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FXUS62 KRAH 230725
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
225 AM EST Wed Jan 23 2019

.SYNOPSIS...
An approaching storm system will bring unsettled weather to central
NC today into Thursday. Drier and colder weather will arrive for the
end of the week.

&&

.NEAR TERM /TONIGHT/...
As of 1000 PM Tuesday...

The Winter Weather Advisory has been extended in time, through
17Z/noon for the nw Piedmont, where the highest probability of
measurable freezing rain and/or drizzle of up to several hundredths
of an inch are expected, and where a developing in-situ cold air
damming regime will likely cause temperatures to remain colder,
longer. There remains the potential for spotty light glazing to the
east of those areas, from Stanly Co. newd across Randolph, Alamance,
Chatham, Orange, and Person Co., where we will highlight that patchy
and lower confidence risk with a Special Weather Statement (SPS).
Observational trends will need to be monitored overnight for
consideration of a short-fused expansion of the Advisory in those
counties.

1038 mb Arctic surface high pressure centered about 60 miles off the
sern VA coast this evening will continue to extend wwd into NC
through early Wed, while drifting toward and just north of Bermuda.
A developing and strengthening WAA regime on the wrn periphery of,
and rising isentropically-atop, that dry air ridge characterized by
02Z surface dewpoints in the single digits to teens in cntl NC, has
caused an area of 1000-4000 ft ceilings to develop from ern GA and
SC nwd into w-cntl NC during the past several hours. It will be that
developing and deepening low level saturation that will likely
produce an axis of light precipitation into the srn and wrn NC
Piedmont late tonight-early Wed - a process that will take some time
and probably not materialize until after 09Z owing to the preceding
Arctic dry. Indeed, some light radar returns have been recently
noted from near GSP to AVL, though none has yet been reported at
nearby AWOS/ASOS.

Opposing physical processes will be at work as those clouds and
precipitation expand newd. Initially, downward IR from the expanding
low clouds, and some light ssely surface stirring, will cause
temperatures to rise as the clouds move overhead tonight, as has
been observed already as far east as HFF, SOP, and SCR in the past
couple of hours, where earlier upr 20s have been replaced by 32
degree readings. The aforementioned development of light
precipitation should then cause surface temperatures to diabatically
cool as they trend toward the wet bulb temperatures in the upr 20s
to near freezing. As was noted earlier, the highest probability of
overlap of sub-freezing surface wet bulb temperatures and light
precipitation will exist over the wrn Piedmont, near the Yadkin
River. And should that measurable precipitation indeed materialize
as expected, the colder NAM/NAM Nest/HiRes NMM/HiRes ARW
simulations, for temperatures holding in the 30s over the nw
Piedmont for most of Wed, will likely prove most realistic.

What could go wrong: If the precipitation is too light and/or
spotty, then incomplete wet-bulbing would result in surface
temperatures hovering aoa freezing, even in the Advisory area. That
is considered a lower probability, but next most likely solution
versus the official forecast discussed above. That alternate
scenario is more likely to the east of the Advisory area (in the SPS
counties), where the expected patchier and lighter nature of the
precipitation there was the primary reason we elected to not expand
the Advisory ewd.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
As of 345 PM Tuesday...

The icing threat will persist in the far NW sections until just past
daybreak, although models indicate temps rising above freezing in
these areas soon thereafter, not unreasonable given that the parent
high will be pushing offshore and in an unfavorable location to
provide a replenishing of cold dry air into our area, rendering it
vulnerable to the strengthening and deepening warm surge aloft. Pops
through much of the day will patchy and light as we await stronger
forcing for ascent with the potent but dampening mid level shortwave
trough moving in from the WSW. Expect widespread showers to move
into the W CWA by early Wed evening, with coverage peaking overnight
in tandem with passage of vigorous upper divergence, and waning by
lunchtime Thu as the trough axis and surface cold front move
through. One to two inches of rain is possible, given the strong
focused lift late Wed night into Thu morning, and a few urban
flooding problems are possible. Expect very gradual clearing W to E
late Thu with downslope low level flow and drying aloft. Highs Wed
will range from the mid 40s NW to the mid 60s SW, followed by a
minimal drop or steady/rising temps Wed night within strong warm air
advection. Highs from the low 50s to mid 60s Thu should occur early,
with steady or slowly falling temps during the afternoon. -GIH

&&

.LONG TERM /FRIDAY THROUGH TUESDAY NIGHT/...
As of 225 AM Wednesday...

Expect a return of cooler, drier weather in the wake of the cold
front. Surface high pressure will slowly migrate eastward over the
region through Saturday. Aloft, the eastern US will be under the
influence of a broad upper level trough through the weekend. A
series of weak upper level shortwaves will swing through the VA's,
however do not expect any precipitation over central NC with any of
them at this time. Highs on Friday and Saturday will generally range
from low 40s NW to around 50 degrees SE while overnight lows will
moderate a bit from mid 20s Friday night to upper 20s/Low 30s
Saturday night. The gradual moderation in temperatures is expected
to continue through the weekend. Highs in the mid 40s to low 50s
Sunday and lows Sunday night in the low to mid 30s.

A surface trough will develop to the west and build eastward over
the Appalachians and NC Piedmont Sunday through and Monday. Warm
southwesterly flow will again set up Monday night as the next
frontal system over the Midwest approaches the Mid-Atlantic. The
upper level trough will shift off to the northeast, resulting in a
return to southwesterly flow aloft as the next upper level trough
amplifies over the Rockies/Plains. Forecast confidence decreases
significantly with the next frontal system, which is expected to
move through the region sometime Tue/Wed. Low confidence in precip
chances and amounts, however expect the moderation in temperatures
to continue ahead of the front with a sharp drop in temperatures
behind it (regardless of whether or not precipitation accompanies
it).

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
As of 1255 AM Wednesday...

Lowering ceilings through 14Z will lead to widespread low end
MVFR/IFR conditions across central NC by daybreak. In addition,
patchy light precipitation is expected to break out over portions of
the Piedmont prior to daybreak. Where sfc temperatures are at or
slightly below freezing, light freezing rain or freezing drizzle
will occur, producing a thin glaze on the cold surfaces. This
freezing precipitation threat appears highest in vicinity of the
Triad terminals through 14Z.

A strengthening low level jet will lead to low level wind shear
conditions across most of central NC after 12Z, likely persisting
into this evening. Winds at 2000ft will be sly between 40-50kts.

The approach of s storm system later today will lead to an increase
in shower coverage west-to-east across the region this evening into
the overnight. In vicinity of the heavier showers, anticipate
visibilities as low as 1-2SM and sfc wind gusts near 40kts.

A cold front will cross central NC Thursday morning, bring an end to
the widespread showers, and veering sfc winds from the SW to the NW.
This NW flow will advect a drier more stable air mass into central
NC, leading to clearing skies NW-SE late Thursday morning through
early Thursday afternoon. VFR parameters expected across central NC
Thursday afternoon into the weekend. The exception will be a small
threat for high end MVFR ceilings in vicinity of the Triad Fri
morning with the passage of a mid level shortwave, and again Sunday
as another s/w approaches from the west.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon EST today for NCZ021-022-038.

&&

$$
SYNOPSIS...WSS
NEAR TERM...MWS
SHORT TERM...Hartfield
LONG TERM...KC
AVIATION...WSS
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