Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Raleigh NC
1029 PM EST Thu Nov 21 2019
high pressure will extend across the southeastern US ahead of a cold
front that will cross Virginia and NC late Fri and Fri night. Cool high
pressure will follow until an area of low pressure tracks across the
region Sat evening.
Near term /tonight/...
as of 1020 PM Thursday...
Downstream of a srn stream mid-upr low near the Nevada/Arizona border and a
positive tilt nrn stream trough extending from the upr Great Lakes
swwd to the nrn plains this evening, a low amplitude srn stream
perturbation over ern Texas and OK will stream ewd across the mid-south
and Tennessee Valley overnight. Related mid-high lvl moisture will continue
to stream across the srn middle Atlantic (va/nc), but with associated
rain now centered over srn MO and nrn Arkansas forecast to remain west
and NW of cntl NC.
1024 mb surface high pressure centered along the sern US coast at
03z will otherwise remain the dominant influence in cntl NC, with
related light sswly stirring between periods of calm, in the
gradient between that high and a cold front approaching from the NW/
from the Great Lakes swwd to the srn plains.
Prolonged calm and relatively clear this evening over our sern
counties, in closest proximity to the foregoing surface high, has
provided for strong radiational cooling and temperatures into the
lwr 40s. Lows there are generally expected to reach the upr 30s,
ranging to mid-upr 40s across the urban corridor over the Piedmont,
where mid-high cloudiness and sswly surface stirring are both
expected to be most prominent.
Short term /Friday and Friday night/...
as of 245 PM Thursday...
As the front slips toward central NC the best forcing with the
parent low will race away from the region, resulting in slowing of
the frontal progression and lessening of precipitation chances. With
good warm southwesterly flow into the area on Friday, highs could
top out in the low to mid 60s. However, winds and wind gusts will
likely increase as well. The best chances for rain during the
morn/aft Friday will be across the north along the Virginia border as the
front slides south toward NC. The best chances will shift more to
the west overnight, though that will be determined by the location
of the front. There are still some questions as to whether/how far
the front will slide into the area late Friday/Friday night. This
will impact both whether some of the cold air behind it will surge
in and where the best rainfall chances will be. There is a chance
that a good portion of the area could remain dry through Friday
night. As alluded to above, temperatures will depend on the
progression of the front, but for now generally expect lows in the
low to mid 40s north and low 50s south.
Long term /Saturday through Wednesday/...
as of 300 PM Thursday...
Southern stream shortwave crossing the south-central US early
Saturday is forecast to reach neutral or slightly negative tilt by
the time it crosses NC Saturday evening, bringing a punch of strong
DCVA and 100+ meter height falls and cold frontal passage Saturday
night. Ahead of the cold front, an in-situ cad airmass should
linger Saturday and be temporarily reinforced by the onset of rain
early Saturday, eventually eroding as warm advection increases
overwhelms the shallow airmass. Latest guidance suggests The Wedge
front will retreat to the northwest Piedmont Saturday afternoon, possibly
with a weak triple point low, before the cold front sweeps across
central NC. Temps should hold in the upper 40s across the north
Saturday afternoon, possibly reaching the lower 60s in the south.
Expect rain to transition to more showery activity Saturday evening,
and whole forecast soundings suggest almost zero instability, the
pattern would suggest the potential at least a brief period of thin
elevated instability that, along with with strong upper forcing,
could support an isolated storm. Uncertainty and coverage wouldn't
support including thunder in the forecast, but it could be an
outside chance. Otherwise, models are just a tad slower with the
timing of the front and main precip band, so have slowed the highest
pops a bit. Quantitative precipitation forecast amounts still look like one half inch to an inch,
highest in the west with additional of the earlier rain.
Behind this system, modified high pressure will build in across the
southern US for sun-Tue, supporting highs slightly above normal. The
midweek system is plagued by wide model spread, but with a primary
storm track west of the mountains and general westerly mid-level
flow, model pops seem too high, with the potential for this system
to present a drier trend in the models with time.
Aviation /00z Thursday through Tuesday/...
as of 645 PM Thursday...
24 hour taf period: generally VFR conditions will continue through
Friday. An increase in mid/high clouds around 8-10 kft is likely
tonight and will persist through Friday as the cold front moves
east. The winds are expected to be primarily southwesterly and
increase during the day Friday to 10-15kts with gusts up to 20kts.
Looking ahead: a cold front will move into and through the area
between late Friday and early Sunday, accompanied by rain, gusty
winds and sub-VFR aviation conditions. A return to VFR conditions is
expected on Sunday, persisting through Tuesday.