Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 231200

Area Forecast Discussion
National Weather Service Raleigh NC
700 AM EST Tue Jan 23 2018

A lead cold front and markedly drier air will cross the Carolinas
and Virginia early today, followed by a secondary cold front, with
noticeably cooler conditions, this evening. Pacific high pressure
will then build across the region through mid to late week.


As of 400 AM Tuesday...

Modified discussion due to unfolding low, but non-zero risk of high
shear low CAPE severe convective episode.

Current convection has organized along a surface confluence axis
resulting in part due to strong low level jet-driven WAA --and
associated underlying height falls-- beneath strengthening sswly/
swly deep tropospheric flow that includes 50 kts as lows as 1500-
2000 ft, per regional VWP and model analysis data this morning.
Associated shear, both in the lowest few kilometers and above, is
extreme. However, generally moist adiabatic/neutral thermodynamic
profiles, above a still-slightly stable near surface layer, are
supportive of only weak instability this morning; though this
instability will likely increase into the 200-400 J/kg range and
become increasingly surface-based as lwr 60s surface dewpoints over
SC continue to advect nwd, immediately in advance of a lead cold
front, analyzed at 09Z from the cntl FL panhandle and far sern AL
newd across nern GA and far swrn NC, and which will cross cntl NC
later this morning, through the midday to early afternoon hours over
the Coastal Plain. Weak diabatic diurnal heating potential will also
exist beforehand, though limited by multi-layered overcast. Both an
isolated tornado and strong to locally damaging wind gusts will be
possible with this lead activity this morning, when the character of
the shear will favor cyclonically-curved hodographs. The (marginal)
tornado potential will decrease with subsequent convection that will
probably organize into a broken line along the cold front, as the
llj axis peels away and surface winds veer to a more sswly
component, thereby straightening forecast hodographs.

The passage of this cold front, one that may be better
characterized as a dry line, will result in clearing; drying in the
low levels; and the deepening of a diurnal mixed layer that will
promote the development of strong sswly winds and associated gusts
that will peak in the 30s kts for several hours immediately behind
the front, with winds then lessening thereafter. Any appreciable
cooling will lag this feature and not occur until a secondary front,
one analyzed at 09Z from wrn PA swd across cntl WV, settles sewd
across cntl NC after 21Z. High temperatures will consequently again
climb into the 60s, to lwr 70s over the ern CWFA.


As of 410 AM Tuesday...

Mostly CAA-driven lows, though with brief radiational cooling
potential over the wrn Piedmont, are expected to dip into the lwr to
mid 30s for most. Skies will be mainly clear, aside from srn stream
upr jet-induced cirrus that will nose nwd across SC, and into at
least srn NC, after midnight.


As of 145 AM Tuesday...

Wednesday through Saturday: Expect dry weather through Saturday. An
upper level shortwave and a reinforcing dry cold frontal passage is
expected Wednesday night/Thursday. Surface high pressure is expected
to build into the area in the wake of the secondary cold front and
remain over the area into Saturday. Meanwhile, the upper level ridge
will move eastward over the Carolinas through Friday night. Highs
will be generally in the upper 40s north to mid 50s south through
Wednesday and Thursday, with moderation beginning Friday. Lows in
the upper 20s to low 30s Wednesday and Thursday nights, increasing
Friday night.

Saturday Night through Tuesday: Another upper level trough and
surface cold front will develop over the Midwest and progress
eastward toward the Mid-Atlantic over the weekend. As southwesterly
flow increases, so will the warm, moist advection into the region.
As a result, cloud cover and chances for precipitation will increase
late Saturday into Sunday as the cold front approaches. Some
differences in the evolution of the upper level trough and the
timing and duration of convection as it moves into/through Central
NC have developed between the medium-range models. The ECMWF is more
amplified, slower, and wetter than the GFS. As a result of the
latest forecast cycle model differences, confidence in the timing of
the front and convection associated with it is low.


As of 700 AM Tuesday...

Generally MVFR ceilings, with pockets of IFR ones, and MVFR
visibility restrictions in showers, will become VFR with the passage
of a lead cold front from west to east between 12-14Z at wrn sites
and 16-17Z at ern ones.

A strong swly low level jet (llj), on the order of 40-50 kts as low
as 1500-2000 ft, with progressively stronger winds above that level,
will continue to migrate newd across e-cntl and nern NC this
morning. The presence of this feature will support the continued
newd development and maintenance of the aforementioned ceilings and
convection. Since the axis of this llj is departing, low level wind
shear should be ending in the next hour or so at RDU/RWI/FAY, if it
has not already. The passage of the aforementioned cold front will
result first in the redevelopment of scattered to numerous showers
along it, then behind it with abrupt clearing, with a veering of
surface winds to swly and a similarly abrupt increase into the 15-25
kt range (sustained), with gusts up to 30-35 kts for several hours
immediately following the frontal passage. These winds will then
lessen and continue to veer to wly or nwly later this afternoon and

Outlook: MVFR ceilings will be possible Sat, with showers Sat night,
ahead of, and in association with, a weekend frontal system.




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