Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 161822

Area Forecast Discussion
National Weather Service Raleigh NC
122 PM EST Sat Dec 16 2017

High pressure will settle across the southeastern US through the
weekend. A series of upper level disturbances will move through the
region through mid week.


As of 1040 AM Saturday...

Only a couple of minor changes were made to the near term forecast
this morning. Sky coverage has been increased through tonight --
though still mostly sunny/clear through this evening-- based on the
observed plume of cirrus on the nrn edge of the srn stream jet now
lifting nwd across the TN Valley/srn Appalachians, and sampled by
12Z RAOBs most-applicably upstream at LZK, OHX, and MHX. High
temperatures have also been raised up to a few degrees given that
the observed subsidence inversion at GSO this morning was about 1000
ft higher than BUFR analysis/forecast soundings indicated; and this
suggests mixing will be deeper today than forecast and consequently
tap into warm(ing) air at the base of the inversion centered just
below 850 mb.

Previous discussion as of 350 AM Saturday follows...

Surface high pressure over Alabama this morning will shift east and
will become centered over the SE states late this afternoon and
tonight, eventually shifting offshore Sunday. Meanwhile,  +100 meter
height rises and resultant strong mid-level subsidence inversion
aloft will spread through the region today. This dry and subsiding
airmass will result in dry conditions and abundant sunshine. While
northern and central areas will see modestly warmer temperatures
today, the southeastern tier will actually see slightly cooler
readings than on Friday. Highs ranging from upper 40s/near 50 north
to lower 50s south.  As the closed low over Mexico opens up and
ejects NEWD into the Southern and Central Plains overnight, high-
level moisture will spread northeast into the area. These high
clouds should have little to no bearing on min temps overnight. Lows
in the upper 20s to lower 30s, with some mid 20s possible in the
typically cooler locations.


As of 350 AM Saturday...

Aforementioned southern stream trough will undergo further de-
amplification on Sunday, shearing apart as it phases with the
northern stream trough across the Ohio Valley. Associated slug of
mid-level moisture will spread east, atop the mid/upper level ridge
in place across the SE US. Fcst soundings suggest these clouds could
be sufficiently opaque and thick enough to temper daytime
temperatures, especially across the western Piedmont where earlier
arrival would more readily coincide with peak afternoon heating.
Highs ranging from lower 50s NW to upper 50s south.

Surge of H8 warm moist advection on the northern periphery the low
to mid-level ridge axis and along the leading edge of a 30 to 35kt
nocturnal low-level jet could result in an isolated shower or
sprinkles across the far southern zones Sunday night. Otherwise,
cloudy with much milder lows generally in the 40s.


As of 243 AM Saturday...

There continue to be significant fluctuations in the precipitation
and temperature forecasts for the extended period as the medium-
range models are having are still exhibiting differences from one
another, particularly with a low/wave moving from the Southwest US
toward the Mid-Atlantic. For now, expect temperatures well above
normal through Tuesday night, highs in the low to mid 60s and lows
in the 40s. Above normal temps will continue through the period,
lowest Wednesday night/Thursday and moderating again thereafter.
Best chances for rain will come Wednesday night into Thursday,
although there will be at least a slight chance over portions of the
area starting Tuesday night and lasting through Thursday night due
to forecast uncertainty.

A fairly complex upper level pattern unfolds during the extended
forecast period. Monday, the high over the Caribbean ridges
northward into the Mid-Atlantic region, while a cutoff low persists
over the Southwest US/Baja/Northern Mexico and a low exists over
central Canada. Over Central NC, the flow aloft will generally be
zonal (west-east) through Tuesday night. The evolution of the upper
level lows/waves and their interaction with one another will
determine much of the weather for Central NC mid-week. It appears
the ridge over Central NC will hold up against the deepening trough
extending south-southeastward from the Canadian low Tuesday/Tuesday
night. Meanwhile, the low over the desert Southwest will progress
eastward into TX, then east-northeastward through the Midwest and
into the TN/OH valley region through Wednesday. The model solutions
really diverge with the evolution of this system beyond 00Z
Wednesday, with the GFS more progressive/quicker and absorbing the
low into the northern stream trough while the EC is slower and holds
onto a closed low/shortwave longer and further north. With the
continued differences, the temperature and precipitation forecast
remain fairly uncertain for Tuesday night through Thursday. However,
both models suggest the best chances for rain will be on Wednesday
night/Thursday. Another trough/low will dig south out of Canada
toward the Southwest US again on Thursday and Friday, resulting in a
return to southwesterly flow and ridging aloft over Central NC.

Differences also exist with regard to the surface pattern, but
generally expect a stationary boundary in the vicinity of the
Carolinas on Monday and Tuesday, with a Piedmont trough developing
ahead of a stronger cold front to the north. The evolution/timing of
the front differ between the GFS and EC. The GFS is faster/more
progressive with the front moving southward through Central NC while
the surface low moves across the Gulf coast and remains south of the
region. The EC is slower with the approach of the front as the
surface low is much farther north and approaches from the west,
possibly moving over or just south of the area Wednesday night or
Thursday. Both solutions forecast a wedge setting up late Thursday
into Friday from High pressure over the Northeast US. Another cold
front approaches from the west Friday and Friday night.


As of 120 PM Sunday...

Under the influence of high pressure that will drift across the
southeastern U.S., VFR conditions are anticipated through
Sunday. A stream of initially thin, high-level clouds/cirrus, in
association with a srn stream upper level jet, will gradually
thicken (to ceilings) and lower to around 10 thousand ft by the
end of the 18Z TAF period. Winds will be light swly to calm, on
the nwrn periphery of the aforementioned surface high pressure
over the sern states.

Outlook: Ceilings will continue to lower, to between 5 and 10
thousand ft, Sun aft, with briefly widespread virga that may
reach the ground in the form of patchy light rain or sprinkles
between 18Z Sun-00Z Mon. The nwd retreat of a warm front, and leading
edge of a more humid air mass, will result in a medium chance of
advection fog and stratus Mon night, mainly at FAY. There will
then be a high chance of rain and sub-VFR conditions, heaviest
and lowest at FAY, late Tue night through Wed night, with the
passage of a low pressure system across the sern U.S.




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