Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201507

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1107 AM EDT Sat May 20 2017

One to two inches of rain and thunderstorms are expected Saturday
night through Monday as a large frontal system passes through the
area.  Frontal passage will bring cooler temperatures with Tuesday
through Thursday highs 5 to 10 degrees below normal.  Some moisture
is expected to return by Wednesday night as a strong system impacts
the area with more rain, thunderstorms, and wind.


As of 1050 am: today`s pattern in our neck of the woods will be
dominated by amplification the upper ridge downstream of deepening
Great Plains upper low. This will keep rather warm mid-level temps
across the area, while surface ridge axis emanating from the
anomalously strong Bermuda high will maintain an area of relatively
low surface theta-e across the forecast area. Meanwhile, a back door
cold front was moving S/SW across the Mid-Atlantic this morning,
with the leading edge somewhere in the vicinity of the I-64 corridor
in central VA. There is a strong consensus in the latest guidance
that this boundary will remain north of our forecast area through
the end of the day. As such, the latest mesoscale model runs depict
a convection-free scenario through the afternoon. Other than
maintaining a slight chance across the far northern zones this
afternoon, we will generally follow suit. Temps will remain well
above climo today.

Aforementioned approaching upper low will drag a surface low and
cold front through the Plains and Lower Mississippi Valley today.
Leftover quasi-stationary boundary across the Virginias may briefly
drop into NC, but the strength of the upper ridge should prevent it
from moving too far south tonight. Did increase pops in northeastern
zones somewhat this evening as the front sags a little south, but
the bigger story will be with the approaching convection along the
front. Slug of tropical moisture will lift from the Gulf into the
Deep South in the WAA regime ahead of the front, and this combined
with the approaching LLJ running smack dab into the SW NC mountains
will provide a good setup for at least brief heavy rain overnight
tonight (toward the end of the near term). GFS has PW almost 2" late
in the period which, perfect progged, would be higher than the daily
PW max for KFFC (way higher for KGSO). NAM is a tad slower than the
GFS and has max PW values just a smidge lower, but still above the
daily max. Deep-layer shear is minimal, with deeper dynamics still
lingering to our west by the end of the period, but moderate
instability may help to enhance rainfall rates. Luckily the area
with the highest QPF is still pretty dry, with residual lingering
drought, but at this time certainly cannot rule out an isolated
flash flood potential.


As of 330 am EDT Saturday: Wedge front makes minimal progresses into
the region from the northeast by Sunday morning as major cold front
approaches from the west.  Good moisture ahead of this front and
strong forcing from synoptic upper low will lead to appreciable
rainfall and thunderstorms continuing Sunday morning and
trailing-off Monday as front makes its passage.  Current timing of
FROPA from GFS has it past the Smokies Monday morning moving
southeastward, and out of the CWA by Monday evening.  FROPA scours
moisture and advects in cooler air with dewpoints dropping 10
degrees from Monday to Tuesday and highs dropping 5 degrees.
Rainfall totals are expected to be 1 to 2 inches for the Saturday
night through Monday period with higher amounts of up to 2 to 3
inches over southwestern parts of the CWA where deep moisture will
be better.  Due to convective nature of much of the precip., local
amounts can be significantly higher, giving some concern for local
flooding with the heavier storms.

Severe potential is limited by low convective instability measures;
with LI to around -3 Saturday and Sunday afternoon, along with low
shear.  Severe chances are probably strongest ahead of the cold
front on Monday where LI may reach -7 along with improved deep layer
shear and lift as system makes its push eastward.

Precipitation with next system may begin as early as Tuesday
morning, though main brunt of that system is not expected until
Tuesday evening.


As of 330am Saturday:  Wet forecast will continue Tuesday evening
through Thursday as next major upper storm system impacts the area.
The Wednesday system is dynamically much more potent over the
Carolinas than the first system this weekend; however, moisture
scouring from the passage of the front on Monday will lead to lower
precipitable water amounts, and likely lower rainfall totals.
Strong surface low will be located west of the area ahead of the
Wednesday system, which may surge some moisture into the area ahead
of the next front, though how effective that will be is uncertain at
this point.  The GFS and EC models differ considerably in the
timing, with the EC having the strong surface low northeast of the
area Wednesday morning with the heaviest precip. over by that time;
while the GFS has the low pulling out with the heaviest precip.
Wednesday night.  The EC solution has a lower chance for severe
weather with it as the more rapid progress gives moisture less time
to advect into the area.  Even with the slower progress of the GFS,
instability levels are marginal Wednesday afternoon, with CAPE less
than 500 j/kg.  Wind shear, though, could be quite good as system
has strong flow aloft, and the GFS develops a surface low center
over the CWA.  If the GFS is correct, late Wednesday has some
potential for significant severe weather.  Rainfall totals Tuesday
through Thursday look to be around an inch, but could cause hydro
issues as it follows closely on the heals of the previous very wet

Passage of front and pull-out of strong system by Thursday morning
will end precip. most places, though some wrap-around showers may
occur over the higher terrain.  Forecast is dry Friday and Saturday
with a warming trend following the Wednesday FROPA that could
reinforce the below normal temps with highs close to 10 degrees
below normal on Thursday.  Due to strong surface low with the Wed.
system, strong gusty winds are also anticipated late Wed. into


At KCLT and elsewhere: IFR cigs advecting up out of the midlands
have made it to KAND, and have included TEMPOs for KCLT/KGSP/KGMU as
the northern edge of the moisture may flirt with these sites before
it dissipates with heating. KAVL has had off-and-on VLIFR conditions
through the night which should also improve with daytime heating.
Generally SKC today with only FEW-SCT passing VFR clouds. Any
convection will be too far to the north to mention at the TAFs,
closer to a nearly stationary frontal boundary. Next system
approaches from the west tonight with increasing low VFR and
eventually MVFR to IFR cigs toward the end of the period. -SHRA will
increase from the west, with VCTS for KAVL/KAND. Winds SSW to start
the period will back SE this evening and could see ESE by the end of
the period, but expect winds to remain on the S side.

Outlook: Showers and thunderstorms with associated restrictions will
continue Sunday and Monday as the front passes. Another system is
expected Tuesday through Thursday, with improving conditions by the
end of the work week.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High  84%
KGSP       High  98%     High 100%     High 100%     Med   70%
KAVL       High 100%     High 100%     High 100%     Med   62%
KHKY       High 100%     High 100%     High 100%     High  81%
KGMU       High  98%     High 100%     High 100%     Med   66%
KAND       High  93%     High 100%     High 100%     Med   68%

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:




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