Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 230800
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
400 AM EDT Mon Apr 23 2018

.SYNOPSIS...
A slowly moving low pressure will bring a strong, moist flow in from
the Gulf and Atlantic today. The abundant moisture will then
gradually decrease 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 TONIGHT/...
As of 325 AM: The most recent RAP runs are exhibiting a slight
uptick in the ESE 850 mb flow today and this, along with the latest
observational trends, indicate that a Wind Advisory above 3500 feet
appears in order across the NC mountains.

Otherwise, water vapor imagery depicts the closed upper low spinning
slowly east over the lower/middle MS River Valley early this
morning. This circulation center will make very little progress
east, reaching only the central TN/KY border area by 12Z Tuesday.
Meanwhile, regional radars are showing the first heavier rain bands
crossing the southwest part of the forecast area. The HRRR and other
convection-allowing models are in agreement with this timing and
they indicate that very best bands of moisture and associated heavy
rainfall will cross the region this morning through mid afternoon.
Based on these developing conditions, and little change to event
total QPF values, we see no reason to adjust the Flood Watch at
present.

The east-southeasterly 850 mb jet will maximize this morning at
nearly 50 kt (with the NAM a seeming outlier at up to 60 kt). A high
elevation Wind Advisory is being posted across the NC mountains
today through this evening before the gradient begins to slowly
relax.

As the rainfall becomes steadier across our area, in-situ or hybrid
cold air damming will get better established through the day. This
should help shield us against much of the upstream convection, and
the operational NAM thus appears too aggressive in uncovering warm
sector instability later this afternoon. A secondary surface low
moving through the Midlands/piedmont tonight could help wrap the
wedge front back to the northwest, but only slight chances of
thunderstorms will generally be featured along the southeast fringe
late today through tonight, and sfc to 3 km shear will lessen during
this period. Expect very little diurnal range on temperatures with
the wedge developing and holding relatively firm.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 330 AM Monday: Rainfall and potentially isolated thunderstorms
(across the southern portions of the FA) will be ongoing at the
start of the forecast period, but overall precipitation will
continue on it`s downward trend throughout the day, tapering off
south to north on Wednesday, as available moisture and the aid of
isentropic lift decreases. Per latest guidance, not much has changed
from the previous forecast package as the main forecast challenges
remain the same for Tuesday, that being the dissipation of the wedge
(temporally) across the Carolinas and the track of the sfc low
before it makes it`s exit out of the Southeast. Looking at the
latest guidance, the tracks of the sfc low differ similar to
previous runs, where the GFS eventually tracks it through central SC
to the Carolina coast (the ECMWF has a similar suggestion) whereas
the NAM takes it just NW of the area with the suggestion of a
secondary low developing across eastern NC Tuesday night.
Additionally, instability seems to remain somewhat present across
the south-southeast portions of the FA on Tuesday as well, but as
noted previously, the values range. It seems both the NAM and ECMWF
are suggesting values of 800-1400 J/kg while the GFS ranges from 40
to 400 J/kg. Upcoming guidance runs could potentially come into
better agreement, but for now have overall kept with the potential
for isolated thunderstorm activity Tuesday morning through the
afternoon hours, with localized flooding the primary threat.

Tuesday night, guidance continues to highlight the sfc low exiting
off to the north as the 500 mb upper closed low opens and the now
trough will swing across NC Wednesday morning. However, that won`t
be the end of the precip as guidance continues to prog a reinforcing
cold front to push through Wednesday into Thursday, allowing for low
end mentionable PoPs to persist through the end of the forecast
period.

With the CAD wedge influencing high temperatures on Tuesday, upper
60s/near 70 degrees south of the I-85 corridor and 50s to lower 60s
elsewhere, temperatures will rebound to just shy of normal on
Wednesday as the wedge dissipates. Overnight low temperatures will
remain slightly above normal. With these temperatures, any precip
through the forecast period will fall as all rain.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 350 AM Monday: The medium range forecast period begins on
Thursday as it seems the unsettled weather is progged to persist
through into the early part of the weekend. Both temporally and
spatially, disagreement persists across most of the guidance, but
the evolving overall scenario remains in that a series of waves are
set to move within (becoming split flow) of the deepening, upper
longwave trough across the eastern CONUS, with a cold front
propagating across the FA Friday into Saturday. With the potential
for isolated to scattered showers to persist through Saturday, will
note that QPF amounts are nowhere near the amounts we are
anticipating for the first part of this week, thus am not expecting
any flooding issues to arise attm. Quiet weather returns on Sunday
as sfc high pressure infiltrates into the Southeast and upper ridge
builds across the central US. High temperatures will remain below
normal Thursday through Saturday, climbing to normal on Sunday under
clearing skies and the welcomed return of sunshine - that is,
according to latest models. Low temperatures will remain below
normal.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Deeper moisture associated with the cyclone
to the west of the Appalachians will invade from the southwest
through the morning hours. Expect a steady uptick in shower coverage
and intensity, along with lowering MVFR cigs. The heart of the
heavier rain bands will cross the region this morning through the
middle of the afternoon and produce visibility restrictions and
persistent IFR to LIFR ceilings. Also, an establishing cold air
damming layer will permit ENE to NE gusts into the 20s at times,
except SE gusts at KAVL. LLWS will flirt with the KAVL region mainly
during the passing of the best SE low level jet circa 10Z to 14Z
this morning. Will generally keep all late day/nighttime
thunderstorm mention well southeast of the terminals with this
issuance.

Outlook: Some improvement is expected on Tuesday as precipitation
continues to shift northeast of the terminal forecast area. Some
moisture will linger, however, and unsettled weather will continue
on Wednesday and Thursday as a trough of low pressure persists
across the eastern part of the country.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  95%     Med   72%     Med   75%     High  88%
KGSP       High  87%     High  82%     High  85%     High  88%
KAVL       High  86%     Med   72%     High  81%     High  98%
KHKY       High  82%     Med   77%     High  81%     High  88%
KGMU       High  86%     High  83%     High  88%     High  83%
KAND       High  87%     High  84%     High  84%     High  94%

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 midnight EDT tonight for NCZ033-048>052-
     058-059-062>064.
SC...Flood Watch through Tuesday morning for SCZ001>007-010.

&&

$$
SYNOPSIS...DEO/HG
NEAR TERM...HG
SHORT TERM...SGL
LONG TERM...SGL
AVIATION...HG
HYDROLOGY...HG



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