Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230846

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
446 AM EDT Tue May 23 2017

A train of Gulf low pressure systems will track through our
area along a stalled front through tonight. As the front moves
southeast, a broad area of showers and storms crosses our area
Wednesday ahead of a closed upper low pressure. Expect temperatures
to be below normal until the deep upper trough moves out before the
start of the weekend. A drying trend will begin by the end of the
work week and continue until the end of the weekend.


As of 300 AM EDT Tuesday: Another day, another round of heavy rain,
and another complicated forecast. As to the synoptic setup, no
surprise here in that we have been advertising this southwesterly
flow aloft head of a digging upper trough over the western Great
Lakes. Multiple waves moving out of the Gulf with anomalously high
PW values.  Upper jetlet with increasing DPVA pushing through today
as well. Surface low(s) will travel up toward the Southern
Appalachians, with still some uncertainty as to the exact track of
the low(s), which would have impact on location of heavier rain.

Widespread rain is moving into the area at AFD time, with embedded
convective activity serving to increase the rainfall rates. Given
the amount of moisture pumping into the area today, it seems
reasonable that the hires models keep rain across the area through
most of the day. Again, details in location of heaviest rain will be
a little sketchy, but given current QPE plus today`s QPF, have opted
to expand the Flash Flood Watch farther north into the NC foothills
and Piedmont. The delineation between flood/flash flood is definitely
muddy, but with the embedded higher rainfall rates, and the fact
that we already had the Flash Flood Watch out for southern zones, it
just made sense to keep that product rather than introducing the
confusion of a Flood Watch as well. So for the 30-hour period from
06z today (now) through 12z Wed, QPF ranges from around 1" in the
northern NC PIedmont to close to 2" in the SW NC mountains. 1"
doesn`t really seem worthy of a Flash Flood Watch on the surface,
but we expect most of that this morning, and remember that this is
basin-average. Some areas have already received 1-2" in the past
couple of hours (already higher than the QPF, so in some areas the
QPF is already invalid, but this is really hard to capture in the
grids), so localized flash flooding will be an increasing concern
through the morning hours.

Now, for better or worse, guidance is in pretty good agreement that
this wave will mostly move out tonight (generally after 00z), with a
bit of a lull in the activity for the end of the near-term period.
Precip associated with the approaching upper trough will move in as
we begin the short term. Difficulty here lies in that, for now, the
heaviest QPF axis for Wednesday is across the mountains, whereas for
today we are looking at more of a Piedmont concern (there will still
be heavy rain across the mountains, but those areas didn`t get the
antecedent precip the past couple of days quite as much). However,
the Flash Flood Watch was in effect until 06z Thursday, and included
none of the mountains. After much deliberation and consultation with
neighbors, have opted to end the current watch at 06z tonight (Wed)
to cover today`s rainfall (and frankly it might not even be needed
that late, but it lines up best with neighboring offices` products).
Later shifts can then reevaluate QPF and issue a new Flash Flood
Watch for impacted areas on Wednesday as necessary.

As for continued convective potential, the widespread rain/clouds
will keep temperatures down (highs today like 10 degrees below
seasonal norms) and limit instability, but some areas across
southern zones could see up to between 500-1000J/kg during peak
heating. SPC Day1 outlook as put Marginal Risk in extreme southern
zones and that seems reasonable, especially with some deep-layer
shear coming across with the upper jet. Profiles remain too tropical
for much in the way of a hail threat, so if anything it would most
likely be some gusty winds.


As of 300 AM Tue: A mature low pressure system will remain
generally over the middle Mississippi and lower Ohio valleys Wed
and early Thu. Its warm conveyor belt will bring abundant moisture
flux and isentropic upglide atop our area Wed, reactivating the
stalled boundary as a warm front once again, which continues
to push north Wed night. A portion of the area thus will see
warm-sector conditions, featuring subtropical moisture and an
uptick in instability. Strong upper level winds will generate
appreciable deep-layer shear, and t-storms may organize into linear
segments. The cold front will approach the area through the morning,
pushing in by late afternoon. With a shortwave moving northward as
it rounds the upper low, both NAM and GFS generate a mesoscale sfc
wave along the front. This could significantly increase the severe
wind threat by strengthening or backing the low level winds. Note
SPC has now expanded the marginal risk to most of the CWFA for
Day 2, and the slight risk to a portion of the Upstate southeast
of I-85.

Furthermore, both NAM/GFS develop an area of heavy rain in response
to the enhanced convergence with their frontal waves. Unfortunately
they do not agree on when this occurs, though they both suggest the
mountain/foothill zones are the most likely locations to experience
the enhanced precip. Confidence is high on a wave developing,
but confidence is low on the placement. We have opted to give the
next shift a chance to evaluate the need for a flash flood watch
for Wednesday, as well as being able to better judge the areal
extent thereof.

The thunderstorm threat will diminish Wed night following the
front, but moisture remains plentiful in its wake. The upper
low itself will drift overhead at that time, which warrants
a continuation of PoP overnight; westerly upslope flow makes
this particularly necessary near the Tenn border. Excellent
lapse rates and diurnal warming on Thursday bring back enough
instability to expect increasing shower coverage. With over 500
J/kg of CAPE progged over portions of WNC and the eastern Upstate,
thunderstorms producing small or perhaps even marginally severe
hail will be possible. Deep mixing and cold advective flow will
produce somewhat gusty winds. Precip chances finally taper off
Thu night, though continued upsloping may allow some lingering
showers in the mountains.


As of 150 AM Tuesday:  Starting at 12Z Friday weak high pressure
will be in control of our weather to start the weekend as the upper
low crosses New England and the next systems gather over the Rockies
and Plains States. The next closed upper low affecting our weather
will rotate over the North Dakota and Canadian border Saturday and
drop down to Wisconsin Sunday night. This feature will be the
driving force for shortwaves moving from the Plains States to the
Appalachians to start next week. The intensity of the convection
affecting our weather will have much to do with the timing of the
associated cold front. The GFS has the front crossing our region
late Monday which may be near peak daily heating for much of the
area. Therefore, instability would be greater with this model
solution. However, the ECMWF currently has the front crossing Sunday
night which would result in lower chances of storms due to less
instability. There would be less cooling with this upper system
since models are consistent in taking the upper system NE to eastern
Canada once it reaches the Great Lakes. At this point, Day 7 of the
current forecast has much uncertainty due to lack of model agreement
on how quickly the cold front crosses our region. There is some
agreement that by 12Z Tuesday of next week that drying will take
place as high pressure moves in behind the front.

Temperatures will be near or just below normal Friday as the upper
low moves away then temperatures a little above normal through the
rest of the medium range.


At KCLT and elsewhere: Raised cigs for the 09z KCLT AMD as most
surrounding obs are IFR (vs. LIFR), though cannot rule out
occasional LIFR still. Had to adjust afternoon PROB30 to VCTS
because of TAF rules regarding time of allowed PROB30. Otherwise,
widespread showers across the area at TAF time, with pockets of
IFR/MVFR/VFR, but the trend will be to IFR or lower cigs shortly if
not already there. Could see some pockets of -TSRA but not enough to
include as prevailing at any of the TAFs. Marginal improvement after
sunrise to MVFR everywhere, with PROB30 -TSRA for a period of time
this afternoon. -SHRA moves out overnight but another round of IFR
cigs/vsby expected. Winds all over the place; generally on the S
side this morning/afternoon but flipping around to the N side this
evening. Confidence on winds is particularly low as this will depend
on trek of surface low across the area.

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

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Low   47%     Med   73%     High  82%     High  86%
KGSP       High  82%     High  84%     High  92%     Med   76%
KAVL       High  91%     High  95%     High  90%     High  84%
KHKY       High  93%     High  98%     High  83%     High  86%
KGMU       Med   61%     High  84%     High  92%     Med   74%
KAND       Med   76%     High  91%     High  85%     Med   70%

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 through late tonight for GAZ018-026-028-029.
NC...Flash Flood Watch through late tonight for NCZ035>037-056-057-
SC...Flash Flood Watch through late tonight for SCZ001>014-019.


SHORT TERM...Wimberley
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