Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Raleigh NC
250 am EST Sat Nov 25 2017
high pressure over the Carolinas will weaken today as a dry cold
front approaches from the northwest. This front will cross our
region tonight. Cool high pressure will build into the Carolinas on
Near term /through tonight/...
as of 250 am Saturday...
Little change required to the near term forecast.
West-southwest low level flow proceeding a sfc cold front will aid
to warm afternoon temperatures a few degrees above normal for late
November. 850mb thermal ridge over the region on the order of 9-10
deg c along with partial sunshine will yield high temperatures in
the mid-upper 60s, 4-7 degrees above normal. While the day will
start out sunny/mostly sunny, a S/W exiting the lower Great Lakes
and scooting into New England will brush to our north. This system
will aid to thicken mid/upper level cloudiness this afternoon
through the early evening hours. Latest GFS suggest enough lift in
the lower half of the atmosphere that would support a few sprinkles
late this afternoon-early evening across the northern Piedmont.
The S/W will aid to push a sfc cold front though our region tonight.
Expect frontal passage in vicinity of The Triad between 7-10 PM, and
across the rest of central NC between 10 PM and 2 am. The veering of
the low level wind to a nwly direction will usher a cooler drier air
mass into the region. This will lead to a dissipation of the cloud
cover NW-se this evening into the overnight. Min temperatures will
generally be in the upper 30s to around 40.
Short term /Sunday and Sunday night/...
as of 250 am Saturday...
Cool sfc high pressure will build into central NC Sunday, and settle
overhead Sunday night. This weather system along with subsidence
aloft in the wake of the exiting mid/upper level S/W will result in
sunny skies Sunday, and mostly clear skies Sunday night. A minor
perturbation aloft will pass to our south late Sunday night,
possibly accompanied by a few cirrus clouds.
In contrast to today, we will have a minor 850mb thermal trough over
central NC Sunday, with temperatures 4-5 degrees c cooler compared
to today. This translates to high temperatures Sunday afternoon 7-9
degrees cooler compared to today, ranging between 55-60 degrees.
With the sfc high expected to be overhead Sunday night, expect clear
and chilly conditions with overnight temperatures in the upper 20s-
Long term /Monday through Friday/...
as of 250 am Saturday...
Fair weather is expected Monday and Tuesday, thanks to high pressure
over the area. The high will move offshore late Tuesday as a
shortwave trough moves across the Great Lakes region. That fast-
moving short wave will push a dissipating backdoor front toward
northern NC early on Wednesday, but no significant weather is
expected across our area with that system. Instead, high pressure
following the aforementioned short wave will quickly rebuild over
our area by Wednesday afternoon. This high will be short-lived as a
more substantial short wave trough in the southern stream approaches
from the west Wednesday night and early Thursday. Like many of the
weather systems that have passed across our area recently, this
system too will have little moisture to work with across our area
thanks to a lack of substantive return flow and moisture advection
ahead of it. It's Worth noting that the 00z GFS and European model (ecmwf) have come
into better agreement with this feature, both position and timing.
So in summary, look for a dry forecast Monday through Wednesday, and
probably Thursday as well.
We'll finally see a better chance for rain Thursday night into
Friday as a northern stream trough and attendant sfc cold front is
progged to sweep across the area. Because there will be so little
time for drying between thursday's short wave and friday's
trough/cold front, models are hinting at better moisture
availability for friday's front, thus an opportunity to see maybe a
1/4 to 1/3 inch of rain with friday's fropa, but still far less than
what we need to catch up.
As for temperatures...both GFS and European model (ecmwf) Show Low level thicknesses
gradually rising through the week before the late-week fropa. Look
for near-normal temps to start the week, but rising to 5-10 deg
above normal by mid-week.
Aviation /06z Saturday through Wednesday/...
as of 1250 am Saturday...
There is a high probability that VFR parameters will persist across
central NC through Sunday. An upper level disturbance and
accompanying sfc cold front will cross our region tonight. While
clouds will thicken and lower later this afternoon through this
evening, ceilings are expected to remain in the VFR category, likely
no lower than 5000 ft. Sfc winds later today ahead of the front will
be swly, mainly less than 10kts. Sfc winds behind the front Sunday
will be nwly and less than 10 kts.
The next threat for sub VFR ceilings appears to be late Tuesday
through early Wednesday, mainly in vicinity of krwi, kfay, and
potentially krdu, as a deck of stratocu may spread into the region
from the coast.