FXUS62 KRAH 180759

Area Forecast Discussion
National Weather Service Raleigh NC
300 AM EST Mon Dec 18 2017

Weak high pressure will extend across central NC today, then move
offshore on Tuesday. A low pressure system will bring unsettled
weather to central NC by Wednesday.


As of 300 AM Monday...

Just minor adjustments made to the near term forecast, primarily to
the sky cover and a slight uptick in the high temperatures today.

Weak, baggy high pressure will extend across central NC from a
parent high centered over the Deep South. The low level wly flow
will advect a notably warmer air mass into our region. This air mass
coupled with periods of partly sunny skies will boost afternoon
temperatures into the low-mid 60s, at least 10 degrees warmer than
Sunday's high temperatures. The main caveat is that there is the
potential for skies to remain overcast or mostly cloudy through mid
day or early afternoon. If the sun fails to appear by early
afternoon, high temperatures will likely be about 5 degrees cooler
(mid-upper 50s nw to the lower 60s se).

Tonight, the tranquil weather pattern will continue with no
precipitation anticipated. A surge of warm, moist air from the south
that the models have been fairly consistent with should lead to
increasing cloud cover with the possibility of fog to develop,
primarily south of highway 64. Min temps relatively mild for
December with overnight temperatures ranging from the upper 30s to
around 40 NW to the mid 40s SE.


As of 300 AM Monday...

Tuesday, weak high pressure at the surface will weaken further while
wly flow persists in the mid-upper levels. This flow will induce the
development of a lee side trough in the western Piedmont. The
resultant low level west-sw flow will continue to usher warm air
into central NC. Potential for afternoon temperatures to reach 70
degrees across our southern tier counties with 60s common elsewhere.
The day will start out rather cloudy with periods of sun probable in
the afternoon. Similar to today, if the clouds persists into the
afternoon, high temperatures will end up 5-6 degrees cooler than

Main adjustment in the Tuesday night forecast was to delay the onset
of precipitation across the western-southern Piedmont until midnight
or a bit later when the moisture is deep enough and lift sufficient
to generate precipitation. Otherwise expect increasing/thickening
cloud cover southwest-to-northeast with mild overnight temperatures
ranging from the mid 40s far north to the lower 50s south.


As of 220 AM Monday...

Wednesday and Wednesday Night: Models are in much better agreement
with the upper level wave moving into the region on Wednesday. High
confidence in a wet forecast for Wednesday, with rain diminishing
overnight as the deamplifying low shifts offshore. A the surface,
though models are in fairly good agreement, small differences in the
location/track of the surface low will have large impacts on the
temperature and precipitation forecasts. The consensus is for the
low to remain south of the area, with the backdoor cold front
sliding south into the area during the day Wednesday. That being
said, the expected rainfall and overcast skies should limit cooling
initially so expect highs in the mid to upper 50s, though if the
more aggressive NAM solution verifies, temps could be a degree or
two lower. High pressure will ridge southward into Central NC
Wednesday night, though clouds and light rain may linger into the
overnight hours. Should skies clear more, lows could be a bit lower,
but for now expect lows generally in the mid to upper 30s, a good 10-
15 degrees lower than Tuesday night.

Thursday through Friday Night: A CAD wedge will set up on Thursday,
lingering over the area, especially in the northwest, into Friday.
With good CAA and clear to partly cloudy skies, temperatures
Thursday will top out in the upper 40s to mid 50s (north to south),
while lows will be in the mid 30s to low 40s. Friday could be a bit
warmer, especially in the southeast, but will depend on if/when the
wedge erodes ahead of the approaching frontal system. With a grain
of salt, expect highs in the mid to upper 50s and lows in the mid to
upper 40s Friday and Friday night. With the southwesterly flow aloft
overrunning the northeasterly cold air advecting into the region at
the surface, expect increasing cloudiness and chances for some light
stratiform rainfall Friday and Friday night. Otherwise, this part of
the forecast will be relatively dry.

Saturday and Sunday: The strong low pressure system that develops
off the Rockies in the Midwest on Friday will strengthen as it
progresses eastward Friday night into Saturday. Central NC will be
under the influence of the upper level ridge, with the increasing
southwesterly flow advecting significant moisture into the
Appalachians. The EC and GFS differ on the evolution of the front
and pre-frontal convection during this part of the period, thus
confidence is relatively low. The GFS is more progressive, moving
the front through the area on Saturday with the frontal zone
stalling over the area, resulting in an extended period of rainfall
for Central NC. However, the EC hangs the front up along the
mountains on Saturday and Sunday, with best chances for rain along
the VA/NC border. As a result of these significant differences,
specifics of the forecast for this time period are very much up in
the air.


As of 1225 AM Monday...

Pockets of MVFR ceilings can be expected across central NC through
13Z. The threat for sub VFR ceilings appears highest in proximity of
KFAY. Otherwise, VFR parameters anticipated through 06Z Tuesday.

After 06z Tuesday, areas of low clouds and fog is expected to
develop across central NC, primarily south of highway 64. The
IFR/LIFR conditions appear highest in vicinity of KFAY, but may also
plague KRWI and KRDU. VFR parameters should return areawide by 16Z

A more persistent round of sub VFR ceilings along with areas of rain
will occur late Tuesday night through Wednesday night as a low
pressure system affects the region. Currently, VFR parameters are
expected to return for Thu and Fri.





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