Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 200655

Area Forecast Discussion
National Weather Service Raleigh NC
155 AM EST Sat Jan 20 2018

High pressure will drift across the southeastern United States
through the weekend.


As of 930 PM Friday...

Just minor updates required with this evening update, mainly
matching up temperature trends which were slower than expected
across portions of the Triad and a bit faster across the sandhills
of North Carolina. Still some concern with isolated fog development
tonight, especially across the sandhills where the surface dewpoint
depression is only 5 degrees in spots. Confidence is low
though, mainly due to the high overhead and the dry airmass aloft,
so have continued to leave it out of the weather grids for now.

Previous Valid Discussion...

Deep westerly flow over the eastern U.S. coupled with a sfc high
anchored over the Deep South will maintain a weak lee side trough
over the Piedmont tonight. While sfc winds will likely decouple
around sunset, this sfc feature should induce a weak south-sw flow
over central NC overnight just above the surface. Thus, should see
overnight temperatures a few degrees warmer. Some members of the NAM
hinting at possible low cloud/fog development above the snowpack as
warmer air just above the surface overrides the colder air at the
surface. Will continue to monitor but believe tomorrow night may be
a better set-up for low clouds/fog. Still expect to see a disparity
between locations where snow covers the ground and where snow is
absent. Minimum temperatures should range from 17-21 where the snow
depth is several inches deep, to the mid-upper 20s elsewhere.


As of 235 PM Friday...

The atmosphere will continue to warm as the deep wly persists and
the sfc high remains positioned over Southern GA/north FL. The
relatively dry air mass will continue to promote wide diurnal
temperature swings from sub freezing early morning lows to mild
afternoon temperatures. Under sunny skies, expect afternoon temps in
the upper 50s to around 60. The exception will be where snow several
inches deep remains. The snow pack of this depth will continue to
have an influence on the air temps, so afternoon temps in the lower
50s will be more likely.

Saturday night, a moistening atmosphere aloft coupled with a
persistent low level warm air advection should promote the
development of fog or low clouds late Saturday night/early Sunday
morning, primarily across the Piedmont where recent heavy snow fell.
If the fog does develop, areas of freezing fog may occur as early
morning temperatures will be slightly below freezing. Min temps 29-


As of 200 AM Saturday...

Monday and Tuesday: Fairly high confidence in the low pressure
system approaching and moving through Central NC as all medium-range
models are in good agreement. An upper low will develop off the
Rockies on Monday, progressing east-northeastward through the
Midwest and into the Great Lakes region through Tuesday. Meanwhile,
the surface cold front will move eastward toward the Mid-Atlantic.
With the increasing southerly flow over Central NC Monday into
Tuesday, high temperatures will range from mid 50s NW to mid 60s SE.
Lows Monday night in the mid 40s NW to low 50s SE. Of bigger concern
than temperatures is the chance for precipitation and possibly some
thunderstorms as the line of pre-frontal convection moves into and
through the area. As of the latest model runs, MUCAPE appears to be
less than 500 J/Kg and the surface low is displaced well to the
north over the Great Lakes. There are still slight timing and
intensity/amount of precip differences between the models, which
will play a role in forecast for possible thunder. For now, given
the uncertainty, will keep mention of thunder out of the forecast.
Both models do indicate precipitation exiting the area by Tuesday
night, with cold air starting to filter in behind the front. As a
result, Tuesday night will likely be dry and temperatures quite a
bit lower than the previous night, in the low to mid 30s.

Wednesday through Saturday: This part of the forecast is likely to
be dry, with a reinforcing dry cold frontal passage on Wednesday
night/Thursday. Surface high pressure is expected to build into the
area in the wake of the secondary cold front and remain over the
area into Saturday. Chances for precipitation will increase over the
weekend, but details are still far from nailed down. Highs will be
generally in the 50s and lows in the upper 20s to low 30s Wednesday
and Thursday night, increasing into the 30s Friday night.


As of 1235 AM Saturday...

High pressure over the southeastern US will result in mainly VFR
conditions through the weekend. The exception will be a good chance
of fog and associated ceilings/visibility restrictions between 05Z-
14Z Sun, with some lingering sub-VFR ceilings possible through the
midday hours on Sunday, as that moist layer lifts and disperses with
diurnal heating.

Outlook: Sub-VFR conditions, and showers, are expected to affect
cntl NC along and ahead of a cold front Mon evening through early




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