Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211830
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
230 PM EDT Fri Jul 21 2017

.SYNOPSIS...
The upper level ridge that has brought such hot weather to our
region of the nation will gradually break down over the weekend as
an upper level trough develops over the eastern part of the country.
Expect temperatures to be a little cooler next week with better
chances for rain.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 215 PM EDT Friday: Convection developing across the NC
mountains nearly as expected. Unexpected is the convection
developing to the south of the CWFA along a weak boundary. The
southern convection should remain south of the area. However, an
outflow boundary from this morning`s OH Valley MCS is moving south
into the NC mountains. This will keep convection going there and
could help spread the convection into the NC Piedmont. The northern
end of outflow boundary is stronger and with more of a push across
VA, but could drop south toward the Piedmont as well. The latest CAM
guidance show the convection dissipating as it moves into the more
stable air over the Piedmont. The guidance shows similar trends
across the Upstate and NE GA as well. Current afternoon and evening
forecast already shows these trends, so will keep them in place.
With very high DCAPE values across the area, and isolated damaging
downburst is possible in the strongest storms.

The guidance has backed off on any lingering convection overnight as
the atmosphere stabilizes and any boundaries move south of the
area. Will keep convection overnight to a more typical diurnal trend
as a result. Mountain valley fog likely again, with only patchy fog
expected outside of the mountains. Lows will be 5 to 10 degrees
above normal.

Chance of convection increases some Saturday as the atmosphere
moistens ahead of a weak boundary dropping toward the area. Again,
the best chance will be across the mountains but with a better
chance across the foothills than recent days. Moderate instability
expected once again although with continued meager lapse rates. Mid
level dry air and DCAPE values will not be as great as previous
days, but will be high enough to keep the potential for isolated
damaging downbursts in place. Hot and humid conditions continue as
well. There may be less mixing than previous days, but still expect
a nearly normal diurnal trend to dew points. This should keep heat
index values below 105 but above 100. Will not issue a heat advisory
at this time, but one may be needed if dew points don`t mix as
expected.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 200 PM Fri: The pattern will change little Saturday night
into early Sunday. Weakly cyclonic flow aloft will exist over our
area, and decent lapse rates will persist just above the boundary
layer. The GFS and SREF expect some convection will continue
overnight in the mountains in response to these conditions, but
the opnl NAM is dry. The forcing however just looks too weak to
expect convective initiation. PoPs thus wane and fall below the
mentionable threshold in all zones.

Heights fall slightly thru Sunday as a shortwave moves across the
northern Great Lakes. The associated cold front should remain to
our north and west during the day, but slightly higher convective
chances are warranted with slightly higher dewpoints and the
potential for activity to propagate off of the front and south into
the area. With somewhat increased shear and the focusing mechanism,
last night SPC included our northern zones in a marginal risk on Day
3. Damaging outflows and perhaps marginally severe hail are within
reason. Max temps should be 2-3 degrees cooler owing to the reduced
thickness and more cloud cover. This will mitigate any excessive
heat concerns that could otherwise arise with the juicier dewpoints.

Some guidance, namely NAM, expects that as the front sags
further south Sunday night, additional rounds of convection could
develop. Hence PoPs will be kept at slight chance or better into
morning. Whatever is left of the frontal boundary appears to wash
out over the area Monday; temps and dewpoints will top out another
degree or two cooler. Nonetheless models depict similar convective
coverage south of I-40. Shear will be sufficient to expect clusters
of storms producing gusty winds, but at this time it does not look
particularly likely they would be severe.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 150 PM Friday: The models show in the 500mb pattern Monday
evening a trough over NY and PA with axis roughly down the
Appalachians. This trough pushes off the New England coast Tuesday
night with the associated cold front lying west to east across TN
and the Carolinas early Tuesday.  This front may even push south
across Georgia Wednesday then linger over our region into late week
as it slowly dissipates. The 06Z run of the GFS had an upper ridge
centered over New Mexico and the TX panhandle having a significant
influence in mid week to the Carolinas.  The new 12Z GFS returns the
suppression of the ridge to the SW and gives the eastern trough the
greatest influence. With the ridge to our west and an upper NW flow
of weather from the top of the ridge to our region, there is the
potential for one or two weakening MCSs to reach our area in the mid
to late week. The dissipating front will provide the focus for
convection each day. Instability should be best on Tuesday
afternoon.  Max temperatures near normal Tuesday and again Friday
but a little below normal Wed and Thursday.  Min Temperatures near
or slightly above normal each day.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Latest CAM guidance shows KAVL and KHKY have
the best chance of seeing any convection from the cells developing
this afternoon. Have included VCTS in those locations. Will monitor
the other sites for possible AMD. Should be VFR through the evening
unless a cell moves through. There will be mountain valley fog again
overnight, but per persistence will leave out of KAVL. KHKY could
see fog, especially if a storm moves through. VFR for the rest of
the sites. Cumulus should develop earlier Saturday with winds
picking up as a lee trough develops. May be a better chance of
convection area wide, but will only include a PROB30 for now at KAVL
where chance is better earlier. Light winds early this afternoon
become light S to SW, then light and variable overnight. SW wind for
Saturday.

Outlook: Trends show increasing chances of convection  continuing
into early next week ahead of an approaching cold front on Monday.
Overnight restrictions will continue in the mountain valleys with
chance increasing elsewhere.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   73%     High  88%
KHKY       High 100%     High 100%     High 100%     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
SHORT TERM...Wimberley
LONG TERM...DEO
AVIATION...RWH



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