount of ongoing showers early wednesday
morning, Wednesday afternoon could be a rather active. As the
stacked low over the Great Lakes digs south toward the TN Valley,
increased forcing for accent and the stalled frontal zone over
west-central NC will trigger convection, though the timing of
greatest coverage is still a little uncertain given that the best
height falls and cooling aloft won`t arrive until later Wednesday
afternoon/evening, possibly toward the end of peak heating. Deep
shear, while strongest to the northwest, will increase to 30-40kt,
especially as the low-level flow backs in response to the
approaching height falls, which may support supercell activity
within a band of convection that appears favored from US-1 east,
based on the available CAMs. All of central NC is in a Marginal
Risk (per SPC) and this could be upgraded if instability appears
to be sufficiently strong.
The other concern for Wednesday evening is isolated flash
flooding given system parallel southwesterly mean flow and PW
near 2 inches. At the moment, Flash Flood Guidance for 3+ hours is
well in excess of 2 inches. The GEFS and EC Ensemble means are
roughly 1-2 inches for the central and northern Piedmont areas,
so urban areas would likely run the greatest risk. However,
confidence in the most favored area is not high enough to consider
a watch at this time.
Even after an evening round of convection "occurs" the ECMWF
indicates another round of precip overnight, which isn`t out of
the question given cyclonic flow aloft and no airmass change. it`s
hard to put much detail into that portion of the forecast until
the mesoscale aspects of Wednesday evening play out, but POPs
will remain high overnight.
Highs Wednesday should be in the mid 70s to lower 80s, with
overnight lows in the mid and upper 70s.
.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 355 PM Tuesday...
A closed mid-upper low now over the Great Lakes will settle SWD and
become increasingly separated/cut-off from the NRN stream flow
across Canada, and ultimately reach the Lower OH/NRN TN Valley by
early Thu. The models agree that the low will linger there through
Fri, then lift NNEWD across the Great Lakes through the weekend.
Meanwhile, a SRN stream jet will help carve a lingering trough axis
across the SERN U.S. and ERN GOM, SWD to the vicinity of the
Yucatan, during the same time. The ultimate placement and strength
of this latter feature may play a role in the eventual track and
strength of the tropical wave now several hundred miles E of
At the surface, a secondary cold front associated with the
aforementioned closed low will have moved into the Appalachians by
early Thu, then drift slowly EWD and merge, over central NC Thu
afternoon, with the preceding frontal zone now stretching along the
NC Blue Ridge. The front, and large-scale forcing and ERN fringe of
colder temperatures/steep lapse rates accompanying the mid-upper
low, will serve as a continued focus for a good chance of showers
and storms on Thu. The merged frontal zone --and associated WRN
bound of instability/convergence/chance of showers and storms-- will
then pivot slowly NEWD through the NE Piedmont and Coastal Plain Thu
night-Fri, before reaching and dissipating along the coast late Fri-
A lee trough and associated light SLY flow will linger over the
Carolinas this weekend, then yield to high pressure forecast to
build across the NERN quarter of the CONUS, beneath NRN stream
ridging aloft, through early-mid next week.
Temperatures are expected to be near to slightly above average
throughout the period.
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
As of 831 PM Tuesday...
Scattered showers and storms will become less numerous over the next
several hours. With a moist low level air mass, IFR stratus looks
likely at KINT/KGSO, while sub-VFR conditions with some patchy fog
are possible elsewhere in the early morning hours. Stationary front
will remain west of the area Wednesday, and showers and
thunderstorms will develop again across the area Wednesday
Looking beyond the 24 TAF Period: The combination of a quasi-
stationary front across the Carolinas and a cut-off upper level low
settling over the Tennessee Valley will keep unsettled weather in the
form of scattered to numerous showers and storms through Thursday.
Conditions are expected to improve Friday through the weekend as
drier air finally spreads in from the west.