Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 231724

Area Forecast Discussion
National Weather Service Raleigh NC
1224 PM EST Thu Nov 23 2017

Initially cool high pressure will weaken and modify as it expands
east across the middle Atlantic and southeast states through Friday
night. Meanwhile, at least a couple of areas of low pressure will
track from the eastern Gulf of Mexico northeastward across FL and
offshore the southeastern US coast.


As of 1224 PM Thursday...

Morning analysis shows plenty of cirrus moving up from the south,
ahead of the short wave over N FL.  This cirrus is thickest from
about the I-95 corridor eastward, then thins out westward. Our
current forecast anticipated this cirrus arrangement, thus our temp
forecast reflects slightly warmer temps (due to more abundant
sunshine) across the western Piedmont and Triad compared to
locations farther east.  Otherwise, we`re still looking at fair
weather the rest of today.  However its worth noting that the latest
HRRR shows that the western edge of the light rain shield that`s
currently located over coastal sections of SC will eventually lift
north and may slide across our far SE zones, mainly southern Sampson
Co.  If this trend continues, we may have to put a slight chance PoP
across that area for this evening...but fortunately any rain that
falls there should be very light and short-lived (only a hour or so
later this evening).

Prev disc as of 345 AM Thursday...WV satellite imagery this morning
depicts a positively-tilted longwave trough stretching from Atlantic
Canada swwd across the cntl Appalachians, Deep South, and swrn Gulf
of Mexico. This trough will be reinforced through tonight, as a pair
of notable shortwave troughs upstream, over the upr Midwest and
nrn/cntl Plains, respectively, amplify sewd in nwly flow aloft. The
models indicate the one now over the upr Midwest will phase with a
perturbation lifting newd from the GOM portion of the positive-tilt
trough, by tonight over the Carolinas. Model cross sections depict
associated strong omega over cntl and ern NC through tonight, but
all above 500 mb (centered between 300-400 mb).

At the surface, an elongated 1024 mb ridge extending this morning
from the lwr Great Lakes swwd into srn TX will continue to weaken
and modify as it migrates ewd, and becomes centered over the srn
middle Atlantic states by 12Z Fri. Meanwhile, a couple of areas of
low pressure are forecast to track, along a frontal zone, from the
ern GOM newd across FL and off the sern US coast.

The main forecast problem and uncertainty will be how cirrus/
cirrostratus in swly high level flow downstream of the
aforementioned positive-tilt trough axis may affect temperatures
today, mainly over the ern half of the forecast area. Full sun
temperatures would favor highs ranging from upr 40s north to middle
50s, but the considerable high cloudiness may serve to keep
temperatures in the srn and cntl Coastal Plain closer to 50 degrees.
The high clouds will edge ewd overnight, as the upper trough axis
likewise progresses ewd, with associated strong radiational cooling
supportive of upper 20s to lower 30s, highest with longest-lingering
high clouds in the Coastal Plain.


As of 400 AM Thursday...

Weak shortwave ridging will briefly build across the Appalachians,
between a positively-tilted trough axis forecast to progress to the
coast through the day Fri, and a quickly-following trough forecast
to amplify from the upr Great lakes to the lwr MS Valley through Sat
morning. At the surface, high pressure will continue to extend swwd
across the srn middle Atlantic states (including NC). Temperatures
will consequently continue to modify in association with the surface
ridge, with highs in the mid 50s to lwr 60s, and lows within a few
degrees either side of 35.


As of 320 AM EDT Thursday...

A series of moisture-starved shortwave troughs traversing eastward
within the Eastern US long wave trough will prove of little
consequence as they move through the region, with very limited rain
chances expected to accompany the attendant sfc cold front moving
progged to move through the region late Saturday. Westerly flow
preceding the fronts arrival will result in mild daytime highs in
the low to mid 60s Saturday afternoon.

The cooler temperatures behind the cold front Sunday and Monday will
be rather short-lived as the upper ridge over the Western US
transition eastwards atop the Eastern US by Tuesday/Wednesday. Highs
Sunday and Monday in the low to mid 50s north to upper 50s south,
then warming back up into the 60s and possibly even near 70 across
the southern coastal plain and Sandhills by Wednesday.

Model spread increases with the timing of upper trough ejecting out
of the Rockies with the EC a full day slower than the GFS. Despite
time discrepancies, this system, at this time, does not appear to be
one that will produce any significant and much needed rainfall.


As of 1224 PM Thursday...

Through 24/18Z: High pressure will continue building over the area,
resulting in light winds dry weather.  Clouds will mostly consist of
high clouds aoa 20k ft agl.  VFR conditions through the 24 hour TAF

After 24/18Z:  A cold front will move across the region late
Saturday and may bring a brief period of sub-VFR conditions during
that time.  Otherwise, VFR conditions expected through Tuesday.





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