Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 150521

Area Forecast Discussion
National Weather Service Raleigh NC
1230 AM EST Fri Dec 15 2017

A pair of frontal zones will merge over south-central NC tonight,
and along which an area of low pressure will develop and track, to
off the srn middle Atlantic coast through Fri. High pressure will
follow and migrate across the southeastern US through the weekend.


As of 920 PM Thursday...

WV satellite and 00Z upr air data depicted this evening a positively-
tilted trough extending from the wrn shore of Hudson Bay swwd across
the upr Midwest, Four Corners, and nrn Mexico. The trough spanned
both the nrn and srn branch of the upr jet, with associated split
stream flow evident across the OH valley and nrn middle Atlantic
states and over the Deep South, respectively. These two streams are
forecast to merge and intensify to near 200 kts over the srn middle
Atlantic states during the next 12-18 hrs, as the nrn stream portion
of the aforementioned trough axis migrates sewd, and the srn stream
bulges slightly nwd in the wake of an inconsequential srn stream
perturbation now migrating across the Carolinas.

At the surface, a lead, weak cold front/trough axis that settled
across cntl NC earlier Thu was analyzed at 02Z from the TX Gulf
coast enewd through s-cntl MS/AL, n-cntl GA/SC, and srn NC. A more
marked air mass change is apparent with a trailing front analyzed,
and evident as a fine line in AKQ radar data during the past couple
of hours, from the nrn OBX nwwd to swrn VA, then swwd through wrn
NC. This frontal zone was also evident at both 925 and 850 mb
between RNK and GSO this evening. In fact, there was a 22 C
temperature difference between CHS and PIT at 850 mb, indicative of
the sharp temperature gradient across the aforementioned frontal
zones - and related to the extreme 200-300 mb winds forecast to
develop over NC/VA on Fri. These two low level frontal zones are
forecast to merge in a trough/convergence axis now developing over s-
cntl NC per recent surface obs, with associated light nely flow, to
calm within the trough axis, likely to encompass cntl NC tonight.

The models suggest the flow just above the surface, in the 925-850
mb layer, will become sly and ascend isentropically-atop the surface
front, such that initial passing mid-high cloudiness will yield to a
scattered to broken stratus layer over the nrn Piedmont and nrn
Coastal Plain, between US 64 and the VA border, by or shortly after
12Z. While associated saturation and warm rain process light rain
may develop there, a relatively dry sub-cloud layer below 1500 ft
will cause most of that precipitation to evaporate before reaching
the ground south of the VA border, should it even develop within
that shallow moist layer.

Otherwise, there may be some patchy fog within the aforementioned
trough axis over srn NC, from near VUJ to FAY to CTZ, owing to
calm/radiational cooling of a marginally more moist surface layer
characterized by surface Td`s generally between 30-34 degrees there,
relative to mid 20s to the north. No changes planned for previously
forecast lows within a few degrees of 30.


As of 253 PM Thursday...

The aforementioned low pressure wave will quickly lift to the NE
Friday morning as a northern stream short wave, currently over the
central Plains, moves east and sweeps across our area during the
afternoon.  As the wave departs and the short wave trough axis moves
across, dry NW flow will return in their wake, clear the sky out by
late afternoon.  Other than clouds early in the day, look for fair
weather with highs from the mid 40s NW to lower 50s SE.

Clear and cold Friday night with high pressure building over the
area, light or calm winds, and good radiational cooling conditions.
That should set us up for lows dipping down in the mid 20s...perhaps
even colder in the usual spots.


As of 247 PM Thursday...

Surface high pressure over the area will produce sunny skies with
slightly cooler than normal temperatures to kick off the weekend.
Highs Saturday will reach the upper 40s north to low 50s south
followed by lows at or slightly below freezing Saturday night. The
high pressure will edge offshore Sunday, with cloudiness increasing
in return flow and associated moisture advection along with low
amplitude ridging aloft on Sunday. The warm air advection and rising
heights will produce modestly warmer highs in mostly the mid and
upper 50s.

Skies will become cloudy with scattered showers Sunday night as a
series of waves begin to lift northeast into the area in deep
southwest flow off the Gulf. The low amplitude ridge will linger
through Tuesday, providing a conveyor for these waves and associated
periods of enhanced shower activity. As we remain in the warm and
moist airmass through the period, highs will be similar both Monday
and Tuesday, ranging from upper 50s north to low and mid 60s south.
Very difficult to forecast how much rain we might get given the
duration of the event and model difficulties with timing and
strength of the waves.

A strong but relatively flat short wave in the northern stream
moving across the Tennessee and Ohio Valleys will push a cold front
east, effectively cutting off our moisture Tuesday night. The upper
flow is basically zonal and the high pressure building in behind the
front is only modestly colder than the air it is replacing, so highs
will take only a mild hit Wednesday and Thursday...mostly in the low
and mid 50s.


As of 1230 AM Friday...

VFR parameters should persist across central NC through 08Z. After
08Z, chances will increase for the potential for a deck of stratus
to develop with MVFR ceilings 1500-2500ft probable, predominantly
across the northern TAF sites. This deck of ceilings expected to
persist until mid day, then gradually dissipate. Some isolated fog
may also develop at FAY near daybreak, but low confidence, as mid to
upper level cloud deck could dictate extent and/or duration.

There is a high chance that VFR parameters will occur Friday night
through Sunday morning. The next threat for sub VFR parameters is
expected late Sunday into early next week as a low pressure system
approaches from the west-sw.




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