Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 140540

Area Forecast Discussion
National Weather Service Raleigh NC
935 PM EST Wed Dec 13 2017

A fast moving upper level disturbance will move just north of the
region tonight, and will be followed by a weak surface cold front
that is expected to pass across central North Carolina Thursday
morning. High pressure will then build into the area by late


As of 935 PM Wednesday...

WV satellite and 00Z upper air data depict a lead shortwave trough
beginning to lift enewd to newd into swrn PA, while a trailing shear
vorticity-dominated one centered invof srn Lake MI was digging
across the lwr Great Lakes and toward the OH Valley. Related to the
lead trough, a narrow band of DCVA/forcing for ascent --and an
associated, approximately 100 mile wide axis of mid level moisture
now over the NC Piedmont, and centered around 10 k ft per surface
observations and 00Z GSO and RNK RAOB data-- will pivot east across
e-cntl NC through 05-06Z. In addition, strongly veering flow through
the lowest couple of kilometers (WAA) --and implied forcing for
ascent-- was evident throughout the lwr OH Valley, srn middle
Atlantic, and Southeast. A band of precipitation, mostly in the form
of snow, not coincidentally was pivoting newd within a zone of
strong QG-forcing for ascent accompanying the lead shortwave trough
and preceding strong lower tropospheric WAA - a truly textbook case
of QG-theory at work.

At the surface, a parent, 993 mb cyclone was analyzed over far nwrn
OH at 02Z, with a preceding warm front analyzed sewd from the low -
across cntl OH and the VAs and on the srn flank of the
aforementioned precipitation band. To the south of the warm front,
sly to swly low level flow exists in the warm sector over the

Light radar returns have steadily lowered, to about 2000 ft in a
narrow, approximately 10 nm wide arc between IGX and RDU in the past
hour. Although extremely narrow, and originating from the
aforementioned narrow band of ceilings centered well above the
surface - around 10 k ft (between 8-13 k ft per KRAX data), a 5-10
minute period of a few flurries may reach the surface as the parent
moist axis shifts ewd.

Otherwise, a strong swly to wswly llj evident on the RTP profiler,
between 30 and 45 kts at 2 and 5 k ft, respectively, will
occasionally mix to the surface and manifest as gusts up to 25 mph,
as has already been the case across several Piedmont sites in the
past couple of hours. Given that WAA is occurring in that layer,
which will promote stability, it is likely that at least temporary
surface decoupling and lessening of the surface wind to 5-6 kts or
less will allow temperatures to dip into the middle 20s to around
freezing by daybreak - about 10 degrees from 9 PM readings.


As of 248 PM Wednesday...

Fair weather is in store for Thursday in the wake of tonight`s
departing short wave trough and the sfc cold front continuing to
push south of our area. With the upper flow becoming westerly, look
for the high clouds currently located over the central Plains to
gradually move eastward and across our area.  Otherwise, with low
level thicknesses remaining close to their pre-frontal values (some
30-40m higher than today), we should see highs at least 8-10 degrees
warmer than today. Overall, Thursday will be partly cloudy with
highs in the lower 50s.

Thursday evening the low level wind field will shift to the N/NE as
high pressure over the Ohio Valley moves east.  Although low level
thicknesses may drop a few meters on this N/NE flow...increasing
high clouds from the southwest should offset the cooler airmass.
Lows in the upper 20s to around 30 under partly cloudy skies.


As of 255 PM Wednesday...

A strong upper short wave moving across the Ohio Valley will nudge a
reinforcing surge of cold air into the area. Surface high pressure
will then build across the area, resulting in plenty of sun, but
suppressing high temperatures to the 40s Friday and Saturday.
Morning lows Saturday will be mostly upper 20s, with some areas
across the south only falling to near freezing. Short wave ridging
will be amplifying north into the Eastern Conus by Saturday night,
with increasing heights and initiation of warm air advection in
return flow as the surface high moves offshore. Cloudiness will be
on the increase by later Sunday with highs reaching mostly low and
mid 50s after a morning low near freezing.

By Monday, a low pressure area will be lifting northeast out of the
Gulf of Mexico. There is a good deal of uncertainty as to how the
details will play out, with model timing issues as the ECMWF has a
strong fast moving short wave race across the Ohio Valley to kick
the system offshore quickly, resulting in precip pretty much limited
to the southeast, while the GFS`s slower short wave allows for a
couple of waves to ride up the upper ridge into the area to produce
more widespread precip and prolonging the duration of precip chances
into Tuesday. Will maintain our ongoing forecast of chance PoPs from
Sunday night through Tuesday and allow details to emerge from later
model consensus. That said, precipitation type will be no problem
with southern stream flow into the area allowing highs Monday and
Tuesday to reach the 50s after morning lows in the low to mid 40s.
Wednesday looks to be a dry and a bit cooler, but still near
seasonable in the 50 to 55 range.


As of 1230 AM Thursday...

There is a high probability of VFR conditions across central NC
through 00Z Friday. A fast moving upper disturbance continues to the
east at this hour, leaving the northern piedmont under clearing
skies and a brisk Northwest wind. An attendant sfc front will cross
central NC later tonight through early Thu. This system will be
accompanied by a deck of mid level clouds with ceilings at or above
10000ft. These clouds will depart prior to sunrise Thu. Low level
wind shear is also a concern in the proximity of KFAY and KRWI as
winds just a couple thousand feet above the surface will be swly 35-
40kts for a short period of time.

VFR conditions are expected to continue through Saturday night.
Another area of low pressure will affect our region late Thu night
through Friday. Ceilings associated with this system are expected to
remain VFR. A low pressure system approaching from the west may
bring a threat for sub VFR ceilings to central NC late in the




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