Scientific Forecaster Discussion
fxus62 krah 051752
Area forecast discussion
National Weather Service Raleigh NC
1255 PM EST Thu Dec 5 2019
high pressure will drift over the region from the southwest through
tonight, before sliding off the southeast coast early Friday. An
upper level disturbance will approach from the west Friday, then
pass just to our south late Friday and Friday night. Another chilly
high pressure will build down the eastern Seaboard into NC over
Near term /today and tonight/...
as of 1105 am Thursday...
The decrease in upper level winds from early this morning,
associated with the offshore movement of the mid level trough axis,
has prompted a diminishing of the orographically enhanced high
clouds across our S sections, leaving mostly clear skies. A few
spots continue to see sporadic gustiness this morning, however this
should decrease as high pressure builds in from the SW through the
day. Only minor tweaks needed to highs, as they're already on pace
to peak in the mid 50s to low 60s. -Gih
Earlier discussion from 640 am: note the downslope west-northwest flow off the
Blue Ridge early this morning. This is a great example of how the
downslope flow can keep the boundary layer to the surface well
mixed. In these cases, the temperatures at night are often 5-10
degrees warmer in the Lee of the mountains. Winds overnight gusted
in the 40-60 mph along the higher ridges around Boone and Jefferson.
Winds out in the foothills were 10-20 mph, with winds 10-15 with
gusts to 20 or so into the Winston-Salem and Greensboro areas. To
the east, a Lee trough formed over the eastern Piedmont into the
coastal plain. Here, winds went calm for several hours allowing for
radiational cooling. Temperatures ranged from 27 at Sanford, 29 at
Albemarle, 30 at Smithfield, 30 at Asheboro, and at least 33 at rdu.
Meanwhile, Greensboro and Winston-Salem were breezy and 42 degrees!
Hickory and Mount Airy in the foothills were windy and in the mid
40s! A very interesting but very difficult temperature forecast with
strong west-northwest winds aloft at 4-5k feet and above, downslope
compressional warming off the Blue Ridge, and decoupling in the
eastern Piedmont (with the calm conditions with the Lee troughing).
Also, note the beautiful standing wave cirrus developing just
downwind of the southern Blue Ridge!
High pressure will bring seasonable temperatures and sunny skies to
the region today. Westerly winds at 10-15 mph this morning will
become light this afternoon. Highs are expected to be in the mid 50s
to lower 60s, with only a few cirrus.
Mainly clear skies this evening will become increasingly cloudy
later tonight. The latest guidance is a bit slower in brining in the
thicker cirrus or high level moisture. This should allow excellent
radiational cooling this evening and strong temperature drop early
tonight, before leveling off later. Lows generally 30-35 expected.
Short term /Friday and Friday night/...
as of 210 am Thursday...
A weakening mid/upper level disturbance is forecast to move east
across the southeastern states Friday and Friday night. As such, mid
and high level moisture is forecast to increase across NC Friday
into Friday night. As the disturbance weakens, it is also forecast
to move east across Georgia and SC, or remain just to our south. Quantitative precipitation forecast is
expected to be limited with the weak system, but with it passing
south that will also limit the light rain chances. We will
essentially keep the current forecast as is, with a limited chance
(30 percent south) or slight chance (20 percent north) of light rain
Friday afternoon into Friday night. The thicker cloud cover will
affect most of the region Friday and Friday night. This should also
help keep temperatures limited to the cooler met statistical
guidance Friday. Highs generally in the 50s expected, with lower 60s
southeast. Again, with low level moisture and even significant mid
level lift forecast - any quantitative precipitation forecast is expected to be very light (less
than 0.05 or so).
Cloudiness should begin to clear from the northwest/north on Friday
night as the disturbance moves offshore. A large high pressure is
then expected to build into NC (down the eastern seaboard)starting
Friday night. Lows should remain rather mild as the cold air advection should
arrive or increase after 12z/Sat. Lows generally in the 35-42 range.
Long term /Saturday through Wednesday/...
as of 310 am Thursday...
Strong (1032-1035+mb) surface high pressure is forecast to build
down the eastern Seaboard Saturday through Sunday. This will bring a
dry Air Ridge (dar) from PA and central Virginia southward into NC/SC.
Clearing skies early Saturday can be expected with cold air advection and daa as
the DAR builds in. This part of the forecast remains fairly high
confidence. Highs should fall below normal this weekend, with mid
40s to mid 50s north to southeast Saturday, then depending on the clouds
Sunday - lower to mid 50s without clouds / colder if cloudiness
arrives early in the day. Right now, the DAR appears strong enough
to help keep at least partly sunny skies for a good part of Sunday
in most model solutions. A weak low pressure may develop offshore
Sunday or Sunday night along the old baroclinic zone. Even if this
occurs, it should have very little impact on central NC sensible
Attention then turns to the west as 500 mb height falls are forecast to
arrive from the west later Sunday into Monday. This will bring
increasing mid and upper level moisture and eventually, low level
moisture as the dry Air Ridge retreats. The approach of the next
system (cold front) from the west Sunday night into Monday night
will bring increasing low level moisture as well as the strong
surface high (dar) shifts offshore - and the low level flow becomes
southeast-S into our region with time Sunday night and Monday. We will keep
the chance pop with the warm air advection resume pattern a top an increasingly
stable low level (developing cad) near surface layer Sunday night
All rain would be the expected p-type as the mid levels warm. Monday
highs will be difficult to determine - given the cad potential if
enough quantitative precipitation forecast occurs in the Piedmont. Right now, we expect a large
temperature gradient with 40s northwest and potentially some 60s southeast.
It is an even lower confidence forecast beyond Monday as the models
and the ensembles depicting major timing/strength/positioning
differences with and after the shot of colder air behind the
initial cold frontal passage (expected tuesday).
It appears a turn to sharply colder weather should occur around
Tuesday night or Wednesday. The longevity of the chill and whether
another system brings precipitation, possibly into the colder air
late week are in question. Some lower to mid 20s for lows
may arrive by Thursday or Friday.
Aviation /18z Thursday through Tuesday/...
as of 1255 PM Thursday...
Confidence is high that VFR conditions will dominate central NC
through early Fri afternoon. Skies will be generally clear through
this evening, then high clouds will overspread the area from the
west later tonight, continuing into Fri morning/afternoon with
steadily thickening and lowering cigs, although they should remain
VFR through the 24 hr period. We'll see a steady breeze from the west-northwest
today, mainly under 10 kts but with a few gusts to 12-18 kts through
mid afternoon, then winds will die off to light/variable through Fri
morning before becoming mainly SW in the early afternoon.
Looking beyond 18z Fri, VFR conditions will hold through at least
the rest of Fri afternoon. Starting late Fri night through Sat,
behind a passing cold front, there is a slight chance of sub-VFR
cigs as cool and stable air builds in from the north. VFR conditions
should resume by late Sat, persisting through sun. We'll then move
into a more unsettled pattern, with a chance for sub-VFR conditions
and increasing rain chances from late sun through Tue. -Gih