Scientific Forecaster Discussion
fxus62 kgsp 201459
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
959 am EST Mon Nov 20 2017
cool high pressure will move in before a weak cold front crosses the
area Tuesday night. Low pressure will develop along the Gulf Coast
Thursday and up the Atlantic Seaboard through Friday. Another strong
cold front will arrive next weekend.
Near term /through tonight/...
as of 930 am EST monday: very few changes to the grids this morning,
mainly just updates to hourly T/TD trends. Skies are clear with some
cirrus starting to move toward east Tennessee. Overall a very quiet day.
Otherwise, the center of the high moves east across the area today
with light SW wind developing during the afternoon. Cirrus will
infiltrate the area from the west during the day as well. Highs will
be up to 5 degrees below normal.
High pressure remains ridged into the area from the east tonight as
an upper trough approaches from the west. Cirrus increases in
coverage and thickens through the night, but the bigger story is the
increase in low level moisture from the SW after midnight. Some of
the guidance is producing precip before daybreak. There could be
some mechanical upslope lift in the southwesterly flow. However, the
moisture is very shallow and isentropic lift remains south of the
area until after daybreak. Therefore, continued the dry forecast but
did increase clouds from the SW after midnight. The guidance has
come in slightly warmer for lows, and this seems reasonable given
the increasing cloud cover. Lows will be up to 5 degrees below
normal, but many locations will be only a couple of degrees colder.
Short term /Tuesday through Thursday/...
as of 230 am Mon: high pressure moves further off the Atlantic
coast on Tuesday as an upper trough dips into the Ohio Valley. In
response, moist southerly flow sets up over the Carolinas and
Georgia. Similar to saturday's isentropic event, the flow is
a bit veered relative to the pressure gradient. Clouds should
easily be generated but unsurprisingly quantitative precipitation forecast response is fairly
small overall. Hence only low pops are being included in the
fcst. Mechanical lift along the south-facing Escarpment is likely to
be a factor, and temperatures there are expected to be right around
freezing at daybreak. However, the better moisture does not advect
in until later in the morning, and therefore freezing rain is not
anticipated. Max temps will remain a couple degrees below normal.
The aforementioned trough swings east Tuesday night and will drive
a weak cold front into the NC mtns early Wed morning. Moisture
is too shallow to expect any upslope-driven precip at that
time. The front weakly pushes across the remainder of the
area wednesday; downsloping and more sunshine will help temps
rebound to about normal. A cold, dry sfc high expands out of the
Mississippi Valley and onto the eastern Seaboard in the wake of
this front. Accordingly, with northeasterly flow, temps tumble for
Thursday. Mins return to near or below freezing over the NC mtns and
northwest Piedmont, and top out only 5-7 degrees below normal area-wide.
There continues to be some discrepancy between global models on
the evolution and timing of a shortwave trough diving into the
deep south near the end of the short-range period. The GFS cuts
off the wave but continues its trend of suggesting the resulting
lift and moisture advection will remain south of the area. The ec
is slower and drier in its depiction. Hence the forecast remains
virtually dry thru Thursday.
Long term /Thursday night through Sunday/...
as of 100 am EST monday: the notably quiet medium range begins
Thanksgiving night as an upper trough axis crosses the forecast
area. At the surface, high pressure will slide in across the
Carolinas as a series of coastal lows drifts across Florida and hugs
the Carolina coast. Guidance has been inconsistent with the handling
of the track of these low pressure systems, with previous runs
depicting scenarios in which some Gulf moisture is shunted into our
area. With the upper trough and cold air solidly in place over the
area on Thursday night and Friday morning, any moisture able to make
it this far north may present some wintry p-type issues. Confidence
is increasing that the moisture will remain to the south and east of
our area at the moment, though, so have kept pops unmentionable for
the time being.
The overall pattern will remain progressive through the medium
range, with another upper trough arriving over the eastern Continental U.S.
Saturday and Sunday. The next low pressure system will pass well to
our north on Sunday, with just a slight chance of wringing out
enough moisture in the higher elevations for some isolated northwest flow
snow showers. Colder air will then advance behind what will be a dry
front for the majority of our area next Monday. Temperatures will
hover near or just below average for the majority of the medium
Aviation /15z Monday through Friday/...
at kclt and elsewhere: no changes to the kclt taf for the 15z
amend. Mainly light north winds become S to SW for the afternoon. Light S
or light and variable wind expected overnight. Cirrus will increase
and thicken through the period. Low level moisture increases from
the SW after midnight with low VFR clouds moving into the SC sites
Outlook: increasing low level moisture could result in cig
restrictions and perhaps -ra on Tue/Tue night. Otherwise, dry/cool
conditions should persist into late week.
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: