Scientific Forecaster Discussion
fxus62 kgsp 170011
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
711 PM EST Sat Nov 16 2019
a deep low pressure system will lift away from the Carolina coast on
Sunday resulting in slightly warmer temps and mostly sunny skies.
A couple of weak and dry cold fronts will move through the area
Monday and Tuesday but temperatures will gradually warm Wednesday
through Friday ahead of the next cold front expected late next week.
Near term /through Sunday/...
as of 710 pm: main update for the 00z taf issuance along with
minor adjustments made to temperatures/pops/winds to coincide
with the latest trends, as the rest of the near term forecast
remains on track. Per latest obs and radar imagery,
precipitation has ended across the area, with temperatures
trending a few degrees warmer than expected - currently in the
upper 40s to lower 50s.
Previous discussion: as the coastal low moves further offshore
tonight, some drying occurs in the currently saturated layer, which
lets the pops fall off. Continued cold advection and reduced cloud
cover will bring min temps a few degrees below climo. Though the
high will migrate eastward on Sunday, the convergence will help
maintain its strength, and the offshore low will continue to drive a
cold NE flow. This should hold aftn maxes two or three categories
below climo in the NC Piedmont, despite insolation. The southwest
County warning forecast area will be less impacted by the cold airmass, nearly reaching
Short term /Sunday night through Tuesday/...
as of 245 PM EST saturday: the short-term (monday and tuesday)
will be fairly benign across the area despite the passage of a
couple shortwaves and attendant cold fronts, for the cold fronts
will be weak and there will be very little moisture to work with.
The first upper-level shortwave arrives early Monday morning around
the base of a larger (longwave) trough and despite its potency
as it becomes negatively tilted over the Ohio Valley, it follows
too quickly behind the departing coastal low to tap into any Gulf
moisture and at worst brings mostly cloudy skies to much of the area
overnight Sunday into early Monday. Given a progressive pattern,
the wave and front quickly move by the area Monday while yet another
shortwave arrives Monday night, dropping much further south towards
the Gulf Coast on Tuesday. This results in yet another round of
cloud cover but again, there is very little moisture to work with.
However, the favorable, upslope northwest low-level flow should
be strong enough in combination with shortwave forcing to justify
a slight chance pop along the central NC/Tennessee state line Tuesday.
Temperatures quickly moderate above freezing Tuesday morning in
the western mountains so expect liquid to be the primary phase
with any isolated shower that develops.
The cloud cover should help to keep min temperatures ever so
slightly warmer both Sunday night and Monday night compared to
Saturday night, with lows at or just below normal. Despite some
insolation each day following the passage of each weak front,
the shortwaves will keep thicknesses in check and highs will stay
stagnant at about 4-6 degrees below normal.
Long term /Tuesday night through Saturday/...
as of 245 PM EST saturday: it seems like the extended forecast has
been a battle between the ec/Canadian and GFS for the past few
weeks, and this continues to be the case. But first, the good
news is that there is decent consensus that upper-level heights
rise significantly behind the short-term shortwave train as a
potent closed-low develops over California, inducing a transient
longwave ridge over the center of the country that moves overhead
quickly by Thursday. This means that skies will remain mostly
clear Wednesday and Thursday and temperatures will finally get
back to normal if not 2-4 degrees above normal by Thursday.
Looks like a nice couple of days to take off from work!
The guidance battle resumes heading towards Friday and Saturday
when the models disagree about how and when the Southern California
closed low ejects out of The Rockies. The ec/Canadian solution
is stronger/deeper and therefore, slower with the propagation of
a 560dm closed low through The Rockies Friday into Saturday while
the GFS more quickly ejects an open trough across the entire Continental U.S.
Friday through Saturday. Because of these differences, there are
differences in how a potent northern-stream trough interacts with
the southern stream energy. Because the ec/Canadian solution is
slower to eject the southern closed-low, it brings the northern
stream trough across our area Friday with a moisture-starved
frontal passage later Friday. The GFS marches a phased system and
attendant cold front across the area early Saturday with a better
quantitative precipitation forecast response, but the ec/Canadian saves the more potent southern
stream system for late Saturday into Sunday. At a minimum, as
high pressure moves offshore, return flow sets up over the area
Thursday afternoon and persists at least into the overnight hours,
justifying at least slight chance pops over the mountains as early
as Thursday evening with low-end pops persisting over the mountains
and parts of the Piedmont through Friday. The forecast continues
to blend the solutions with a nod to the ec/Canadian consensus,
saving the better pops for later Saturday into Sunday morning as
the second system approaches.
Regardless of the solution, expect another mild day on Friday
with cooler, slightly below-normal conditions on Saturday because
there is consensus that either weak cold air advection will be in place behind
the ec/Canadian front or cloudy/showery conditions will persist
as the front is moving through (gfs).
Aviation /00z Sunday through Thursday/...
for kclt and elsewhere: aside from possible MVFR cigs at kclt
late overnight into early Sunday morning, expect VFR to prevail
through the valid taf period as coastal low continues to pull
further away from the coastal Carolinas. Few/sct/bkn mid to high
clouds will continue to linger at the taf sites through the
overnight hours, with VFR/marginal MVFR cigs possible late overnight
into the early morning hours at kclt due to an inversion in place.
On Sunday, conditions will improve to VFR at kclt into late morning,
with VFR prevailing at all taf sites through the end of the valid
taf period. Northeasterly winds will continue 10 to 12 kts with
higher gusts tonight, gusts subsiding Sunday morning, with sustained
winds dropping below 10 kts by the afternoon hours.
Outlook: breezy west/northwest winds and brief low cigs focused across the
mountains including kavl will be possible with couple of mainly dry
disturbances early in the coming week. Otherwise expect VFR.
00-06z 06-12z 12-18z 18-00z
kclt high 100% high 94% high 95% high 100%
kgsp high 100% high 100% high 99% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 98% high 91% high 95%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% high 100% high 100%
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: