Scientific Forecaster Discussion
fxus62 kgsp 170854
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
354 am EST sun Dec 17 2017
moisture and a gradual warm-up will return to the area today. A
series of disturbances will pass across the area next week, with a
prolonged period of unsettled weather.
Near term /until 6 PM this evening/...
400 am EST update...made localized downward adjustments to hr/ly
temps. Upper clouds are increasing in coverage from the west and the
rate of cooling has decreased somewhat during the past couple hrs.
Min temps still look good.
Recent obs show only high clouds affecting the fcst area and
upstream over nrn Georgia and most of Tennessee. Regional comp radar has a light
to mod rainfall shield over the MS valley associated with an upper
level disturbance. This area of precip will advance toward the area
this morning likely reaching the SW/rn NC mtns arnd 15z. The precip
will begin as -sn or -rasn transitioning over the -ra by 17z or so
with no sigfnt accums. The latest hires models are at odds wrt to
the movement of the precip line with the hrrr the most aggressive in
developing -shra across the ern zones by 18z. Not sure this is
likely based on the hrrr soundings showing a very dry layer below
h75 during this time. Thus...have held pops in the slight chance
range east while bring pops up across the higher terrain to mid
chance thru most of the day. Rainfall amounts will generally be low
with arnd a half inch or so over the srn NC mtns.
The op models show diminishing precip returns thru the overnight
period except the NAM which keeps isol/sct mech lift showers all
night. With this uncertainty...will keep low-end chance pops
mainly over the mtn zones. Some -snsh are possible as temps
cool across the nrn NC mtn peaks overnight...but again no sigfnt
accums are expected due to the short duration and temps near
the mixed range. Max and min temps will return to normal levels
as llvl SW/ly flow sets today and cloud cover holds min temps up
compared to the recent below freezing levels.
Short term /6 PM this evening through Wednesday night/...
as of 215 am EST sunday: a broad upper ridge will be in place over
the Caribbean at the start of the period, with split flow around a
Desert Southwest cutoff and the northern tier broad trough. Moisture
from the departing near-term wave will quickly diminish across our
area, though some moisture will remain to the south over a
quasistationary boundary across the deep south. Monday should be
mainly dry across the area, with highs a good 6-8 degrees warmer
than those today, several degrees above climo. Boundary layer
moisture remains high leading to Monday night lows generally 10
degrees above normal.
The Desert Southwest low will be kicked east Monday night into
Tuesday, with a large area of upper level diffluence resulting in
widespread synoptic lift across the Southern Plains spreading east
into the lower MS valley. Low level warm air advection will lead to temperatures
approaching 70 across portions of the Piedmont on Tuesday. Warm
frontal moisture will work toward the southern Appalachians Tuesday
night (surface low will be over eastern Texas at this point). All
guidance keeps the moisture plume across our area through the end of
the period; ecwmf is farther north with the surface low in Tennessee
whereas GFS is across MS/al/GA. The upper low fills and damps,
becoming an open wave near the NC coast by the end of the period. By
Thursday morning, should get a decent 1-2" storm total across the
area, perhaps up to 3" in the mountains. Have increased pops to
categorical early Wednesday, but timing of best chances may adjust
still. Should be a bit cooler for highs on Wednesday, but position
of the warm front will of course determine actual highs, but for now
generally 5-10 degrees cooler than on Tuesday (but still several
degrees above normal).
Long term /Thursday through Saturday/...
as of 230 am EST sunday: weak and transient damming will develop on
Thursday in the wake of the short term disturbance, with
temperatures forced down closer to seasonal normals for highs. The
upper ridge will flatten out during the day, but a positively-tilted
deepening upper trough across The Rockies and High Plains Thursday
night into Friday will increase the amplitude of the downstream
ridge, allowing temperatures to rise a few degrees by Friday.
Generally dry as we start the period, but cannot rule out some
isentropic showers over the cool dome.
The system approaching for the weekend will have a southern stream
component with moisture lifting out of the Gulf and across the
southeast Friday, and northern stream moisture across the Great
Lakes. Global models coming into better agreement on timing of the
front approaching the southern Appalachians Friday night into
Saturday, though the GFS still a little faster with the upslope
precip. The front is slow to cross the area as the Caribbean upper
ridge remains stubbornly in place, leading to a prolonged period of
SW flow across the southeast, but the depth and strength of the
incoming longwave trough will eventually force the upper ridge out
of the way, though this beyond the end of the period. Should see
another good shot of rain over the weekend. Definitely some question
as to how quickly the cold air advection will push into the mountains Saturday
night, and cannot rule out some brief snow showers. The extended
ends wet with continued above-normal temperatures, but this will
likely change once the front moves through.
Aviation /09z Sunday through Thursday/...
at kclt and elsewhere: passing CI to begin the period...then some
lowering of clouds across the mtns as an ulvl S/W approaches the
area. Expect MVFR ceilings/visibility with showers that develop mainly over
kavl...also possible across the upstate terminals this afternoon.
Periods of light rain may linger thru the latter half of the period
at kavl with lowering visibility to IFR possible near 12z. Kclt shud
remain VFR thru the period as forcing and good moisture supply
remains west across the higher terrain....although a few short-lived
-shra may occur arnd 00z with passing S/W energy.
Outlook: potential for restrictions Tuesday into Wednesday as a
moist frontal system moves thru the area. More restrictions possible
over the weekend as another moist cold 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: