FXUS62 KGSP 280238

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1038 PM EDT Sun Sep 27 2020

Warm and fairly humid conditions will persist early this week,
with rain chances increasing gradually until a cold front pushes
through Tuesday. Dry and somewhat chilly weather is expected behind
the front for the second half of the week. Expecting the coolest
nights to be Friday night and Saturday night.


As of 1035 PM: The open wave passing over northern areas is
continuing to support isolated showers with brief heavy rain across
western portions of the Upstate of SC into northeast GA. This trend
will probably continue as the activity lifts north into the
mountains with further expansion possible with the added orographic
lift. The activity in these areas will likely delay the onset of any
fog development until later tonight with most guidance showing the
greatest shot east of the I-26 corridor to the I-77 corridor.
Trimmed back rain chances southeast of I-85 for the overnight period
in collaboration with the Columbia WFO as subsidence aloft as
denoted by a dry pocked on water vapor imagery should limit shower

Otherwise, a dampening short wave is expected to lift northeast past
the region this evening...and thus begins our period of active
weather. Most of the better forcing and deeper moisture should
actually move northeast across the Midlands and Coastal Plain
through the evening hours as a weak upper jet pulls out to the
northeast. Other mid-level forcing closer to the mid/upper wave, and
some weak instability, will maintain the possibility of some shower
activity through the late afternoon and evening mainly over the
higher terrain. Some of the mesoscale models develop more shower
activity in the early morning hours and move that steadily across
roughly the western half of the fcst area through daybreak or
thereabouts, so a chance of precip will be maintained for the
mtns/foothills overnight, expanding east around daybreak. Low temps
should be mild. Monday looks like something of a toss-up right now.
On the one hand, the weak mid/upper wave will pass to the northeast
early in the day giving way to short wave ridging for the rest of
the morning into midday, but on the other hand we retain an air mass
rich in low level moisture, so shower activity could develop at any
time. Precip chances should improve in the afternoon with the
approach of the next vort lobe ahead of the upper trof digging over
the Plains/mid MS Valley region. Precip probs will be kept in the
chance range. Might even get some thunder in the afternoon, but
instability looks too low to be concerned with severe threat.


As of 130 PM Sunday...Better agreement is seen in the overall
pattern between the op models, however the GFS is now a little slower
and deeper than the other guidance wrt to the upper low and it has
trended slower than it/s previous run. This wont have a great impact
on the expected sensible weather, but it does support more of a
hydro issue as opposed to severe storms Tue afternoon with
training cells possible across the NC mtns early, then over the
I-77 corridor in the afternoon east of of a developing llvl
theta-e ridge.

With continued sw/ly moist flow and training across the mtns
Tue...the srn NC mtns and srn BR areas look favored for flooding
concerns where localized 2-3 inches of rain may fall by the
afternoon before the frontal zone finally shifts east. The overall
instability trend is down east of the mtns Tue afternoon, but values
in the 600-900 J/kg would be enuf to spawn semi-organized lines of
convection given the large scale dynamic lift and bulk shear levels
reaching 50 kts during max heating. Deep convection could also
become trained over that area for a few hrs which would produce
areas of rain arnd 1.5 to 2.0 inches, which could be enuf for minor
flooding of shallow streams/creeks and low-lying urban areas. Most
guidance pushes the llvl convg zone east by 06z Wed, however the GFS
is clinging to a wrapped around Atl moist feed due to it/s slow
moving stacked low. Yet, not anticipating much additional precip
Wed, likely 0.5 inches or less across the far ern zones. Right now,
it doesn/t look like a flash flood watch will be needed for the FA,
but we`ll continue to monitor the trends of this situation.

Max temps will be tricky Tue due to the FROPA timing, with the mtns
remaining below normal to near normal temps over the ern zones. A
notable drop in highs, abt 10 degrees F, is on tap Wed as winds
shift nw/ly and mostly cloudy skies linger associated with the upper
low. Mins will remain abv normal Mon night within high moisture
levels and sw/ly flow, then drop a cat or so below normal Tue night
as a cooler airmass begins to mix in, yet cloud cover remains
relatively high.


As of 200 PM Sunday...The ext range begins with the possibility of
linger -shra across the NC zones as the latest GFS is slow to lift
it/s upper low away from the Carolinas. The model actually produces
the arnd another inch or so of upslope precip across the ern
escarpment thru 09z, but this has been mostly discounted due to
model comps and the GEFS QPF probs showing less than 20 percent of
0.25 inches during this time. So, all that to say, no impacts are
anticipated early Thu while a stg Canadian sfc high ridges in from
the northwest. There could be some wrn mtn upslope -shra late Thu
evening in trapped upstream moisture, but precip amts will be low
end. Pretty much a dry forecast Fri thru the weekend is expected
with the broad sfc high reinforcing a deeply dry airmass Fri night.

Max temps will be fallish Thu with highs in the mid 70s across the
non/mtns and arnd 70 F over the mtn valleys. Highs Fri thru Sun will
struggle to get out of the 60s as the cooler cP air continues to
advect in from the northwest. Mins will be also be 10-15 degrees
below normal thru the period with the lowest temps beginning Thu
night, which will support the develop of frost across the NC mtns
each morning thereafter.


At KCLT and elsewhere: Intermittent showers are possible through the
period, focused near the mountains through 15Z and becoming more
widespread with isolated TSRA after 17Z. Moderate confidence in
IFR/LIFR cigs/vis developing after 06Z and improving around 15Z for
most terminals. Low end southerly wind gusts are possible as early
as 15Z.

Outlook: Unsettled weather expected thru at least the midweek. A
strong cold front should push thru the area by the end of the week,
bringing drier weather and VFR conditions.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  93%     High  86%     Med   73%     High 100%
KGSP       High  81%     High  82%     Med   75%     High  96%
KAVL       Med   79%     High  84%     High  80%     High  98%
KHKY       Med   78%     High  93%     Med   79%     High  98%
KGMU       High  81%     Med   77%     Med   75%     High  96%
KAND       High  81%     Med   77%     High  83%     High  95%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:






NWS GSP Office Area Forecast Discussion