Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222057
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
457 PM EDT Sat Jul 22 2017

.SYNOPSIS...
The upper level ridge that has brought such hot weather to our
region of the nation is retreating to the desert southwest as an
upper level trough develops over the eastern part of the country.
Expect temperatures to be a little cooler toward mid week and
better chances for rain over the next few days.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 430 PM EDT Saturday: Have made several updates to pops through
the evening and overnight hours, mainly to reduce pops through the
Piedmont and to speed up the dissipation trend overnight, but also
to add in slightly higher pops across the northern mountains
overnight as CAM guidance, especially the HRRR, has been
consistently bringing a small convective complex down the flow
aloft. Will continue to monitor near-term convective trends upstream
and increase pops further if necessary.

Otherwise, should be another round of mountain valley fog becoming
dense in some areas overnight. Patchy fog elsewhere. Lows remain
above normal.

Overall, expect convection to be more widespread Sunday as the
atmosphere becomes very unstable and forcing may increase with
upstream outflow boundaries moving in from the NW. Shear may
increase as heights fall slightly allowing MCS associated short
waves to brush by the area. DCAPE and sfc delta theta-E values
remain very high. Freezing and wet bulb zero levels remain high, so
it will still take some very tall storms for severe weather to
develop. However, with the increased coverage and potentially better
organization, storms with damaging downbursts will be more likely
than the past few days. Highs should be a couple of degrees cooler
than Saturday with increased convective debris and slightly lower
thickness values. Even with dew point mixing less than any day this
past week, the lower temps should keep heat index values from
topping out over heat advisory levels. Still, HI values over 100
will be likely outside of the mountains.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 200 PM Sat: A weak trough axis is expected to cross the
area Sunday night or Monday morning, along with a weak baroclinic
zone. Widespread convection is depicted by various models over
the Tennessee Valley late Sunday in response to those features,
and the activity will spread along gust fronts into the Deep South
and possibly across the Appalachians overnight. With warm low
levels maintaining unstable profiles at that time, we will need
to advertise some nondiurnal PoPs. It is not out of the question
that with profiles continuing to be quite dry aloft, the stronger
storms--particularly those that cluster along outflows--will be
capable of producing wind damage.

On Monday diurnal instability should redevelop across most
if not all of the area. However, the NAM family of guidance
depicts convective overturning following the overnight storms,
which has a lingering stabilizing effect through the day over the
northern portion of our area. Furthermore, the NAM and Canadian
GDPS show a weak vort max extending across north GA and SC along
the baroclinic zone (though by that time the latter is very hard
to pick out). Precip thus appears most likely over the southern
zones. I will point out that the GFS is slower and further north
in showing this feature entering the area, which suggests a more
even distribution of PoPs on Monday.

With shear having weakened following the departure of the trough,
storms Monday afternoon should return to pulse mode. Furthermore
evening stabilization should occur and PoPs will eventually taper
off. The models depict little change in the upper pattern for
Tuesday, but whatever is left of the baroclinic zone should have
drifted further south. Thus I expect PoPs to return to a more
typical distribution, but with values perhaps 10 percent above
climo; pulse threats are again expected.

Max temps look to be a bit cooler but still slightly above
normal. Though sticky dewpoints in the upper 60s to near 70 will
make for muggy afternoons, heat indices should stay below advisory
criteria.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 145 PM Saturday: A closed upper low located over central
Canada Tuesday evening will move to eastern Canada into the second
half of the week while 500mb heights fall over the eastern part of
the nation. By next weekend, the ridge remains over the Desert
Southwest while models indicate a broad upper trough becomes
established from the Mississippi Valley eastward.

Old nearly stationary frontal boundary located west to east across
our region Wednesday and dissipating Thursday. This will give our
area enhanced convective activity Wednesday then mainly restricted
to the mountains late Thursday. By Thursday night, the next cold
front crosses the Ohio Valley and reaches the VA and NC border early
Friday. This front is expected to become stationary west to east
next weekend from near Memphis to near Norfolk providing the focus
for enhanced coverage of diurnal convection going into the weekend.
There is also the possibility of more mesoscale convective systems
moving southeast from the top of the ridge crossing the Mississippi
Valley toward our area. Timing of these MCSs are very difficult days
in advance and they usually weaken as the systems reach our area. Of
course, this could in short term forecast situations increase POPs
in a nondiurnal convective enhancement.

Max Temperatures are forecast to be about 5 degrees below normal on
Wednesday, then around normal Thursday and Friday and perhaps just
below normal Saturday.  Min temperatures will be near or just above
normal due to the extensive cloud cover.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Only change to the KCLT TAF for the 21z
AMD is to drop Cu bases from 060 to 050. Otherwise, high based Cu
will continue to increase through the afternoon. Best chance of
convection will be at KAVL and KHKY. Have included VCTS there.
Convection diminishes this evening. Expect another round of mountain
valley fog and have gone MVFR for now at KAVL, but it could be IFR
or lower. Patchy fog outside of the mountains, so no mention for
now. Convection develops again Sunday afternoon and could be more
widespread. For now, PROB30 limited to KAVL and KHKY where an
earlier start is possible. S to SW wind today becomes W to SW Sunday.

Outlook: Increasing chances of convection will continue into the
middle of next week ahead of a series of weakening cold fronts.
Overnight restrictions will continue in the mountain valleys with
chance increasing elsewhere.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   63%     High 100%
KHKY       High 100%     High 100%     High  94%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...RWH/TDP
SHORT TERM...Wimberley
LONG TERM...DEO
AVIATION...RWH/TDP



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