Scientific Forecaster Discussion
fxus62 kgsp 180543
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1243 am EST Mon Dec 18 2017
weak high pressure will linger over the southeast tonight
through Monday. Low pressure and abundant moisture will return to
the region from the west Tuesday night through Wednesday, with dry
high pressure following Thursday through Friday. A more vigorous
cold front will arrive from the west next weekend and usher in a
period of unsettled weather.
Near term /through today/...
as of 930 PM EST sunday: latest radar imagery shows some
lingering showers across the western upstate. Latest cam guidance
keeps isolated showers over this area through midnight then
dissipates them as forcing wanes. Have followed these trends for the
rest of the night. Should see patchy areas of fog overnight, mainly
over the mountains and foothills with widespread cloud cover most
locations. May even see patchy dense fog where low clouds fail to
develop. Given cloud cover, expect overnight low temperatures to be
slightly warmer than we've seen over the past couple of nights, with
lower 40's across the upstate down into the mid to upper 30's across
the mountains and foothills.
On Monday, expect lingering clouds throughout the day as residual
moisture spreads across the area, though some guidance suggests the
mid levels levels may dry out much sooner. With a subtropical ridge
building into the area, expect overall quiet conditions with a
warmer day in store as temperatures climb into the upper 50's/lower
Short term /tonight through Wednesday night/...
as of 2pm EST sunday:
Tuesday morning begins mild with generally zonal flow aloft with dry
northwesterly 850mb flow. However, clouds aloft will be increasing
as a well-defined shortwave exits from northern Mexico and crosses
south Texas on Tuesday, picking-up Gulf moisture as it does so.
This wave will bring potentially significant rainfall to the area
beginning Wednesday morning, and lasting into Wednesday evening.
Timing is fairly consistent in the ec and GFS models with the
heaviest rain Wednesday afternoon impacting all parts of the cwa,
and i'm inclined to go with fairly high pops on Wednesday. The
mountains will receive the most rainfall with 1 to 3 inches with
amounts tapering southwest with an inch or so in the Piedmont. Late
in the event Thursday morning, when precipitation amounts have
become light and scattered behind the cold front, temperatures will
cool enough at higher elevations to produce some scattered light
System produces a well-defined closed low/triple point over western
NC Wednesday morning with a cold front trailing southwest, and a
warm front extending eastward. Areas along and north of the warm
front will be associated with the most rain, and this area will
include much of the County Warning Area.
Warm advection pattern south of the warm front on Tuesday will lead
to warming in addition to moistening with highs perhaps 5 or fewer
degrees shy of seasonal records.
Dewpoints increase to the mid-50s by Wednesday, and GFS actually
predicts a smidgen of cape Wednesday afternoon. Have decided to
keep any mention of thunder out of the forecast at this time as
BUFKIT soundings show any possible convection to be very shallow,
and area with potentially useful cape only impacts a handful of
counties in northeast Georgia and the upstate.
Long term /Thursday through Sunday/...
as of 2:45 PM EST sunday: at the start of this period the models are
similar in that the lead wave of energy will be east of our forecast
area (fa) Thursday morning. This will allow a Flat Ridge aloft to
shift across the area, while surface high pressure takes up a
damming configuration from Quebec Canada south into the Carolina's
and northeast Georgia. The model soundings show some shallow
moisture in parts of our south fa, as a result spotty light rain or
drizzle will be possible.
The damming configuration should last Thursday night into part of
Friday, and then break down. This is a result of the next strong
spoke of energy lifting from the southwest Continental U.S. To the western
Great Lakes Friday. The flow at most levels will be in the process
of backing to the southwest (slowest at the surface) during Friday.
Moisture will become deeper through the column later Friday and
Friday night, in the southwest flow. Therefore we should note a
subtle expansion of shower activity, but still well in the chance
At the start of the weekend, and beyond, is where we have noted the
greatest divergence (or changes) in how the models are handling the
next front and associated precipitation. Today is no exception as
the new European model has shifted slower, even slower than the GFS
and Canadian. However, to some degree they appear to be following
sensible weather. A broad high pressure system in the western
Atlantic to the Gulf of Mexico, combined with a southwest flow
aloft, should signal a slower moving frontal pattern. The first wind
shift from the lead energy source will allow some type of wind shift
to approach the forecast area Saturday or Sunday. The European is now super
slow with its front tucked well west right through the weekend.
We will allow a plume of moisture, to overspread our area, in the
deepening southwest flow aloft and ahead of the initial wind shift
change. A blended approach yields some high chance and low likely
probability of precipitation Saturday into Sunday. It would seem the colder air would only
ooze into our area (if it even makes it this far southeast) by the
end of this forecast cycle. Considerable implications where
everything lines up, since we are going slower, pw's are very high
for this time of year, thus how much rainfall.
We suspect there will be more adjustments needed this far out, as
the models line up with consistency from run to run.
Aviation /06z Monday through Friday/...
at kclt and elsewhere: MVFR cigs in place across the upstate will
continue through most of the overnight hours, with perhaps some
brief clearing at kclt just before daybreak before the MVFR cigs
finally work their way out by mid-morning. Kavl cigs should remain
VFR but cannot rule out brief MVFR vsby. Khky, with clearer skies,
will likely drop to vlifr vsby overnight, with a prolonged period of
LIFR. Improving everywhere through the day with winds picking up out
of the SW 5-10kt, except kavl where northwest winds will prevail, with some
low-end gusts this afternoon. As winds drop off this evening at
kavl, some low level wind shear not out of the question, but not included at this
time. Cigs remain VFR through the end of the period, but low-level
moisture will remain in place underneath a developing inversion, and
so expect potential for fog development overnight tonight and have
introduced at least MVFR conditions everywhere but kavl (ifr at kclt
beyond the 24h period).
Outlook: expect restrictions to increase late Tuesday into Wednesday
as a moist frontal system moves thru the area. Expect brief
improvement Thursday and Friday, but more restrictions possible over
the weekend as another moist cold front crosses from the west.
15-21z 21-03z 03-09z 09-12z
kclt high 81% high 82% Med 68% low 56%
kgsp Med 70% Med 75% high 83% Med 64%
kavl high 94% high 85% Med 70% high 87%
khky high 100% high 83% Med 66% high 100%
kgmu Med 76% high 86% high 85% Med 63%
kand Med 76% Med 65% high 90% Med 61%
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: