Scientific Forecaster Discussion
fxus62 krah 151945
Area forecast discussion
National Weather Service Raleigh NC
245 PM EST sun Dec 15 2019
broad high pressure will extend across North Carolina today into
Monday, with deepening low pressure expected to move north of the
area on Tuesday and bring a cold front across the region late
Near term /through Monday/...
as of 220 PM Sunday...
18z surface analysis shows high pressure centered just off the Florida/Georgia
coast, a Lee trough along the eastern slopes of the Appalachians,
and a diffuse synoptic cold front stretching from central PA through
central Tennessee to NE Texas. Clear skies observed on satellite imagery...and
through the window... will continue this evening, with sporadic SW
wind gusts to 15-20kt and temps topping out in the 55-60 range for
the most part.
Return flow will slowly increase tonight between the offshore high
and the cold front west of the mountains. The northern end of the
front will slip south through Virginia tonight but should settle just north
of the NC/Virginia border. Models indicated some nocturnally enhanced
isentropic accent over the western Piedmont and increasing low
clouds, though with varying degrees of moisture depth. Some href
members produce light qpf over the far northwest Piedmont, but this
is likely overdone and should be focused north of the shallow
boundary. Skies should vary from mostly clear west to nearly clear
east by Monday morning, with lows accordingly ranging from coolest
in the upper 30s slightly mild in the low/mid 40s south and west.
South-southwest low level flow will continue to increase on Monday in repsonse
to an upper trough swinging through the mid-MS Rover valley region
and a low pressure ejecting out of the deep south. Models show a
couple weaker low pressure ripples moving along the front, but
again all precip remain northwest of The Triad through at least
Monday afternoon. Morning low clouds should scatter and lift a bit
in the northwest Piedmont, though persistent warm advection will
keep enough low clouds around to hold temps back in the upper 50s or
lower 60s, quite a bit cooler than in the east when highs may reach
the lower 70s under mainly high clouds aloft. Have held off on
introducing pops prior to 00z Monday evening, but light rain could
end up reach portions of The Triad by early evening.
Short term /Monday night/...
as of 245 PM Sunday...
Mon night should be warm with increasing clouds, especially W, and
increasing pops overnight. Low level flow from the S and SW, around
the periphery of the offshore-centered surface high and ahead of the
incoming cold front, will continue to draw moisture into the area,
while aloft, passage of a mid level perturbation (well out ahead of
the negatively tilted trough over the central conus) will produce
increasing high clouds. Dry/stable air will linger for a longer time
through the 800-500 mb depth, however, noted on 700 mb progs showing
warm air advection and low relative humidity at 700 mb early Mon night. This should restrict pops
Mon evening to isolated at best over the northwest third of the forecast
area, although The Triad could see some steadier showers prior to
midnight, and a few insignificant sprinkles can't be ruled out.
Latest model runs agree fairly well with our earlier forecast
bringing higher pops in overnight as the column moistens ahead of
the front with surging pw, so will stick with the trend toward
likely to categorical pops northwest ranging to lower chance southeast after
midnight. Temps will not drop much Mon night with the thickening
clouds and steady warm south-southwest breeze, so still anticipate very mild
lows from the lower 50s northwest to near 60 se, near or just over the
normal high temps for this time of year. -Gih
Long term /Tuesday through Sunday/...
as of 245 PM Sunday...
Will continue likely to categorical pops across central NC as the
front progresses eastward Tue. Storm total rainfall is expected to
be mostly under an inch, based on the fairly swift frontal movement
through NC, although a period of heavier rain remains possible with
very high pws near 1.5". Minor timing differences persist among the
models, with the faster GFS outpacing the NAM/Canadian and
especially the slow European model (ecmwf). But overall it looks like the front will
be pushing offshore an hour or two before or after midnight, with
precip chances mostly east of the forecast area overnight. Still expect
clearing to be gradual given that the mid level flow will remain
from the SW through daybreak Wed ahead of the lagging and positively
tilted longwave trough. Expect warm highs Tue ahead of the front, 60
to 70, followed by plunging temps Tue night behind the front into
Even if the slower European model (ecmwf) comes to pass, skies should still clear out
by noon Wed from northwest to southeast as the mid level shear axis and upper jet
core shift to our east. Surface high pressure will build over the
eastern US from the west Wed through Fri with below normal thicknesses,
bottoming out Thu morning at 30-40 M below normal. The Wed-Fri
period should be chilly but generally quiet with plenty of sunshine
beneath mid level ridging shifting over the southeast, although some
mid level Pacific-NW-rooted moisture may stream across the area
within the northwest steering flow Thu/Thu night, perhaps inducing some
orographically enhanced cloudiness. Temps will be cool with highs in
the 40s to lower 50s.
Uncertainty remains high regarding next weekend's forecast, as model
differences continue with respect to a mid level trough moving into
the central Continental U.S. Early Fri. The latest GFS and European model (ecmwf) are southern
stream-dominated and both deepen a low over OK/North Texas Fri before
tracking it east-southeast over the Gulf states and FL, although the GFS is a
full 12-24 hrs faster with this movement than the European model (ecmwf). The
Canadian, on the other hand, keeps the trough phased and open,
shifting quickly east and off the East Coast on Sat. Both the GFS and
European model (ecmwf) favor deep surface cyclogenesis over the NE Gulf or near the
southeast coast over the weekend, a solution supported by the gefs,
albeit with a wide spatial range of its surface low positions by
Sunday. Based on the preceding chilly surface ridge centered to our
N, extending down the eastern Seaboard with a strengthening low
level easterly flow into the southeast/FL, it would seem that
increasing baroclinicity and cyclogenesis are good possibilities.
Still lacking a clear favorite at this point, however, will stay the
course with both temps and pops not departing too much from
climatology for both Sat and sun. Hopefully the models come into
better agreement very soon, given that we're heading into the heavy
pre-Holiday travel period. -Gih
Aviation /18z Sunday through Friday/...
as of 1255 PM Sunday...
High confidence in VFR through this evening (00z/16th). High
pressure settled near the Florida/Georgia coast will continue to shift east,
allowing swrly flow to increase across central NC through Monday.
Guidance indicates a good chance of low clouds at int and gso Monday
morning, varying from IFR to high end MVFR. While confidence is not
high enough for IFR tafs, MVFR with a possible short period of IFR
by 12z Monday seems likely. Skies should remain most clear at Fay
and rwi, with VFR through the period. At rdu, some of the low
clouds to the west could perhaps drift east to rdu by 12z, but
confidence is low.
While ceilings lift by mid-morning Monday, MVFR may persist at int
and gso. SW winds will increase to near 10kt and gust to near 20kt
Looking beyond 18z monday: an approaching cold front will bring
deteriorating aviation conditions Monday night through Tuesday night,
with periods of rain and low ceilings vsbys. The cold front will
push through the area Tuesday night, with VFR expected through
Thursday. A coastal system may bring poor aviation conditions again
by next weekend.