Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 202017

Area Forecast Discussion
National Weather Service Raleigh NC
317 PM EST Fri Jan 20 2017

several waves of low pressure and upper level disturbances will
impact the region through early next week.


As of 300 PM Friday...

A sharp boundary has set up over the Sandhills NE into the central
Coastal Plain this afternoon. Partly sunny skies with temperatures
in the lower 70s were found over the south and east, while northwest
of the boundary - there were low clouds, mist, and cool temperatures
in the upper 40s to lower 50s.

Multiple guidance sources show high probabilities of fog tonight
into Saturday morning with saturation near the surface all night.
Lows generally in the 50s expected, with some upper 40s NW. The
Piedmont Damming region may be the place that has the best chance of
dense fog, although areas to the south and east may also have that
chance increase later tonight.


As of 300 PM Friday...

Most guidance is agreement in depicting a nearly stationary front
over southern and eastern sections of our region Saturday into
Saturday night. There should continue to be lingering in-situ
damming conditions northwest of the boundary - with areas of fog and
overcast conditions. To the southeast, conditions will warm again
depending on the amount of sun that can break through. The first in
the series of mid/upper level waves will approach the region from
the south-southwest late in the day. Showers and isolated
thunderstorms are expected, with likely POP forecast along and SE of
the boundary in the mid to late afternoon into the evening. It
appears that any thunderstorms should remain below severe criteria
in the SE, with NIL threat NW. Highs generally 55-60 NW ranging to
70 SE.

This first round of showers/isolated thunderstorms should move down
east and away overnight. This will leave cloudy skies with a chance
of light rain or drizzle. Showers and even isolated thunderstorms
are then expected to begin to affect the SW portion of the region
before 12z/Sunday. We will begin to ramp up the POP again with the
stronger system`s approach from the SW for Sunday. Lows should hold
in the 50s to lower 60s SE.


As of 310 pM Friday...

...Storms, some potentially severe, and heavy rain expected
Sunday and Sunday night...

A deep upper trough will evolve over the southern US this
weekend, eventually becoming a closed low over the Southeast by
Sunday, per latest guidance. The main energy feeding the
evolutions is just now entering the southwestern US. The
associated surface low will track across the Deep South and then
deepen near the southern Appalachians Sunday. The NAM features
a Miller B surface low pattern on Sunday with a low west of the
mountains and secondary low forming over the Coastal Plain,
while the GFS/ECMWF/Canadian all show essentially one low
tracking somewhat atypically through western NC. It looks like
there will be two main episodes of precip; one early Sunday
associated with a leading, lower amplitude wave in the
southwesterlies, and then a second with the closed low and
associated DCVA/cold front. There are still a lot of small
scale differences and uncertainties in the timing and magnitude
of impacts, which may include severe storms and/or flooding.

Severe threat: Forcing for ascent will not be lacking on Sunday
given lead disturbances aloft and the eventual 100+ meter
height falls late Sunday Deep layer shear is plenty strong in
excess of 50kt, and low-level hodographs are long and
increasingly cyclonically curved owing to a 40-50kt LLJ and the
isallobaric response to the deepening surface low. The CAD
airmass that developed this morning may still be around in the
western Piedmont, and thus the early morning convection is more
likely to be elevated than later in the day when backed low
level flow has a better chance to advection the warm sector
inland. The late day convection should evolve into a squall line
as the forcing becomes more linear. Highs in the mid 60s
northwest to mid 70s southeast, along with dewpoints potentially
in the low/mid 60s, below mid-level lapse rates of 6.5-7C/km
could definitely support CAPE values of near 1000 J/KG.
Destablization is always a tough thing for models to handle in
the pattern, and they typically over mix the eastern edges of
the CAD airmass and overestimate SBCAPE. There is always the
potential for convection along the Gulf Coast to alter the
downstream mass fields and disrupt moisture transport. However,
the strengthen of the surface low does warrant some concern for
damaging winds and a few tornadoes inland to maybe the US-1
corridor later Sunday?

QPF and Flooding: Total QPF is between 2-3 inches through the
weekend, which may not be a significant flooding threat when
spread out over a couple of days. However, PW around 1.25" and
a cutoff low to our SW are favorable for at least some minor

The upper low will pull out on Monday bu may still support some
convection Monday afternoon as the cold pool aloft crosses the
area. POPs trend down more appreciably Monday night.  Height
rises over the region Tuesday and Wednesday will result in a return
to near or just above normal temps. Models then project a
sheared shortwave moving through the Midwest that would result
in a frontal passage and a low end chance of rain around


As of 1258 PM Friday...

24 Hour TAF period: VFR ceilings will exist in the KFAY region this
afternoon, with all other sites expected to remain IFR to MVFR CIGS.
Areas of fog and drizzle will continue over the KINT and KGSO areas.
With light wind overnight, areas of fog and low overcast will
develop, mainly after 03Z/21. Most TAF sites should fall into the
LIFR VSBY and CIG range between 09z and 13z/21. Some minor
improvement will occur after 13z - with some MVFR CIGS from KFAY to
KRWI, but lower IFR conditions from KRDU to KINT through into the

Long term: Sub-VFR conditions are expected with showers and even
some thunderstorms at times Saturday nigh through Monday morning.

A return to VFR conditions expected Monday afternoon and Tuesday.




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