Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211812

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
212 PM EDT Sun May 21 2017

An area of low pressure will move away from the area tonight, with
another system approaching the region Monday night. With the
approach of a deep upper trough and attendant cold front Wednesday,
another round of storms will be possible. Expect dry high pressure
and milder temperatures to return on Friday and into next weekend.


As of 100 pm Sunday: Meso-low is currently located between KCEU and
KGVL. NE winds across the northern part of the Upstate, resulting in
part from establishment of in-situ cold air damming is focusing an
area of strong low level convergence along an E/W axis through the
middle of the Upstate. An ridge of moderate surface based
instability is upstream of our area is resulting in heavy-rainfall
producing convection along the I-26 corridor in the Upstate, and
into the NC foothills. The latter area will be of particular
concern, as moderately strong E/SE upslope flow in the 1-5 kft AGL
layer may focus some especially problematic rainfall rates there.
The Watch in this area looks quite good, but we may need to start
trimming it along the western flank, as the back edge of the precip
shield was in the process of clearing the Smokies/surrounding areas
as well as our GA mtn zones.

Despite warming into the 80s, southeastern areas have only been able
to realize 500-1000 J/kg of sbCAPE thus far, and with the rain
shield just off to their west, there`s not much additional
insolation to be had. Thus, the severe weather threat is minimal.
However, areas along the I-77 corridor will see some rather healthy
rainfall rates as convection passes through late this afternoon and
evening, so localized flash flooding of urban areas along the I-77
corridor will remain of some concern over through that time.


As of 200 PM EDT Sunday: Cycloidal motion will continue among the
closed low centers over the Great Lakes Monday night, with
associated vorticity lobes digging southward across the Midwest and
sharpening up the central CONUS trough through Tuesday. An MCV, or
other small-scale wave, developing in the base of the trough over
the lower Mississippi River Valley Monday night will likely move
quickly into the forecast area from the southwest around daybreak
Tuesday. The next slug of deeper moisture will accompany this
feature across the forecast area through Tuesday. QPF should be
enhanced as a low level circulation associated with this feature
rides northeast along the stalled frontal boundary and generally
moves up the I-85 corridor. Many locations will see another 1 to 2
inches of rain from late Monday night into Tuesday night, and the
hydrology threat will increase once again. Mid-level lapse rates
will be fairly modest on Tuesday and widespread clouds and
precipitation will limit instability, but backing and strengthening
flow will improve shear and the overlap of any instability with the
brief spike of 30 to 40 kt surface to 3 km bulk shear could produce
some rotating updrafts and isolated severe weather.

The upper trough will then sharpen up to the west Tuesday night
through Wednesday and deepen into a closed low over Illinois. Yet
another round of deeper moisture should surge northward across the
forecast area Wednesday to continue the threat of heavy rainfall.
Locations southeast of I-85 could also see sbCAPE recovery to 1000
to 2000 J/kg by late Wednesday afternoon just ahead of the next
passing cold front. Given the ongoing flooding threat in the near
term, no new Flood Watches will be featured yet for Tuesday or
Wednesday, but the HWO mention of heavy rain will continue.


As of 200 PM EDT Sunday: The sharp upper trough will finally migrate
east of the Appalachians on Thursday. Steep lapse rates of 7.0+ deg
C/km will accompany the passing trough axis, but low level flow will
veer more westerly post-fropa and lower levels will start to dry
out. Scattered showers and thunderstorms may continue during the day
Thursday, and northwest upslope flow moisture may keep some western
mountain showers going into Thursday night.

Broad ridging between systems is expected over the southeast Friday
through Saturday. Northwesterly downsloping flow will aid drying and
warming somewhat on Friday before low level flow starts to back
southwest into next weekend. The ridge will then flatten out on
Sunday with a return of embedded shortwaves in deep layer WSW flow
aloft. Instability will also build over the southeast half ahead of
the next cold front approaching from the northwest and moisture
return will be slightly better over the higher terrain to warrant
chance convective PoPs over the mountains and slight chances in the
piedmont. Temperatures will steadily warm through the period.


At KCLT and KHKY: Heavy rain showers and TS will sweep over the
terminals over the first couple of hours of the forecast period,
then continue into the evening before winding down mid/late evening.
Cold air damming air mass has seeped into KHKY, and IFR cigs/NE
winds should continue there through at least the afternoon. Expect
these conditions to remain north of KCLT, but could  certainly see
periods of IFR in +RA, and would not be surprised to see periods of
E/NE winds as well. Otherwise, low levels remain moist, but
mid/upper levels begin to dry out late tonight. This should allow
for development of fog/low stratus, esp near KHKY.

Elsewhere: Persistent heavy rain showers will gradually taper off
over the next few hours, but brief periods of heavy rain will remain
possible through early evening. More problematic will be the cigs,
which have developed at the IFR range from time to time at the
Upstate SC terminals and KHKY. These may well persist through the
next six hours, but only had confidence to advertise a tempo through
21Z. NE winds at the Upstate terminals should gradually shift
through NW and eventually return to SW this evening. Low levels will
remain moist, but mid/upper levels begin to dry out late tonight.
This should allow for development of fog/low stratus.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through about Wednesday.
Drying is expected to finally occur Thu through the end of the week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  84%     Med   73%     High  88%     High  88%
KGSP       High  84%     High  83%     Med   73%     Med   79%
KAVL       High  85%     Low   54%     Med   75%     High  80%
KHKY       Med   65%     Med   64%     Med   64%     High  81%
KGMU       High  83%     Med   73%     Med   69%     High  82%
KAND       High  87%     High  85%     Med   78%     Med   78%

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:


GA...Flash Flood Watch until 2 AM EDT Monday for GAZ010-017.
NC...Flash Flood Watch until 2 AM EDT Monday for NCZ033-048>050-052-
SC...Flash Flood Watch until 2 AM EDT Monday for SCZ001>003.


SHORT TERM...Wimberley
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