Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
FXUS62 KRAH 291151
Area Forecast Discussion
National Weather Service Raleigh NC
650 AM EST Tue Nov 29 2016
Broad cyclonic flow aloft over the Central US will result in
southwesterly flow over the Southeast US through Wednesday. This
front will sweep eastward through the area Wednesday night.
.NEAR TERM /Through Tonight/...
As of 350 AM Tuesday...
Cyclonic flow aloft encompasses much the CONUS, with southwesterly
flow now established over the Southeast US. On the eastern side of
this trough, a notable shortwave is lifting through the Midwest
states, and an approaching band of convection associated with a cold
front aloft and intense 50-60kt LLJ is crossing the Appalachians.
Given increasingly less focused forcing and no instability, the band
has shown significant weakening this morning, with only patchy rain
just now reaching the western Piedmont. Warm advection has
increased with the arrival of the LLJ, so showers will continue to
be possible this morning, mainly west of US as forward speed of the
system slows. There`s really no sign of a focus for much qpf today,
with hires and larger scale models all showing just smatterings of a
tenth on an inch or less across mainly the Piedmont. Other than a
period early this morning in the Triad, it`s hard to find any reason
for more than 30-40 POPs by midday and just a slight chance this
The bigger story will likely be the wind, as the combination of a
lack of diabatic stabilizing and warm advection should allow for
decent mixing and tapping into the intense, albeit weakening, wind
field this afternoon. GFS and NAM soundings show around 40kts at
1500 this afternoon, and there have already been a few wind gusts to
20-25kt this morning. A veil of mid clouds will likely limit some
of the mixing potential, so it does not look like a wind advisory
scenario, but it will certainly be breezy.
The early rain in the northwest Piedmont has caused temps to drop
a few degrees, but with a strong southerly wind in place and
dewpoints well into the 50s, it`s hard to see much of a cold air
damming airmass developing. This, some faith will be put in a blend
of statistical guidance, which suggests highs of 67-77, and seems
reasonable given the pattern.
Fast but uniform southwesterly flow will continue tonight, with no
discernible forcing and just broad warm advection. 20-30 POPs look
most appropriate over the western piedmont based on a general model
consensus. very unseasonably mild lows of 59-63.
.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 350 AM Tuesday...
A series of shortwave perturbations, now stretching from the Four
Corners region NNWwd across Alberta, will amplify through the base
of an associated broad trough centered over the Nrn Plains, then
lift NEwd from the MS Valley early Wed to the OH valley and Ern
Great Lakes Wed night. The progression of these shortwaves are
forecast to cause the parent trough/low center to reform near the
Srn border of Ontario and Quebec by 12Z Thu. A subtropical ridge
will meanwhile persist near the Bahamas; and the height gradient
between the two features will maintain anomalously strong SW flow
through much of the Ern U.S.
A cold front, and preceding confluence axis related to the earlier
vigorous squall line over the TN Valley, will be draped in close
proximity from the OH Valley to the Lower MS valley at 12Z Wed;
progress into the Appalachians by Wed aft-eve, then sweep to the
coast by 12Z Thu - slightly faster than previously forecast.
While much of Wed is still expected to be largely devoid of anything
more than scattered showers owing to a lingering influence of the
aforementioned subtropical ridge, the reformation of the mid-upper
low over SE Canada will cause slightly stronger (than previously
forecast) height falls aloft --about 30-60 meters at 500 mb-- to
glance NC late Wed and Wed night. Associated forcing for ascent will
support the maintenance of a squall line/katafront along the
aforementioned pre-frontal confluence axis/just ahead of the surface
cold front; and this feature is expected to move into the Wrn
Piedmont and focus the relative best chance of rain, particularly
meaningful amounts of greater than a quarter inch, generally west of
U.S. Highway 1 late Wed afternoon through early Wed evening. The
models continue to indicate this band of convection will diminish in
coverage and intensity with time and Ewd extent, as the larger scale
forcing lifts Nwd and away from the Carolinas. Momentum transport
from speedy low-mid level flow may result in surface wind gusts up
to 40-45 kts, but a lack of any more appreciable instability
suggests this HSLC case will probably be a non-event for Central NC.
It will be warm/in the 70s, and breezy with gusts up to 35 mph ahead
of the front, with CAA and temperatures in the mid-upper 40s W to
mid-upper 50s E by Thu morning.
.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 410 AM Tuesday...
Beneath confluent flow aloft between the upper low initially over SE
Canada and a cut-off low forecast to evolve over the SWrn U.S., high
pressure and associated dry and cooler conditions are forecast to
build/extend from the Central Plains to the Middle Atlantic coast
through Sat. Dry air associated with the ridge will be topped by
periods of high clouds along a nearby WSWly Srn branch upper jet Fri-
Forecast confidence remains low for Sun-Mon, with large model spread
regarding how quickly the Swrn U.S. cut-off low moves E. The 00Z EC
has trended slower, while the GFS has trended slightly more
progressive, so perhaps a middle ground is in site, which would
favor thickening and lowering cloudiness, and a chance of rain late
centered around Sun night.
.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
As of 650 AM Tuesday...
Showers currently moving into western NC continue to weaken, with
just a few areas of light rain in the western Piedmont and just MVFR
ceilings. Forecast soundings suggest ceiling may still lower to IFR
by 15Z as rain continues to move into the area. However, the rain
should be intermittent, and ceilings may be able to rise back to
MVFR this afternoon and then fall back to MVFR or IFR tonight.
MVFR ceilings and some patchy rain may extend east to RDU and FAY by
15Z, but confidence is lower, and MVFR is much less likely at KRWI,
if even at all. These sites may see MVFR ceilings (re)develop
tonight as moist southerly flow continues.
Meanwhile, winds will strengthen this morning, slowly becoming more
southerly and likely gusting to 25-30kt, possibly to 35kt at times
if cloud cover does not inhibit boundary layer growth. In fact,
there have already been a couple reports of 30kt winds near
the Triad, and LLWS was reported at FBG, where surface winds
are still light beneath a 50kt low level jet. These strong winds will
weaken tonight, but still remain around 10kt sustained.
Outlook: Another band of convection and a cold front will approach
on Wednesday and cross the area on Wednesday, with showers and a few
thunderstorms resulting sub-VFR conditions. VFR conditions are
expected to return by Thursday.