Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 281720
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
120 PM 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 1035 AM Wednesday...

Conditions still look favorable for severe storms and heavy rain
late this afternoon through at least early Thursday.  The quasi-
stationary synoptic front was analyzed over the Foothills yet again
this morning, having made little progress east without better
support from the upper low settling toward the Tenn Valley.  The
front isn`t expected to move much today either, leaving central NC
well within the warm sector, which will be characterized by MLCAPE
upwards of 2000 J/KG owing to strong diabatic heating and warm
advection from the south.  Clusters of storms may contain supercells
by late this evening as mid-level winds increase to 40-50kt and near
surface flow backs/strengthens in response to the approaching upper
low, with increasing threat of a tornado toward the NC/VA border
where winds will likely be backed the most.

The biggest uncertainty is a focus for convective initiation given
just broad isentropic accent around 300K and the lack of strong
height falls/DCVA until later tonight.  Thus, convection is likely
to form on mesoscale boundaries (ie differential heating or
localized convergence), which makes it hard to prog a specific area
for hazard threats.  Mostly southwesterly deep layer flow will
cause bands to be oriented southwest to northeast and may train over
some unlucky areas.  PW is 1.75-2" and isolated storms have produce
2-4" of rain the past couple of days. The flash flood threat will be
greatest in urban areas. CAM ensembles still favor the US-1 corridor
for multiple rounds, and the threat of severe/flooding looks to be
greatest from 21z-03z, given best overlap of peak heating and
increasing forcing, though the forcing aspect will likely continue
some convection through he overnight hours.

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. Lows
65 to 70.

&&

SHORT TERM /Thursday through Friday/...
As of 120 PM Wednesday...

Additional showers and storms with training moderate to heavy rain
remains likely over Central NC Thu, with a gradual shift to the ENE
on Fri.

The powerful mid level low will continue to sit and spin over KY on
Thu before drifting/wobbling slightly northward to IN on Fri, with
lobes of DPVA sweeping through western and central NC, along/atop
the very slow-moving north-south oriented cold/occluded surface
frontal zone. PW values remain elevated (over 1.5") over all but the
far west Thu, shifting to east of I-95 Fri, beneath nearly
continuous bands of pronounced upper divergence. The WRF-ARW/NMM
both start the day Thu with a band of convection bisecting the
forecast area, drifting slowly eastward through the far eastern
Piedmont and Coastal Plain through Fri, filling in with better
coverage with time, with a cell motion that presents a threat for
training and resultant localized flooding. Will retain, with minor
tweaks, the overall pattern of the earlier forecast, with lower
chance pops west and high pops in the central/eastern CWA Thu,
trending lower WSW to ENE through Fri but remaining high in the far
eastern/NE forecast area Fri, tapering off further Fri night.

Temps both days will be held down by clouds/precip in the east and
by lowering post-front thicknesses in the western CWA. Highs in the
upper 70s to lower 80s Thu and in the mid 70s to lower 80s Fri,
still slightly above seasonal normals. Lows 60-68 Thu night and 54-
65 Fri night. -GIH

&&

.LONG TERM /Saturday 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/BS
SHORT TERM...Hartfield
LONG TERM...Vincent
AVIATION...CBL


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