Scientific Forecaster Discussion
fxus62 krah 231130
Area forecast discussion
National Weather Service Raleigh NC
630 am 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 Florida and
offshore the southeastern US coast.
Near term /through tonight/...
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 Texas 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 Florida 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.
Short term /Friday and Friday night/...
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.
Long term /Saturday through Wednesday/...
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.
Aviation /12z Thursday through Monday/...
as of 630 am Thursday...
Cool and dry high pressure will build east across the Carolinas and
result in a high probability of VFR conditions, with periods of
considerable high level cloudiness (above 20 thousand ft),
especially at ern sites, through early tonight. Mainly light nely
surface winds around the high will become light and variable to calm
by late this afternoon and evening, as the high settles overhead.
Outlook: waves of low pressure will track northeastward, along an
offshore frontal zone Fri-sat; and there is a small chance that some
associated MVFR range moisture/ceilings and light rain will edge as
far wwd as Fay and rwi by late Fri or Fri night. There will be a
similarly small chance of a few showers in cntl NC, in association
with the passage of a cold front, early Sat night.