Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 230253
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
953 PM EST Sun Jan 22 2017
Numerous waves of energy will bring rounds of heavy rain and
thunderstorms to the area tonight. A vigorous low pressure system
will sweep toward the region tonight and move northeast of the
Carolinas on Monday. Mild high pressure will move in Tuesday and
Wednesday until a cold front crosses the region Wednesday night.
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
0300 UTC Update: Winds were updated with a blend of the latest NAM
and ADJMAV. Thunder chances were updated from the latest GFS and
NAM Cape values.
As of 245 PM EST Sunday: Complicated forecast today. Plentiful low-
level moisture has kept quite a bit of cloudiness across the area
and inhibited instability increase. The latest mesoanalysis has some
small amounts of sbCAPE creeping into our southern zones (100-250
J/kg). Not a lot of low-level shear just yet since the surface low
is still to the west, but plenty of deep-layer shear already in
place (50-60kt). So for the synoptic setup, we have the deep upper
closed low over the ArkLaMiss area that will continue to dive
southeast into the Deep South before lifting northeast again toward
the Carolinas. The attendant surface low currently in SE Alabama
will lift northeast toward the Southern Appalachians through the
period. Warm frontal convection ahead and northeast of the surface
low will begin spreading into our area this evening, with a
secondary round of convection possible on its heels associated with
the deep surface low itself. Latest trends have been for a decrease
in the look of the severe weather threat across most of our NC
counties, except the CLT metro area, but very little change across
our Upstate SC and NE GA counties.
So, if the models are to be believed, sbCAPE will increase across
the area over the next few hours, with the max around 03z. This is
not collocated with the best low-level shear and so resulting SigTor
Parameters, on the more bullish NAM, are at best around 4 in extreme
southern zones (closer to 1 on the GFS). This is hardly a reason to
let down one`s guard, but obviously the greater threat is to our
south where a significant severe weather/tornado outbreak is
expected. However, we all know that if the shear is high enough, it
doesn`t even take 500 J/kg sbCAPE to get things going in an HSLC
event. Overall, as temperatures aloft cool over our area, the hail
threat should increase through the evening, as will the damaging
wind and tornado potential. Right now thinking isolated tornadoes at
best but will have to continually reassess the threat through the
afternoon and evening as we see how CAPE trends evolve.
Other issue is the QPF. WPC does have us in a slight risk of
exceeding flash flood guidance, and for the next 24 hours or so
looking at widespread 1" amounts with 2-3" over the mountains. Add
that to what we`ve already had this weekend and it`s possible we
could see some isolated flash flooding. Some areas across the upper
Savannah Valley have seen over 2" in the past 24 hours, but so far
area waterways seem to be handling this precipitation with only
minor rises. For now, do not see the need to issue a Flash Flood
Temperatures will remain warm, with once again overnight lows closer
to seasonal average highs. Should see more reasonable temperatures
across the mountains tomorrow, though still not quite seasonal,
with the Piedmont in the upper 50s to lower 60s.
.SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/...
As of 100 PM EST Sunday: Fairly robust cyclonic flow will remain
atop the region Monday night in the wake of departing closed upper
low. Additional pcpn chances will increasingly be limited to NW NC
as the night wears on given that showers should become forced solely
by upslope/CAA along with the depth of moisture becoming
progressively shallow. Model llvl wind progs 45-50 kts at 85H
leading to the ongoing probability of strong high elevation wind
gusts into Tuesday morning. The snow shower threat should be highly
elevation dependent as it looks to be a race between lowering
freezing levels and lessening moisture. At this point, feel that
high elevation snowfall of an inch or two is not out of the question
throughout the Tenn border counties.
Expect the upper flow to deamplify on Tuesday and sfc pressure
gradient to loosen considerably. Model crosssections are progging
dry conditions throughout the depth of atmosphere, so expect
sunshine with max temperatures 8-10 deg f above climo. Lower level
return flow/lee troughing looks to develop on Wednesday with a
questionable amount of moisture return ahead of encroaching yet
weakly forced cold front. Sunshine should progressively fade behind
increasing clouds that day, but there should be a downslope aided
boost to max temperatures, maybe even category or two warmer than
Tuesday`s readings. Any small later day shower chances still should
be limited to the mountains.
.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 200 PM Sunday: The recently posted op 12z EC is consistent
with it/s earlier run in effectively and quickly brushing baroclinic
zone to the coast during Thursday. This will keep the sensible wx
fcst unchanged from previous thinking featuring token shower
chances, especially Wed nite into Thu am. Also, the onset of a
cooler period of weather is on tap to start off the period with a
shift to below normal max temperatures for Friday, the first
occurrence since 10 January. Coincident with the chillier airmass
overspreading the region, a period of NW flow snow showers remains
possible Thu nite into Friday. The large scale pattern for next
weekend will feature an eastern conus l/wv trough and below normal
temperatures. At this point, pcpn chances will be limited to just
periodic nw flow snow showers.
.AVIATION /03Z MONDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: Guidance is supportive of MVFR vsby restrictions
s this evening as rain spread north over the area, with some brief
IFR at all sites but KCLT. Some improvement in vsby occurs overnight
at KCLT/KGSP/KGMU/KAND as winds increase ahead of an approaching
front. Guidance takes Mountain and Foothills sites down to IFR vsby
by dawn, with KCLT MVFR. VFR vsby returns from KAND to KCLT by
midday, while KAVL and KHKY go MVFR, reaching VFR in the afternoon.
Cigs are expected to bounce between IFR and LIFR in the Mountains
and Foothills overnight, while KCLT remains IFR. After dawn, even
KCLT`s cig goes LIFR. Improvement to MVFR occurs by midday, with
VFR returning east of the mountains in the afternoon.
Winds will veer from NE to this evening at all sites but KHKY, the S
around after midnight. KHKY veers S by dawn. Winds veer from SW to
NW on Monday. Gusts will be greatest around midnight, and late on
Convective chances will increase from South to North this evening,
especially along the Interstate 85 Corridor from KAND to KCLT.
Convective chances diminish after dawn, but rain chances persist
almost through the day on Monday.
Wind shear will be briefly possible at KAVL this evening.
Outlook: Expect gradual improvement by Tuesday but with moisture
returning ahead of the next cold front Wednesday into Thursday.
03-09Z 09-15Z 15-21Z 21-00Z
KCLT High 82% Low 45% Med 62% High 96%
KGSP Med 68% Low 42% Med 70% High 100%
KAVL Med 67% Med 73% High 83% Med 76%
KHKY Low 52% Med 60% Low 59% Med 66%
KGMU Med 76% Med 65% Med 71% High 98%
KAND High 90% Low 58% Med 73% 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: