Area Forecast Discussion
Issued by NWS Greer, SC

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376
FXUS62 KGSP 231817
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
217 PM EDT Tue May 23 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 155 PM EDT: Water vapor satellite imagery shows a closed low
pressure system drifting southward over Iowa this afternoon with the
associated trough amplifying from the southern plains to the lower
MS River valley. Coupled upper jet divergence east of the trough
axis will remain over our foothill and piedmont zones through the
late afternoon hours before lifting off to the northeast later this
evening. Regional radars also depict the best surface wave and
associated stronger convection moving through central GA and this
should brush our piedmont through early evening before moving off to
the northeast. Limited drying is expected to develop from the
southwest behind the wave later this evening. This should herald a
lull in the better QPF across the region for the nighttime hours,
with mainly light rain continuing in the western mountains as 850 mb
flow veers more westerly tonight before backing southerly by
daybreak Wednesday.

A more potent 120 to 130 kt upper jet will move over the region on
Wednesday as the trough sharpens up to the west. In addition,
southerly flow at low levels will improve upglide and coverage.
Locations along and southeast of I-85 will see the best chance of
having an uncovered warm sector airmass Wednesday afternoon with
instability bubbling sbCAPE to near 2000 J/kg. In addition, Piedmont
850 mb wind speeds should reach 25 to 35 kt by late afternoon during
peak heating to briefly enhance bulk shear values. Model QPF is not
terribly impressive, but the convective rates could once again raise
the hydro threat. See the Hydro discussion section for details on
the Flash Flood/Flood Watch through process for the ongoing hydro
threat.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
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.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
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.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Very moist profiles continue around the
region but with ceilings quite variable in most areas. Still expect
ceilings to settle down into the MVFR to tempo IFR range through
late afternoon and become IFR to possibly LIFR most areas overnight.
Any lingering thunder threat appears to have shifted mainly east of
the terminal forecast area so will carry only showers going forward.
Winds are another tricky aspect to the forecast. They should be
light away from the heaviest showers, but with mainly ENE at
KCLT/KHKY and southeast at the Upstate sites and KAVL. Convective
chances return through the day on Wednesday with the approach of the
upper trough and continued surface front in the area.

Outlook: Scattered showers and thunderstorms will continue on
Thursday as the upper trough passes. Drying is expected to finally
occur Thursday night through Saturday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   77%     High  86%     High  81%     High  83%
KGSP       Med   76%     Med   77%     High  90%     Med   74%
KAVL       Med   79%     High  90%     Med   67%     High  92%
KHKY       Med   69%     High  87%     High  82%     Med   73%
KGMU       Med   73%     High  80%     High  87%     Med   78%
KAND       Med   76%     High  90%     High  88%     High  87%

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 anticipated lull in the precipitation for tonight is vastly
complicating Watch issuance going forward. We have an existing Flash
Flood Watch through 06Z tonight for the foothills/piedmont that can
probably be pared away from the southwest as the surface wave and
associated heavier precip moves by. Any watches for Wednesday onward
would likely be needed for the entire forecast area with good
coverage west over the mountains but better rates east over the
piedmont. New Watches are currently being considered in
collaboration with the forecast neighbors.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flash Flood Watch until 2 AM EDT Wednesday for GAZ018-026-028-
     029.
NC...Flash Flood Watch until 2 AM EDT Wednesday for NCZ035>037-056-
     057-064-065-068>072-082-501>510.
SC...Flash Flood Watch until 2 AM EDT Wednesday for SCZ001>014-019.

&&

$$
SYNOPSIS...DEO
NEAR TERM...HG
SHORT TERM...Wimberley
LONG TERM...DEO
AVIATION...HG
HYDROLOGY...HG



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