Area Forecast Discussion
Issued by NWS Greer, SC

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046
FXUS62 KGSP 232353
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
753 PM EDT Mon Apr 23 2018

.SYNOPSIS...
A slowly moving low pressure system will continue to bring moist
flow in from the south into Tuesday. The atmosphere will
generally dry out Wednesday before another moist area of low
pressure crosses the region Thursday through Friday.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 750 PM EDT Monday: Heaviest rainfall has shifted east of the
area as the warm conveyor belt has moved east. Still, weaker moist
upglide remains over the area with patchy rain moving through.
Analysis shows wedge still in place across the area with instability
to our west and south. this is keeping the thunderstorms over GA
from maintaining themselves as the move toward the area. Looks like
the chance of convection will remain limited as best overnight.
There will still be pockets of heavier rain moving across the area,
so will leave flood watch in place, but any flooding will be from
slow rises. Winds aren`t especially strong across the higher
elevations, but have had reports of some trees down due to the
combination of wind and wet soils. Will leave the wind advisory in
place as well. Temps are generally on track with only some minor
adjustments.

Otherwise, what appears to be in-situ type cold air damming has set
up across the area, which is reflected in the solid surface NEly
flow in the wedge of high pressure and a deck of low cloud across
the entire area. A wedge of low-level dry air has worked into the
eastern part of the forecast area, but cloud cover has prevented
insolation all day and scouring of the wedge at this point in the
afternoon seems unlikely, especially considering the substantial
isentropic upglide and strong northeasterly flow at the surface.
This is reflected in model CAPE fields, that have a safe boundary
between our area and any instability that has been able to develop
to our south. Strong SEly 850mb winds are still in place over the
FA, and with gusts to 45kt+ still being reported at the highest
peaks, high elevation wind advisory for the NC mountains is still
well-placed. The advisory was extended to go until 600Z (2am)
tomorrow morning due to suspicion that gusts may not drop off quite
as quickly as previously thought, as models have been hanging on to
stout 850mb flow past midnight. The best pressure gradient will
begin to relax after midnight, but will do so relatively slowly,
keeping gusts elevated across much of the forecast area overnight.

A triple-point low is in the early stages of development over
south/central Georgia, and will continue to undergo cyclogenesis
through the evening and tonight as it moves towards the northeast.
This low should help to scatter out the wedge early tomorrow morning
as it moves through our forecast area, though with the upper low and
its attendant moisture making little progress eastward by tomorrow,
chances for showers and even potentially a thunderstorm in our far
southern and eastern zones will remain through the end of the near
term period.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 220 PM EDT Monday: the short term fcst picks up at 00z on
Wednesday with a closed upper low opening back up over the region as
a broader area of trofiness persists across much of the Eastern
CONUS. By late Wed/early Thurs, the above upper trof lifts NE of the
area as another upper trof digs down across the ArkLaTex region.
This trof axis is expected to be approaching the CWFA as the period
ends late Thurs. At the sfc, the large low that has been affecting
the region is expected to be centered right over the Carolinas or a
bit north of there. By early Thurs, the low will lift northward and
towards New England and deepen in the process. In the low`s wake,
broad high pressure will gradually move over the area from the west
as an area of deeper moisture lingers to our west. For the remainder
of the period on Thurs, the sfc high appears to weaken as another
low tries to spin up over the Southeast. The models are not very
clear wrt the pattern evolution for the later part of Thurs, with
the GFS keeping drier air over the fcst area compared to the other
operational models. The NAM spins up another sfc low along the
Northern Gulf Coast on Thurs, with the other models hinting that a
low will develop over that area. As for the sensible fcst, PoPs will
taper off on Wed with only a small amount of QPF expected over the
CWFA. PoPs increase again on Thurs as the deeper moisture mentioned
above moves over the area and another sfc low tries to develop to
our southwest. Temps will be right around climatology on Wed with
highs dropping about a category below normal for Thurs.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
of 230 PM Monday: The extended forecast starts at 00z Friday with
a progressive full-latitude trof pushing thru the Plains and
Midwest. This trof will be picking up a southern stream wave over
the central Gulf states, lifting that feature across the CWFA early
Friday. The GFS has generally been the strongest with this wave and
even inducing a compact, but moist, sfc low over the Carolinas. The
last few runs of the EC had stubbornly stayed weaker and drier for
Friday, but now the 12z run has come in a lot like the 12z GFS. The
latest SuperBlend has a shotgun 30-40% CHC of rain on Friday, and
this looks like a good compromise. However, if the GFS is right,
PoPs will likely need to be higher. Either way, QPF shouldn`t be
enough to cause any flooding problems. The main trof will then push
into the area Friday night, with the southern end trying to break
off somewhat and slow down over the Carolinas. Again, the GFS is the
most bullish on this, and shows continued showery activity thru the
day on Saturday. Meanwhile, the EC is fairly dry with the passage of
this wave. I have some lingering PoPs into Saturday. Temps both days
will feature a depressed diurnal curve with slightly above normal
min temps and slightly below normal max temps.

The model guidance is generally in good agreement on the longwave
upper trough finally shifting east, as an upper ridge moves in
across the MS Valley to the Great Lakes Sunday thru Monday. Dry
weather and a warming trend back to slightly above normal temps is
expected by the end of the extended fcst period.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Cigs and vsby generally varying between MVFR
and IFR this evening as patches of moderate to heavy rain move north
across the area. Expect cigs to fall to LIFR overnight with IFR cigs
as rain diminishes in intensity but good coverage continues. Gusty
ENE wind continues as well, SE at KAVL. Expect some improvement
across the west after daybreak and at KCLT for the afternoon.
However, MVFR cigs may linger through the day with IFR in showers or
TSRA at KCLT. Gusts should diminish during the morning but direction
remaining NE for all but KAVL where they go N and KAND where they go
W.

Outlook: Moisture lingers along with unsettled weather on Wednesday
and Thursday as a trough of low pressure persists across the eastern
part of the country.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  84%     High  88%     High  86%     Med   78%
KGSP       Med   74%     High  86%     High  87%     Med   74%
KAVL       High  81%     High  94%     High  94%     High  87%
KHKY       High  82%     High  89%     High  92%     High  87%
KGMU       Med   64%     High  92%     High  88%     Med   70%
KAND       Low   56%     High  87%     Med   77%     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:

www.weather.gov/gsp/aviation

&&

.HYDROLOGY...
Although precipitation rates do not appear sufficient in any 3 to 6
hour windows to produce flash flooding, duration flooding remains
possible, especially in the most upslope preferred locations and
over the adjacent foothills. Storm totals of 3 to 4 inches are still
expected, with localized 4 to 6 inch amounts in the most upslope
preferred areas.

Flooding of the most common flood-prone and low-lying areas may
steadily develop from the southwest today, with main stem flooding
of the upper French Broad River and associated tributaries quite
possible. Flooding of main stems and streams in the Upstate may also
develop, especially along the Broad and Saluda rivers, with crests
not occurring until Tuesday and perhaps early Wednesday. Additional
significant rises may develop along the Catawba River chain in
western NC during and immediately after the heavy rainfall. Water
level forecasts for these rivers are accessible at
http:/water.weather.gov/ahps2/index.php?wfo=gsp.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch through Tuesday morning for GAZ010-017-018-026-028-
     029.
NC...Flood Watch through Tuesday morning for NCZ033-049-050-053-059-
     062>065-501>510.
     Wind Advisory until 2 AM EDT Tuesday for NCZ033-048>053-058-
     059-062>064.
SC...Flood Watch through Tuesday morning for SCZ001>007-010.

&&

$$
SYNOPSIS...SBK
NEAR TERM...Carroll/RWH
SHORT TERM...JPT
LONG TERM...ARK
AVIATION...RWH
HYDROLOGY...



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