Scientific Forecaster Discussion
fxus62 krah 181946
Area forecast discussion
National Weather Service Raleigh NC
246 PM EST Mon Dec 18 2017
weak high pressure will extend across central NC today, then move
offshore on Tuesday. A low pressure system will bring unsettled
weather to central NC by Wednesday.
Near term /through tonight/...
as of 200 PM Monday...
With the residual stratus/fog over The Triad having finally
dissipated earlier this afternoon, just high thin clouds and a few
mid clouds are all that's left over central NC this afternoon. A
batch of sheared vorticity will pass just to our south the rest of
the afternoon, yielding a bit more opaque cloud cover south than
north, but skies will generally remain mostly clear through early
this evening. But uncertainty regarding the remainder of the evening
and night grows, with disparity among the latest models as to how
much fog/stratus may develop overnight. While the GFS has been more
subdued with thinner fog and fewer clouds, the NAM and the latest
hrrr are suggesting fog/stratus developing in The Triad by mid
evening before spreading east and southeast over the rest of the area
overnight. Favorable factors for this development, including very
light winds and rising dewpoints, are balanced by the downward-infrared
effects of a few high and mid clouds as well as the dry and
downsloping winds just aloft at 950-850 mb overnight to potentially
instigate mixing of drier air near the surface. Given the trends of
the hrrr, have nudged the forecast in the cloudier/foggier direction
by bumping up cloud cover a bit tonight (nw first, then spreading east
and se), with areas of fog. Still anticipate a mild night, with lows
of 39-46. -Gih
Short term /Tuesday through Tuesday night/...
as of 230 PM Monday...
The uncertainty regarding tonight's extent of overnight clouds
extends into Tue, with considerable difference among models
regarding the persistence of any early-morning clouds. Considering
that the flow just off the surface will remain from the west or WNW,
inducing downslope drying and potential quick mixing-out of what
should be a shallow near-surface moist layer, the faster-clearing
solution (gfs) appears the most plausible, contrary to the NAM which
locks in a highly stable and moist surface layer much of the day.
However, there is still a chance that the warming just off the
surface may serve to actually increase surface-based stability
through the day, via differential thermal advection in the vertical.
Will opt for the more optimistic solution of the GFS, showing at
least partial clearing by late morning. Thicknesses should be much
above normal, by 35-40 M (although some of this layer contains part
of the warm nose aloft). Notwithstanding the shallow mixing, it
still supports highs in the upper 60s to lower 70s, although if the
longer-lived cloudiness of the NAM ends up correct, these temps will
be too warm by several degrees. By Tue night, an upper low located
near Tucson Arizona this morning will have tracked east-northeast across OK/TX,
reaching MO/Arkansas by Wed morning. Its approach will tighten up a west-east
frontal zone across the Carolinas, and it is along this
frontogenetic zone that we'll see precip start to break out late Tue
night. This development looks to be trending slower, waiting for a
40-50 kt southwesterly jet at 850 mb to track eastward over the Gulf
states, nosing into SW NC toward dawn Wed. Will bring in a trend to
overcast skies, with a slight chance of rain in our far SW Tue
evening, expanding eastward and increasing to likely over the rest
of the west Piedmont overnight as moist upglide strengthens. Very warm
lows in the upper 40s to lower 50s expected. -Gih
Long term /Wednesday through Monday/...
as of 244 PM Monday...
A tale of two forecasts in the long term as a very high confidence
forecast early in the period diverges into very different scenarios
for the weekend and the Christmas Holiday. First off, there is very
good agreement between models on a rain making low pressure system
moving across the deep south and through the Carolinas. Quantitative precipitation forecast will be
the big story with this system as much as an inch and a half of rain
can be expected with this system on Wednesday and Wednesday night.
The totals may vary depending upon the exact track of the low but
the entire forecast area should receive at least a half of an inch
of rain. The system moves out to sea on Thursday and will be briefly
replaced by a cad wedge as high pressure transits across southern
Canada. That is where the similarities end. Temperatures during this
period will be near normal values for this time of year, mainly in
the lower 50s. Lows in the mid 30s.
Markedly different solutions for the following system for the
weekend result in very different forecast scenarios as a cold front
approaches the area from the west with a parent high over the Great
Lakes. The differences are not in the type of the system, but rather
on how far east the front will make it before stalling out. Current
GFS output shows the front moving all the way through the area by
Saturday night before retrograding back over the forecast area by
late Sunday and remaining overhead through Monday night. The European model (ecmwf)
solution on the other hand keeps the front to our west throughout
the forecast period, keep central NC in the warm sector but also
quite a bit drier as the bulk of the precipitation stays west of the
County Warning Area. For now have favored this slower timing and therefore keep
temperatures and dewpoints on the higher end of the spectrum and
pops on the lower end. This forecast is likely to change throughout
the week until models get a better grasp on the progression of the
front. Temps well above normal, upper 50s to upper 60s in spots.
Lows in the mid 40s to near 50 degrees.
Aviation /18z Monday through Saturday/...
as of 120 PM Monday...
The fog and stratus that held on across The Triad terminals
(int/gso) through the lunch hour has dissipated, leaving VFR
conditions across central NC. These conditions are likely to hold
through sunset, however forecast confidence drops thereafter, with
considerable uncertainty as to how much (if any) fog/stratus forms
tonight. At this time, it appears that the most likely scenario is
of MVFR fog developing after 03z in int/gso and after 06z elsewhere,
becoming IFR later overnight as surface winds remain light from the
SW with a cooling surface down to the slowly rising dewpoints. There
is expected to be considerable dry air just aloft, however, such
that it's not clear how thick the fog will become or if it will
transition to stratus. Any fog and patchy stratus tonight is
expected to lift and mix out by 14z Tue, followed by VFR conditions
and 5-9 kt surface winds from the west or west-southwest.
Looking beyond 18z Tue, VFR conditions should hold through Tue and
much of Tue night, although high level cigs will be thickening and
lowering Tue evening and overnight as an upper level disturbance
approaches from the west, with cigs likely becoming MVFR at all
locations just before sunrise Wed. This will culminate in a good
chance of sub-VFR conditions in widespread rain starting in the pre-
dawn hours Wed at int/gso and by mid morning elsewhere, lasting
through much of Wed night. Low-end VFR or high-end MVFR cigs will
hold through Thu, before dropping back to sub-VFR Thu night, likely
lasting into Sat as an upper trough approaches from the west. -Gih