Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
318 PM EDT Wed Aug 1 2018

.SYNOPSIS...
A broad area of low pressure will persist across the Tennessee
Valley through at least Friday. This low will transport deep
moisture from the Gulf of Mexico and produce abundant cloud
cover and continued rounds of precipitation, which will hold
high temperatures below normal.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 245 PM EDT Wednesday: At the top of our agenda remains the
severe weather threat for the afternoon and evening. The midday
update to the Day 1 Convective Outlook split the Slight Risk and
kept parts of the Upstate and metro Charlotte in the risk area,
and it seems reasonable to be concerned about the area along and
south/east of Interstate 85. The latest mesoanalysis shows the
breaks in the cloud cover have allowed that region to destabilize
to about 2000 J/kg of sfc-based CAPE. We are in a gradient of
effective bulk shear, but part of the area still had over 40 kt
with 0-3km storm relative helicity around 150 m2/s2. The 12Z run
of the HREF still suggested some rotating storms, so it would be
wise to not give up on a few HP supercells. Ultimately, our tornado
potential might depend on whether or not any boundaries will be
present to locally enhance the shear/helicity, and for the time
being my confidence is not particularly strong that it will be the
case. So, don't completely disregard the severe thunderstorm threat
while we concentrate mainly on the flash flood threat. Speaking
of which...now that we are seeing some lines/clusters beginning to
organize in the direction of the deep layer shear, our flash flood
threat will ramp up into the late afternoon and evening. Temps
need an upward adjustment over the east where the clouds have
scattered out.

The latest model guidance, thankfully, continues to play along with
our ongoing forecast with no surprises. The upper level pattern
remains locked in place with a trof axis over the MS River valley
and a large anticyclone over the wrn Atlantic. The pattern will
continue to focus a plume of deep tropical moisture from the Gulf
of Mexico to the srn Appalachians and western Carolinas thru the
near term period. Precipitable water should remain at or above the
upper end of the PW climatology (i.e. well above normal), while
the atmosphere will be characterized by a deep warm cloud layer,
and our meso-beta element vectors remain small. All this is to say
that we have many of the necessary ingredients for flash flooding,
so the ongoing Flash Flood Watch remains in good shape. Precip prob
will remain likely to categorical into the evening while we work
thru the diurnal cycle. There may be something of a relative lull
late tonight/early Thursday morning, but the lull might be more
of a dropoff in intensity, not coverage. We ramp back up after
daybreak on Thursday as we continue to bring mid-level PVA across
the region, add some daytime heating, and bring some increased upper
divergence to bear. That should increase our flood threat thru the
day, perhaps higher than today as we continue to pile up the rain
and soak the ground. As for severe thunderstorms, we cannot rule
them out once again over the eastern fringe of the fcst area in the
afternoon as we once again destabilize to 1500 J/kg of sfc-based
CAPE and retain seasonally strong wind shear. Temps should be on
the cool side of normal where clouds and extensive precip linger.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 245 PM EDT Wednesday: At the start of the forecast cycle our
area (FA) still sits in a blocking pattern. Once again the key
players: the upper level ridge in the west, troffing from the Great
Lakes to the Gulf of Mexico, and the western Atlantic high pressure
system.

The "wave train" continues Thursday night with embedded pockets of
energy shifting through the region in a southwesterly flow aloft.
The right entrance region of the upper level jet continues to show
its hand allowing lifting mechanisms to continue.

It looks like we have to wait for this upper level low in the
Bahamas to rotate around the western periphery of the Atlantic high
pressure system. Once this happens the upper level high will shift
slowly west, and the upper level trough will deamplify considerably.

Meanwhile, precipitable water values remain elevated Thursday night,
at 2.0 inches or greater in spots. Various mechanisms of forcing
will allow for the potential of additional heavy rain. As a result
we will keep high POPS going Thursday night.

For Friday we kept POPS elevated with lingering precipitable waters
still high, and the last vestiges of the upper trough lifts out. At
this point hard to pin point whether any one area would need an
extension of the Flash Flood Watch at this time. The mountains could
be candidates, but not enough evidence at this point

As we start the weekend, although heights will be on the rise,
diurnal showers and thunderstorms should erupt. The higher
elevations look to have the better involvement, but other areas are
still possible.

Once again we kept temperatures in range with the suite of guide,
with less diurnal range than usual.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 155 PM Wed:  The Bermuda High will exert more of an influence
on our weather during the weekend with a good southerly flow of air
into our region. From late Saturday to late Sunday the center of the
Bermuda High gets closer to the Carolina coast. As a result, the
weak 5 to 10 kt southerly 925mb flow becomes even lighter on Sunday
and shifts more toward the west to start the week. Less moisture
transport through time. Precipitable water levels drop from 1.8 in
the lower Piedmont Saturday evening to 1.5 to 1.6 across the area on
Monday. Instability  around 2000 CAPE in piedmont Saturday evening
then 2000 to 2500 late Sunday along the I-85 corridor. The next
trough crosses the Upper Mississippi Valley Monday and Monday night
with the cold front having a roughly 200 mile wide area of frontal
convection.  Height falls across our region in mid week as the
trough becomes established over the Eastern Half of the nation.
Frontal convection expected over our region on Wednesday with the
greater cloud cover.Temperatures will be near normal

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
At KCLT and Elsewhere: Another mixed bag this afternoon. Clouds
continue to scatter out at the lowest level east of the mtns while
boundary layer heating raises the cloud base up thru the MVFR
category. With the exception of KAVL, most places will be in and
out of a low-end VFR ceiling with occasional MVFR as showers pass
overhead. The stronger convection will have brief IFR ceiling
and visibility. Thunderstorms will become more numerous thru
the late afternoon and the fcst becomes an exercise in picking
the most likely time. We begin with a four hour window in the
late afternoon and early evening and will amend from there. More
restictions will continue with widespread rain this evening and
into the overnight hours. At some point we should see a lull early
Thursday, during which time confidence is high that an IFR stratus
deck will develop. Once formed, it will persist through at least
late morning on Thursday. Details of its demise are left out for
now. We finish with a PROB30 for convection that ramps back up with
daytime heating. Wind will be south to southeast thru the period,
with stronger gusts near thunderstorms.

Outlook: A wet pattern will remain in place over the region
through late week. Numerous SHRA/TSRA will develop across the
area, especially each afternoon and evening, along with associated
restrictions. Limited drying may occur over the weekend.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High  90%     High  81%     High  86%
KGSP       Low   51%     High  84%     Med   74%     High  86%
KAVL       High  83%     High  90%     Med   77%     High  96%
KHKY       Med   67%     Med   74%     High  82%     High  86%
KGMU       Med   64%     Med   79%     Med   76%     High  85%
KAND       Med   71%     High  86%     Med   76%     High  88%

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

&&

.HYDROLOGY...
The current hydrology situation this afternoon is comprised of
two primary areas of concern.  The first is over the far western
mountains including Graham, Haywood, Jackson, Macon, Rabun, and
Swain counties, where instability is poor (SBCAPE <500 J/kg) but low-
level covergence closer to the proximity of the surface front and
overall upslope flow suggest some forcing exists.  This combined
with high PWs of 1.7-1.9" suggest efficient, warm-layer rain
processes are still in play. However, at this time, rainfall rates
are generally less than 1"/hr in the SW mountains; therefore, the
flash flood threat here is not imminent but antecedent conditions
are priming if higher rainfall rates materialize later today into
tonight.

The second area of concern is along and east of the Blue Ridge
Escarpment into the Foothills and Piedmont of the Carolinas and NE
GA where instability is greater (SBCAPE >1500 J/kg) and PW values
are still near climatological maxes (1.8-2.1"). Also, Flash Flood
Guidance values are lower here, especially along the Escarpment, due
to previous activity (<2"/hr). Therefore, the concern for localized
flash flooding is greater in this area as ongoing activity is
producing rainfall rates of 1-3"/hr and some brief anchoring has
been occurring, producing 1-2" of rainfall in as little as 30-45
minutes.  Over urbanized areas, this will produce quick
street/parking lot/underpass flooding and will need to be closely
monitored.

As rainfall continues area wide, antecedent conditions will continue
to deteriorate as we approach the best chances for heavy rainfall
tonight into Thursday.  Therefore, the flood risk will continue to
increase into Thursday before tapering off (but not disappearing
completely) into Friday and the weekend. Meanwhile, as basin-average
rainfall amounts increase into tomorrow, mainstem river flooding
becomes a greater possibility, especially in the climatologically
favorable areas of the upper French Broad River, but rapid response
along the lower Tuckasegee, Swannanoa, South Fork Catawba, and
Pigeon River basins is also possible. At this time, Minor flooding
(i.e., flooding of low-lying areas, greenways, boat-access ramps,
recreational areas) is the primary concern.  Monitor the latest
forecasts and stream levels at:

https://water.weather.gov/ahps2/index.php?wfo=gsp.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flash Flood Watch through Friday morning for GAZ010-017-018-
     026-028-029.
NC...Flash Flood Watch through Friday morning for NCZ033-035>037-
     048>053-056>059-062>065-068>072-082-501>510.
SC...Flash Flood Watch through Friday morning for SCZ001>014-019.

&&

SYNOPSIS...DEO
NEAR TERM...PM
SHORT TERM...TS
LONG TERM...DEO
AVIATION...PM
HYDROLOGY...JMP

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