Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 210900

Area Forecast Discussion
National Weather Service Raleigh NC
400 AM EST Sun Jan 21 2018

High pressure will drift across the Carolinas through tonight, then
offshore. A frontal system will cross the region Monday night
through early Tuesday.


As of 359 AM Sunday...

Today: WV satellite imagery and 00Z upr air data depict continued
split flow across the ern two thirds of the CONUS, downstream of
increasingly merged/phased streams over n-cntl Mexico, at the base
of a deep nrn stream trough over the Rockies and related developing
closed low invof the Four Corners region. Downstream, 00Z upr air
data indicate the the trough over the FL panhandle remained a closed
mid-upr feature, but it should become an open wave as the strong srn
branch of the jet over s-cntl FL causes the low to shear ewd and off
the sern U.S. coast today. Any appreciable high level cloud
associated with the split stream flow will consequently flank NC to
the north and south. Instead, a scattering of high-based
stratocumulus may spill from the VA Blue ridge sewd, in generally
nwly to wnwly 850 mb flow, into mainly the nrn NC Piedmont this
afternoon. Mainly sunny sky conditions may consequently become
partly cloudy there after noon.

At the surface, a 1026 mb surface high pressure analyzed at 09Z over
sern GA will migrate newd across the Carolinas today. Continued
modification of the ridge will result in further moderating
temperatures that are expected to peak within a few degrees either
side of 65 this afternoon.

Tonight and Monday: As the trough over the Rockies migrates ewd, the
difluent exit region of the phased upr jets will likewise spread
ewd, and across the cntl Appalachians and middle Atlantic states
late tonight and Monday. Considerable high cloudiness will
overspread cntl NC from the wsw late tonight and early Mon, likely
peaking during the morning hours, before probably moving east during
the afternoon.

The surface ridge will have settled directly over cntl NC tonight,
before then drifting offshore on Mon. Coastal frontogenesis will
occur in return flow on the swrn periphery of the ridge; and this
warm front --and associated more humid air mass characterized by
leading edge of surface dewpoints in the 50s-- is expected to
retreat nwd across GA/SC on Mon, and into cntl NC by Mon evening-
early Mon night. The approach of that boundary and associated low
level warm/moist advection will result in both a nwd development of
a shield of stratocumulus and cloudy or mostly so conditions during
the mid-late afternoon and evening hours, and a slight chance of a
shower over the far sern RAH CWFA by 00Z Tue. Any pre-(cold)
frontal/warm conveyor convection should not arrive even in the wrn
Piedmont until Mon night based on the latest model guidance.

Despite the aforementioned increasing high clouds late tonight,
strong radiational conditions beforehand and otherwise will favor
lows mostly in the mid-upr 30s. High temperatures Mon are expected
to be similar to those of Sunday, though perhaps a degree or two
warmer in the southeast and cooler in the west, owing to presence
and timing of the aforementioned cloudiness.


As of 225 AM Sunday...

Fairly high confidence in a cold front approaching and moving
through Central NC as all medium-range models indicate it. The
uncertainty lies with the timing and intensity of precipitation as
in moves through the area. The upper low that develops off the
Rockies Sunday night will progress eastward through the Midwest on
Monday before moving northeast into the Great Lakes region Tuesday.
Meanwhile, the surface cold front will move eastward toward the Mid-
Atlantic. The GFS is a bit more progressive/quicker with the fropa,
while the ECMWF is slower, lagging 6-12 hours behind the GFS. For
now, expect best chances for convection between midnight and noon on
Tuesday. Winds are expected to strengthen ahead of the fropa, with
sustained winds increasing to around 15 kts gusting into the 20-30
kt range Tuesday afternoon, although there is potential for some
isolated higher gusts. Instability still appears to be limited and
with the bulk of the convection currently expected overnight,
chances for thunder will be somewhat limited across the western half
of the area. There is a better, but not great, chance for some
isolated thunder across the east late Tuesday morning, but
confidence is still not terribly high at this time. Have included a
slight chance for thunder with the latest forecast package.

With the increasing southerly flow over Central NC Monday into
Tuesday, lows Monday night in the upper 40s NW to mid 50s SE will
likely occur at midnight Tuesday and increase through the night. High
temperatures will range from upper 50s NW to mid 60s SE. With the
front expected to be through Central NC by Tuesday night, clearing
skies and cold advection in the wake of the front, expect lows in
the low to mid 30s.


As of 200 AM Sunday...

Wednesday through Sunday: Expect dry weather through Saturday, 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 as another
frontal system approaches the region, but details are still far from
nailed down at this time. Highs will be generally in the 50s through
Friday and lows in the upper 20s to low 30s Wednesday and Thursday
night. High and low temperatures will increase Saturday and Sunday
as the next system approaches.


As of 120 AM Sunday...

High pressure over the southeastern U.S. will result in mainly VFR
conditions through the 06Z TAF period. The one exception to VFR will
be patchy fog over the Piedmont, mainly around INT and GSO, this
morning. Initially mainly clear skies may become scattered with high-
based stratocumulus (5-6 k ft) later today, but these will be well
within VFR range and with limited coverage and duration.

Outlook: The threat of fog, and perhaps associated low ceilings,
will exist for late tonight-Mon morning again over the Piedmont, but
confidence in occurrence is below average given the latest trends in
both observational and model data. Otherwise, a better chance of IFR-
MVFR ceilings will exist with the northward retreat of a warm front
across cntl NC Mon evening and night. These conditions will persist
until a cold front and accompanying band of showers cross cntl NC
from west to east on Tue, with a subsequent return to VFR, generally
between 12Z-15Z at Triad sites and around 18-20Z at ern ones.




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