Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 281153
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
745 AM EDT WED SEP 28 2016

.SYNOPSIS...
A potent upper level low over the Great Lakes will drift southward
through the Ohio valley today, stall over the Cumberland Plateau in
eastern Tennessee and Kentucky on Thursday and Friday, then
retrograde north through the Ohio valley and lower Great Lakes
this weekend.

&&

.NEAR TERM /TODAY AND TONIGHT/...
As of 310 AM Wednesday...

As the mid/upper level cyclone drifts south towards the TN Valley,
models indicate a weak surface wave will develop along the quasi-
stationary frontal zone in place across Western NC. Daytime heating
and result moderate destabilization within  the seasonably moist
PWATS 1.5-1.7" east of the frontal zone will support the re-
development of showers and storm across the area this afternoon and
evening, with increasing upper level diffluence and the arrival of
better H5 height falls after 00z yielding the potential for
convective regeneration and a south to north training band of
convection overnight.

Central NC remains in a marginal risk for severe storms this
afternoon and evening, as the approach of the mid level speed
maximum into the Southern Appalachians will increase 0-6km shear
values of 30 to 40kts, highest north and west of the Triangle.
If we indeed destabilize as strongly as the NAM suggest (2500-
3000J/Kg of MLCAPE), wouldn`t be surprise to see SPC upgrade the
marginal risk to a slight risk as some of the stronger updrafts
could exhibit rotation and bring the threat of wind damage, hail and
isolated tornado. Storm intensity should decrease between 03 to 06z,
as the BL cools/stabilizes from loss of heating.

Additionally, as the mid/upper level flow becomes parallel to the
southerly low-level inflow, convective training would bring the
threat of heavy rainfall of 2 to 3 inches or higher with a threat of
flash flooding in a few locations.

With no airmass change, highs today will be very close to what we
saw on Tuesday ranging from upper 70s north to mid 80s south. Lows65
to 70.

&&

SHORT TERM /THURSDAY AND THURSDAY NIGHT/...
As of 425 AM Wednesday...

The deep cut-off low over the Southern Appalachians and the quasi-
stationary frontal zone over Western NC will continue to support
scattered to numerous showers and storms across the area Thursday
and into Thursday night. Models generally agree that the plume of
above normal 1.5-1.7" PWATs will get shunted slightly eastward as
the dry slot advances eastward into the western and possibly Central
Piedmont by late Thursday/Thursday night. This should eventually
lead to an eastward shift in the higher convective rain chances,
centered along and east of Highway 1.

While favorable deep layer shear of 35 to 40 kts will be sufficient
to support severe storms, ongoing/lingering convection across the
area from daybreak on could greatly hamper destabilization. Will
need to monitor the severe aspect over the next day. Additionally,
the potential for some locations to see multi-days of heavy rainfall
will result in a continued threat for isolated flooding.

Highs Thursday similar to the last 2 days, potentially slightly
cooler across the Sandhills and southern coastal plain given more
robust rain chances. Highs in the mid/upper 70s NW to lower 80s
south. Drier air working its way into the western piedmont could
allow for slightly cooler overnight lows in the lower 60s, with mid
to upper 60s elsewhere.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
As of 430 AM Wednesday...

Overview: High confidence in dry/pleasant conditions over the
weekend and early next week gives way to below normal confidence
mid/late next week as a highly meridional synoptic pattern evolves
over the Continental US and a potentially significant tropical
cyclone (yet to develop) could threaten portions of the Southeast
coast.

A potent upper level low over the Great Lakes will drift southward
through the Ohio valley today, stall over the Cumberland Plateau in
eastern Tennessee and Kentucky on Thursday and Friday, then
retrograde north through the Ohio valley and lower Great Lakes this
weekend. An attendant cold front will track east of the Appalachians
on Thursday, then stall over eastern portions of the Carolinas
Friday into the weekend, gradually washing out late this weekend and
early next week. Forecast confidence is above average at the
beginning of the long term period (Fri-Sun), but decreases markedly
thereafter (Mon-Wed), primarily w/regard to the evolution of the
stalled/retrograding upper level low. The 00Z GFS indicates that the
upper low will track northward through New England Sun night, at
which point it is absorbed by the northern stream jet (Mon) and
tracks rapidly NE through the Canadian Maritimes into the northern
Atlantic (Tue), with subsidence in the wake of this feature aiding
amplification of a deep upper level ridge along the Eastern Seaboard
(Wed). The 00Z ECMWF is a little slower and a little further west
when retrograding the upper low northward over the weekend. As a
consequence, the upper low fails to rendezvous with the northern
stream jet (Mon-Tue) and cuts-off along the New England coast (Tue-
Wed), precluding significant amplification of an upper level ridge
along the Eastern Seaboard as depicted by the GFS.

If the robust tropical wave currently approaching the Windward
Islands develops into a significant tropical cyclone over the
Carribean late this weekend/early next week (as long range guidance
continues to suggest), then the evolution of the aforementioned
upper level low will likely become a critical piece of the puzzle in
determining it`s track and potential impact on the Southeast US
coast mid/late next week. -Vincent

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
As of 745 AM Wednesday...

IFR to MVFR ceilings in place across the western and central
terminals are expected to lift to VFR between 15 to 18z. As the
mid/upper level cyclone drifts south towards the TN Valley,
models indicate a weak surface wave will develop along the quasi-
stationary frontal zone, with re-development of showers and thunderstorms
expected across the area this afternoon. As upper-level support
increases overnight, showers and storms could persist well into the
overnight hours. Given continued moist low-level airmass in place,
areas of IFR to MVFR stratus will also be possible.

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.

&&

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

&&

$$

SYNOPSIS...Vincent
NEAR TERM...CBL
SHORT TERM...CBL
LONG TERM...Vincent
AVIATION...CBL


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