Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 151151

Area Forecast Discussion
National Weather Service Raleigh NC
650 AM EST Sun Jan 15 2017

A cold front will drop southward across central NC today. High
pressure will build in behind the front and reinforce a cold air
damming airmass over the area through Monday night.


As of 350 AM Sunday...

Despite low clouds over VA and fog over SC, central NC is mostly
clear this morning owing to westerly low-level downslope flow.
There have been a couple periods of orographic cirrus just north of
the VA border, but not enough to eliminate a small chance of some
patchy fog around sunrise.

Today: Given a lack of precip and diabatic enhancement of the CAD
airmass the past day or so, the cold front over central SC this
morning will continue to weaken under good heating today. However, a
secondary cold front over central VA this morning will surge south
into the norther Piedmont by midday and through all of central NC by
early evening, as a 1032mb surface high shifts from the Midwest to
the Mid-Atlantic states. As was the case on Friday (which today`s
pattern seems to strongly resemble) the cold advection isn`t
terribly strong and may lag the northeast wind shift by a few hours.
The front should induce some low-clouds between 2-4k ft, especially
in the coastal plain per NAM/GFS forecast soundings, but the flow is
pretty weak throughout the low- levels, so it`s hard to see
widespread stratus until later tonight. Highs should feature a large
range from north to south again, as the over all lack of greater
cloud cover will allow for good heating, specially across the south
through early afternoon. Most guidance suggests 50-65 for highs,
though the RAP/HRRR has highs of 70 in the southern Piedmont.

Tonight: The cold front will push south of the area by this
evening, with weak cold advection and a light northeast wind
overnight. The H9 flow will swing around to southerly after
midnight, resulting weak (and shallow) isentropic lift in the
western Piedmont. As a result, some patchy light rain or drizzle
should break out and drift east through the Piedmont through
Monday morning, with low clouds eventually engulfing the entire
area. Lows 38-45.


As of 350 AM Sunday...

CAD will persist Monday as the parent surface high shifts to the
Mid-Atlantic coast and then eventually offshore. The main impacts
look to be from just widespread low clouds since the H9-H85 flow
is weak (< 30kt) and the flow aloft is flat. Low clouds will
result in highs several degrees cooler than today, but the lack of
precip should keep highs within a couple degrees but on the cooler
side of guidance; 45-52. The aforementioned surface high will
shift offshore Monday night, but our CAD airmass should linger
without an erosion mechanism. Warm advection will start to
increase over the western Piedmont, on the eastern flank of a
prefrontal LLJ over the Tenn Valley. Most of the associated rain
will be west of the mountains, but their could be a few areas in
the Foothills and flirting with the western piedmont by Tuesday
morning.  Lows 42-46.


As of 400 AM Sunday...

The models are in relatively good agreement with the large scale
pattern across NOAM through the start of this forecast period (early
Tue), and this pattern will maintain continued cool and cloudy
conditions --from in-situ CAD-- that will linger through midday
Tue. A warming SSW low level flow will develop later Tue, and
continue until a cold front accompanying a cyclone crossing the
Great Lakes settles into the Srn Middle Atlantic states with an
associated chance of showers centered around the day Wed.

Primary forecast uncertainty for the remainder of the forecast
period is how much nrn stream flow amplification of a series of
nrn stream, split flow shortwave troughs occurs from Canada, and
subsequently 1) to what degree they interact with a longer
wavelength/less progressive srn stream, and 2) possibly slow and
form a closed low across New England and the adjacent Canadian
Maritimes. The handling of these features will play a role in the
timing and degree of longwave riding and possible blocking over
the ern U.S., and to what degree, if any, upstream shortwave
energy can penetrate the ridge.

It appears at this time that surface ridging, beneath the ridge
aloft, will generally dominate late in the week and early next
weekend, with one deamplifying shortwave trough likely to migrate
into the ridge with an associated increase in cloudiness and
chance of rain late Fri or Fri night. It will be a mild pattern,
however, with temperatures remaining above freezing throughout the
forecast period.


AS OF 650 AM Sunday...

VFR early this morning, with clouds at 6-8k ft drifting east across
the area and some very isolated MVFR vsbys. A cold front will surge
south across the area between 15z and 21z today, causing the
currently light and variable winds to become northeasterly at 6-
10kt.  Models have struggled with this weather pattern the last
couple of days, and seem to still be in disagreement on how quickly
MVFR ceilings will redevelop, but recent trends suggest some
scattered MVFR ceilings this afternoon and then a better chance of
cigs lowering at FAY/INT/GSO/RDU after 00Z. The lowest ceilings
should be from FAY to GSO/INT after 03Z, once the low level flow
swings around to southerly again, though it`s not clear how low cigs
will get,  Given how weak the low-level winds are, the current TAF
will only reflect MVFR ceilings overnight, with light rain at

Outlook: Conditions are expected to at least MVFR by 12Z Monday, and
possibly IFR at INT/GSO, as southerly flow reinforces a cold air
damming airmass over the area.  Low cigs should persist into
Tuesday, when there will be a chance for low clouds to scatter ahead
of a cold front approaching from the west.  The cold front and
associated rain and low cigs/vsby will should impact the area
Wednesday.  VFR Conditions will return to all terminals in the wake
of the cold front Wed night and Thursday.]




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