Area Forecast Discussion
National Weather Service Raleigh NC
358 PM EDT TUE SEP 27 2016

.SYNOPSIS...
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/...
As of 330 PM Tuesday

The near term is a little complicated and uncertain, though likely
low impact. The synoptic cold front, which has essentially
stalled, stretches from Birmingham through the NC Foothills into
central VA. A pseudo warm front/differential heating boundary
shows up across northern Piedmont of NC this afternoon, with this
morning's rain having reinforced the upper 60s to mid 80's
temperature gradient across the area. This boundary should mix
north a little this afternoon, but fog seems likely to the north
across the northern Piedmont and northern coastal plain tonight.
Meanwhile, one line of shallow convection is progressing into the
coastal plain, while another area of showers/storms has been
getting going over upstate SC. Water vapor imagery shows a speed
max crossing the mid-Mississippi River valley region and a buckle
in the moisture channel over northern AL, both of which may help
to maintain the the convection to our west into the Piedmont
later this evening. As such, we will maintain 30-40 POPs well into
the overnight period. Severe parameters don't particularly line up
well today, with localized 30-35kt effective bulk shear across the
northern CWA in the vicinity of the front and better instability
to the south. Lows will be in the mid to upper 60s.

&&

.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
Barbados. 

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-
Sat.

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 /19Z TUESDAY THROUGH SUNDAY/...
As of 210 PM Tuesday...

Low level moisture in the vicinity of a front draped across western
NC and central VA has maintained MVFR ceilings at INT and GSO, with
VFR conditions having developed elsewhere. Satellite imagery suggests
better potential for clearing at INT than at GSO in the next few
hours, before ceilings redevelop overnight and possibly drop back
down to IFR or LIFR. Low vsbys will also be possible mainly from
KINT to KRWI northward, especially if clouds aloft thin out. 
Confidence in the nature of possible sub-VFR conditions is much lower
at FAY.  Otherwise, scattered showers or isolated storms are still
possible this afternoon and evening, mainly at INT/GSO/FAY, though
models trends have been showing less potential.

Scattered showers are possible Wednesday morning, with numerous
showers and storms Wednesday afternoon and evening.

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...SMITH
NEAR TERM...SMITH
SHORT TERM...SMITH
LONG TERM...MWS
AVIATION...SMITH



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