Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180729

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
329 AM EDT Sat Aug 18 2018

An upper level trough will persist over the eastern half of the
nation through the weekend with abundant moisture coming up from the
Gulf. A strong and reinforcing trough will arrive early next week
creating a greater boost to shower and thunderstorm activity around
Tuesday. The passage of the cold front on Wednesday will bring drier
air to start the second half of the week.


As of 320 am: Patches of light rain left over from earlier
convection will continue to gradually dissipate over the next hour
or so. No significant resurgence of convection is expected thru
daybreak. Will have to watch for the development of patchy fog,
but for now low stratus seems to be favored.

Today looks like it should be more convectively active than
yesterday. A short wave rounding the bottom of the mid/upper
trof over the Midwest will bring the trof axis across the region
late today while providing a shot of dpva. Meanwhile, weak upper
divergence will also move overhead. Both will make the overall
environment more conducive to deep convection. The CAMs respond in
kind, especially the 00Z HREF, which supports the idea that precip
prob should be in the likely range given the numerous coverage. So,
unlike yesterday, when the guidance suggested that our fcst was
overdone, today it looks like we are on track with most people
seeing at least a passing shower or thunderstorm in the afternoon
and evening. Severity does not look all that bad as lapse rates
continue to be relatively weak and mid-level dry air is lacking,
but a few strong/severe multicells will not be ruled out as the
short wave could help to organize storms late in the day. High
temps will be close to normal. The convection should gradually
weaken thru the evening hours as the wave passes to the east. The
GFS especially ends the precip by midnight or so. The fcst holds
onto a small chance thru the night over the mtns in deference
to the NAM which keeps some energy off to the west thru daybreak
Sunday. Low temps will be seasonally mild once again.


As of 150 AM EDT Saturday: A s/w upper level ridge will traverse the
FA to begin the short range. This will have the effect of drying out
the mlvls while levels below h5 remain quite moist. The added
subsidence will create shallow inversions aloft which will inhibit
sbCAPE values and limit the potential for stg/svr tstms.
However...precip coverage across the mtns will be abv normal with
general tstms possibly creating localized hydro issues. On Mon...a
h5 wave digs into the ridge and deep layered moisture returns. With
PWATS reaching arnd 2.0 inches in increasing mech lift...afternoon
shra/tstms will efficiently develop early on across the mtns and
non/mtns by mid/afternoon. These storms will be good precip makers
with high rainfall rates likely. Thus...another day on tap where the
possibility of localized flooding will be a concern. Max temps will
remain below normal in layered cloud cover and mins will be held a
cat or so abv normal.


As of 210 AM EDT Saturday: A deeply moist pattern will persist to
begin the ext period as a developing h5 trof axis swings toward the
fcst area. With good upper difl flow and continued GOM moist
adv...the atmos will continue to support shra/tstms capable of
producing heavy rainfall esp across the higher terrain Tue afternoon
and perhaps into Wed. The upper trof axis is progged to cross the
region late Tue into Wed as the llvl flow responds and veers
sigfntly ahead of an approaching broad area of cP hipres. There will
likely be an airmass mix with this feature beginning Thu into Fri
which will limit the precip potential while lowering PWATs to
seasonal levels. Expect lower max temps during this time with sfc
td/s mixing into the lower to mid 60s by the end of the week...which
will feel good for mid/late August.


At KCLT and elsewhere: Still dealing with the remnants of a recent
convective flare-up over Upstate SC, so those TAF sites get a
brief TEMPO to handle the MVFR-level ceiling contained therein. The
extensive cloud cover moving over the NC mtns suggests a bit less
of a likelihood of fog but maybe more of a chance of low stratus
thru daybreak. Will monitor for amendments. Otherwise, a light SW
wind overnight will become stronger on Saturday and occasionally
gusty in the afternoon. The passage of an upper wave will help
make the environment more favorable for more numerous thunderstorms
compared to yesterday, and the CAM guidance lights it up fairly well
this afternoon. Will use a four hour TEMPO to highlight the most
likely time range for thunderstorms, followed by a 4-6 hr PROB30 to
handle additional storms into the evening. Prevailing ceiling will
be VFR. We should see a bit more in the way of fog late Saturday
night assuming the storms are more numerous during the daytime.

Outlook: Mainly diurnal showers and thunderstorms remain in the
forecast through early next week. Restrictions will mainly be
limited to areas in heavy showers and low clouds/fog across the
mountain valleys each morning.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  91%     High  84%     High 100%     High 100%
KGSP       High  90%     High  80%     Med   74%     High 100%
KAVL       Med   73%     High  80%     Med   61%     High  85%
KHKY       High  87%     Med   79%     Med   77%     High 100%
KGMU       High  89%     Med   74%     Low   56%     High  82%
KAND       High  87%     Med   78%     High  91%     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:




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