Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 210248
AFDRAH

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

.SYNOPSIS...
Several waves of low pressure will impact the region through
Monday.

&&

.NEAR TERM /OVERNIGHT/...
As of 950 PM Friday...

Evening upper air analysis depicts a narrow s/w ridge of high
pressure over our region aloft while a wavy surface boundary
stretched along our border with South Carolina. A very saturated air
mass in the lowest 2500 ft has resulted in a deck of low clouds
across the nw half of the forecast area.

Across the southeast half, patchy high clouds were drifting
overhead; though, as the night wears on, expect low clouds and/or
areas of fog to develop over this region. A few sites have reported
visibilities as low as a quarter of a mile. Checking in with county
law enforcement officials reveal that the dense fog was spotty and
not causing any travel issues at this time. Currently have a SPS for
pockets of dense fog through 1 AM. Do not plan to issue an advisory
at this time. If I had to, it would be over the se third of the
forecast area though once the low clouds develop or move in, the sfc
visibility will improve above a quarter of a mile. Will continue to
monitor.

Do not expect to see much on the radar until closer to daybreak over
our far southern counties. Thus, aside from a patch or two of
drizzle, expect no rain overnight. Min temps mid-upper 40s northeast
third to the lower 50s south.

&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
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.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
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
flooding.

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
midweek.


&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
As of 630 PM Friday...

24-Hour TAF period: Aside from a brief period of clearing at KFAY,
all TAF sites remain MVFR/IFR as of 00Z. Expect conditions to
continue deteriorating overnight and remain sub-vfr through the TAF
period. By 06Z, expect cigs in the the 100 to 300 ft range with
visibilities at or below 1 SM. Expect fog to develop for at least a
few hours tonight at all terminals. Guidance suggests some slight
and brief improvement Saturday afternoon, but conditions should
remain sub-vfr. Winds will also be largely calm overnight and light
Saturday afternoon.

Looking ahead: Sub-VFR conditions are expected with showers and even
some thunderstorms at times Saturday night through Monday morning. A
return to VFR conditions expected Monday afternoon and Tuesday.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...MWS
NEAR TERM...WSS
SHORT TERM...BADGETT
LONG TERM...SMITH
AVIATION...KC/BADGETT


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