Scientific Forecaster Discussion
fxus62 kgsp 190451
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1151 PM EST Sat Nov 18 2017
a cold front crosses our region tonight with breezy to windy
conditions expected on Sunday. Cool high pressure will be in
control for much of the week. Another cold front arrives next
Near term /through Sunday/...
as of 1035 PM EST saturday: the long awaited frontal band of
convection has arrived across portions of extreme western NC and
northeast Georgia. Fortunately according to the latest Storm Prediction Center mesoanalysis
cape remains non existent and nearly all upper divergence has lifted
northward with the ejecting upper jet Max. Therefore not expecting
any deep convection to support thunderstorms therefore will brush up
the severe weather potential statement in the next half hour to remove any ts wording. However, a
50-60kt llj along/ahead of the convective axis could be tapped for
wind gusts upward of 45-50mph, possibly as high as 60mph, especially
during any heavier precipitation. All said, the front looks to be
on time with modest caa moving in approx 75-100 miles behind the
front across the Cumberland Plateau at this time. This caa regime
is still fcst to sweep into the high terrain of NC/Georgia within the
next hour/two leading to falling snow levels, and thus a changeover
to snow showers atop the highest peaks/ridgetops, continuing into
Sunday with any accums restricted to elevations above 5kft. Outside
of the mtns, the front will quickly sweep east into/through the
fthills/Piedmont regions of NC/SC overnight, clearing the fcst area
by daybreak. All said, pops/sky/T/TD and winds were tweaked to
align with the current/fcst convection through the remainder of
tonight and into Sunday morning.
Previous discussion: while winds will be howling (gusts as high as
60 mph) this evening on the exposed peaks and ridge tops of the
southern Appalachians, winds elsewhere should be quite tame until
fropa occurs btw 03-09z. Strong winds should then descend into the
lower elevations within the cold advection regime. While advisory
criteria gusts will likely be hard to come by in the mtn valleys
even then, guidance tends to under-do the winds for the first 1-2
hours Post-fropa during strong cold-advection surges, so would
expect at least occasional advisory criteria gusts in the lower
elevations during that time. Meanwhile, the high peaks and ridge
tops should continue to howl, with gusts as high as 60 mph likely
occurring through tomorrow morning. That being the case, see no
reason to deviate from the current Wind Advisory.
Snow levels drop quickly across the high terrain from late evening
on, while a moist northwest flow quickly develops into the southern
Appalachians. Expect scattered showers, with high elevations snow
showers to develop late tonight/early Sunday. However, ingredients
are such that accumulating snowfall will be difficult to come by,
with moisture depth and residence time of moisture coincident with
cold air being the primary limiting factors. As such, any accums
should be limited to elevations above 4000' near the Tennessee border,
if not 5000', and even there less than an inch is expected.
Clearing quickly occurs Sunday morning, with most locations
(the Tennessee border area being the primary exception) likely seeing
clear/mostly clear skies by sunrise. Max temps will be 10-15
degrees below normal in most areas.
Short term /Sunday night through Tuesday/...
as of 130 PM saturday: not much going over the short range period.
The models are in good agreement with the synoptic pattern which
features a 1026 mb sfc high traversing the area west to east while
subs remains strong thru most of the period. This high will center
off the the atl coast early tues...however strong llvl ridging will
remain over the forecast area Tue morning into the early afternoon. Thus...very
little moisture flux will be available before the ridge breaks down
in the afternoon as a low pressure trof advances from the west.
Soundings show little precip potential across most areas except
within an isent lift area aligned over the srn bl and srn balsams
later Tue. Not expecting much precip with this scenario as deep
layered lift remains low-end as a broad area of vort energy
producing mainly upper and mid level clouds crosses a slowly
Max temps will be held about 5-7 degrees below normal both days as a
llvl thermal airmass is slow to modify. Mins will also be about 10
degrees below normal with freezing conds supporting a wintry mix
over the SW/ly upslope zones early Tue...however advecting llvl
moisture will likely be too late for any sensible wx to materialize.
Long term /Tuesday night through Saturday/...
as of 215 PM EST saturday: the medium range period around the
Thanksgiving Holiday remains uncertain across the southeast due to
the continued run-to-run inconsistencies among the models. Nearly
all solutions have a broad upstream trough at the start of the
period over, or just east of, the MS River Valley Tuesday night.
Differences then develop quickly through Wednesday. In a continued
role reversal from previous day solutions, the 12z GFS now features
a strong cutoff low developing over the deep south. The 00z European model (ecmwf)
solution featured a more progressive open trough, however, the 12z
data has trended back toward the deeper southern tier GFS low for
wedneday night through Thursday. Fortunately, even the deeper,
slower solutions keep the abundant moisture shunted mainly toward
the coastline Thursday as the northern Gulf of Mexico surface low
redevelops over the southeast coastal waters. Will keep mainly
slight chances for light rain going across the southeast parts of
the forecast area for Thanksgiving day.
Deep layer northwest flow should set up behind the departing system
on Friday with Lee troughing developing in surface high pressure
over the southeast. A strong cold front will then approach from the
northwest late next Saturday but with dry conditions likely
persisting through late day. Temperatures will gradually trend back
up toward climatology just ahead of the approaching front.
Aviation /06z Sunday through Thursday/...
at kclt and elsewhere: a mixture of VFR/MVFR leading into solid
VFR through this taf period at all sites. A fast moving cold
front is currently moving across the region with a rather dynamic
llj along/ahead of the boundary. Therefore continued with 40kt
2kft llws mention at all sites aside for kavl given passage.
Flow will veer wly/nwly behind the front, with gusts subsiding for
a few hours. As for the convection itself, precip timing at each
site was based on camguide and current radar coverage, with all
sites dry by 08z at the latest. Skies/cigs will remain at low VFR
levels through much of the overnight aside for when heavier precip
is ongoing with MVFR cigs favored via tempos. Toward sunrise skies
will clear out rather rapidly with nwly winds increasing yielding
low end gusting amidst improving caa through mid/late afternoon.
That said, with moist upslope flow favored along the Tennessee line,
did hold onto MVFR stratus at kavl as upstream clouds advect down
valley until around midday.
Outlook: expect VFR from Sunday morning through at least the middle
of next week, under the influence of cold Continental high pressure.
17-23z 23-05z 05-11z 11-12z
kclt high 87% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 94% high 100% high 100% high 100%
khky high 100% high 100% high 100% low 58%
kgmu high 100% high 100% high 100% high 100%
kand high 94% high 100% high 100% high 83%
The percentage reflects the number of guidance members agreeing
with the scheduled taf issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:
Georgia...Wind Advisory until noon EST Sunday for gaz010-017.
NC...Wind Advisory until noon EST Sunday for ncz033-048>053-058-059-