Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 141447

Area Forecast Discussion
National Weather Service Raleigh NC
945 AM EST Sun Jan 14 2018

Chilly high pressure will extend across the Carolinas today and
tonight. The high will retreat to our northeast Monday, bringing
more seasonable temperatures to our region on Tuesday. A strong
upper level trough will cross the region Tuesday night through
Wednesday night.


As of 945 AM Sunday...

Forecast looks good. Aside from shallow moisture around 850 mb
beneath the inversion aloft noted on the 12z GSO sounding, dry air
is in place and upstream, so expect little more than a few flat cu
and wispy high clouds today as subsidence dominates. Despite the
abundant sunshine, low level thermal profiles and cold air advection
today support highs of just 33-40, 15-20 degrees below normal highs
for this time of year. Gusty winds from the NNE should continue into
the afternoon before slowly decreasing late, as the MSLP gradient
slackens. -GIH

Earlier discussion from 300 AM: Chilly high pressure centered over
the northern Mid-Atlantic and New England will extend south into
central NC today and tonight. 00Z upper air analysis at 850mb
depicts a deep thermal trough over the eastern half of the nation
with the 0 degree isotherm well south into south Texas and the
northern Gulf. Nly low level flow will maintain cold air advection
over central NC today. This cold air advection already evident by
pre-dawn temperatures in the 20-25 degree range across the Piedmont.
With early morning temperatures likely starting in the 17-23 degrees
range, even a 15 degree temperature recovery translates to afternoon
temperatures just above freezing across the northern counties, and
mid-upper 30s across the south. While skies will be sunny-mostly
sunny through most of the day, an approaching weak upper level s/w
may result in patchy high clouds streaking in from the sw late in
the day.

Tonight, aside from brief periods of thin/patchy mid and upper level
clouds, expect clear/partly cloudy skies and a very dry/cold low
level atmosphere. Currently think that clouds aloft will be too thin
to have much affect on our temperatures so will lean toward the cool
side of guidance. Min temps 15-20, with 10-15 above at the normally
colder spots in the Piedmont.


As of 300 AM Sunday...

Monday and Monday night, sfc ridge weakens but will maintain an
orientation south southwest-north northeast across central NC. This
will maintain the bone dry air mass in the lower levels of the
atmosphere. 850mb flow attains a wly direction Monday, initiating
warming in the lower atmosphere. Aloft, wly flow will begin to
stream moisture ahead of a shear axis crossing the mid MS/lower OH
Valley. Thus, expect periods of filtered sunshine through the veil
of high clouds, particularly across the western Piedmont. Although
we will start out quite cold, the moderating atmosphere and
developing wly flow will aid to boost afternoon temperatures into
upper 30s/around 40 NW to the low-mid 40s SE. The moderating air
mass continues Monday night under mostly clear/partly cloudy skies.
Min temps generally 20-25 degrees, though slightly colder in the
outlying areas of the Piedmont.


As of 355 AM EDT Sunday...

The initial inland sfc low associated with the high amplitude
positive-tilted trough over the NE-central US will fill over the
Eastern Great Lakes/NE US late Tuesday-Tuesday night, eventually
giving way to a series of coastal low development along the Mid-
Atlantic Coast Wednesday-Thursday as the potent upper trough assumes
a neutral tilt as it pivots east across the region.

There continues to be a large model spread WRT to the timing and
strength/amplification of the trough and it`s associated ana-frontal
precip band, driven by strong shortwave dynamics and f-gen that`s
progged to move west-east across central NC late Tuesday night and
into the day on Wednesday.

Operational versions of the EC and GFS, have trended drier with
liquid equivalent amounts over the past several runs. Meanwhile, the
GEFS and EPS avg mean liquid QPF is generally around a 0.10" across
much of the area, with slightly higher amounts of near 0.20" across
the far northern/VA bordering counties.

With afternoon temps on Tuesday expected to moderate into the 45 to
50 degree range Tuesday afternoon ahead of the attendant sfc cold
front, current timing and thermal profiles, still supports precip
starting as rain or a rain-snow mix at onset Tuesday night with west
to east  change-over to snow early Wednesday morning and into the
afternoon(east)as the expansive Arctic High builds in from the west.
As previous forecast discussions have noted, one model pitfall and
potential forecast concern is that NWP models tend to advect low-
level cold air east of the mountains too quickly, which in itself
can be a self limiting process as considerable dry sub-cloud layers
develop. Given continued high uncertainty, will make only minor
changes to current forecast. Stay tune.

The Arctic blast is very strong but short-lived with H8 temps
progged to crash to -12 to 13C Wednesday night as the parent high
settles over the Gulf Coast region. Highs generally in the 30s both
Wednesday and Thursday, with quick moderation as temperatures return
above normal by the weekend.


As of 640 AM Sunday...

There is a high likelihood of VFR parameters across central NC
through 12Z Monday. Chilly high pressure at the surface will
continue to build into central NC today. Nly sfc winds 7-12kts will
occur with infrequent gusts 16-20kts probable, especially through
18Z, and mostly in vicinity of KRWI and KFAY. Sfc winds will subside
to less than 10kts by sunset, and remain light and variable on

VFR parameters should continue through late Tuesday. An upper level
trough will increase the cloudiness late Tuesday through Wednesday,
lowering cloud bases to low end VFR and possibly high end MVFR close
to daybreak Wednesday. Ceilings should lift/dissipate Wednesday
night into Thursday as central NC experiences subsidence/sinking air
in the wake of the departing upper level trough.





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