Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240916

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
516 AM EDT Tue Apr 24 2018

A slowly moving low pressure system will be northeast of our region
by tonight, reaching eastern Virginia early Wednesday. The
atmosphere will generally dry out Wednesday before another moist
area of low pressure crosses the region Thursday through early
Saturday. High pressure builds across our area during the weekend
and to start next week.


As of 505 AM: The Flood Watch has been cancelled for southern-most
sections of the CWA, but continued along and immediately adjacent to
the mountains until 8 AM.

Otherwise, water vapor satellite imagery shows the closed 500 mb low
pressure system lifting very slowly northeast across the
Tennessee/Kentucky border this morning. Meanwhile, at the surface
the existing weak wedge frontal boundary remains draped along/near
the southeast fringe of our forecast area. Upglide forcing continues
over the shallowing cold air damming layer, but it will steadily
diminish going forward. The existing Flood Watch has thus been pared
back toward the escarpment as mentioned above.

The latest numerical models continue to feature a surface low
traversing our lower piedmont as the cold wedge decays today. There
is good agreement on a little plume of higher sbCAPE values, on the
order of 500 to 1000 J/kg, pushing northward into Greenwood County
this morning and then expanding along the I-77 corridor through the
day. As this instability and some mid-level drying wrap in, deeper
layer shear should shift north and east of the forecast area to
provide only minimal overlap of a couple of hours around daybreak
east of I-77. Thus, the window for any severe potential seems fairly
narrow. The better shear may go away this afternoon, but I-77
corridor instability will likely bubble up further to require a
piedmont thunder mention throughout the day.

The rain event will likely end with a period of moist northwest flow
along the TN border and the western mountains tonight behind the
departing surface low. PoPs will thus steadily diminish, but linger
the longest across the western mountains. The downsloping flow will
also decay any residual cold air damming through the period, with
temperatures exhibiting a larger diurnal range than observed on
Monday as maxes recover some 10 degrees in piedmont locations.


As of 320 AM Tuesday: Latest guidance remains in fairly good
agreement with the placement of the once closed 500 mb low/now open
wave over the Carolinas Wednesday morning as it`s parent sfc low
pressure system temporarily sits across eastern NC into eastern VA.
Throughout the rest of the day, the sfc low is progged to ride up
the eastern seaboard as the upper wave propagates eastward off the
Carolina coast. With residual moisture wrapping around the backside
of the exiting system and the anticipation of a reinforcing cold
front progged to push through the area late Wednesday night into
early Thursday morning, do anticipate most of any lingering
precip/light rain shower activity to remain confined to the
mountains, though would not entirely rule out isolated light showers
elsewhere. Otherwise, a few breaks in the clouds may occur across
the Upstate and NW Piedmont on Wednesday, but will be short-lived.

On Thursday, attention turns towards another system back to the west
as guidance depicts another approaching 500 mb closed low progged to
dive through the central plains and into the lower MS Valley
Thursday aftn, while at the same time, sfc low pressure develops
MS/AL/GA. This is where slight discrepancy comes into play between
the GFS and ECMWF - the GFS brings the sfc low up through the
central Carolinas, while the ECMWF keeps the track closer to the
Carolina coast (keeping in mind this is based off the older run of
the ECMWF attm). Tapping into Gulf moisture, especially at the lower
levels, as the upper trough moves across the TN Valley into the
Carolinas (all while blending the GFS/ECMWF sfc low tracks), overall
anticipate increasing rain chances Thursday afternoon and night.
While the final track remains unknown attm of the sfc low, have kept
on the more cautious side of things in regards to QPF amounts attm,
but overall am anticipating a low end QPF event. Limited instability
may move into the FA, but given uncertainty, have kept out any
mention of thunder attm.

With the CAD wedge out of the picture, expect warmer temperatures on
Wednesday, with lower 70s across the NW Piedmont and
Upstate/northeast GA, and cooler across the mountains. Overnight
lows will be at or just slightly above normal. Slightly cooler
temperatures expected on Thursday with the passing of the
reinforcing cold front.


As of 150 AM Tuesday: The extended forecast starts at 12z Friday
with medium range models showing a rather deep trof over the eastern
half of the nation with a high amplitude ridge over the Rockies and
western Canada. Both the GFS and ECMWF have a small and rather
compact low over the Carolinas early Friday. This low will be
affecting the I-77 corridor and east early Friday and the low moves
well east of the area by Friday afternoon. Instability values have
decreased in association with the low pressure and upper trof. Only
CAPE on GFS is over the western Upstate at 00Z Saturday with only
200 on the latest GFS run. The trof axis will be approaching from
the west Friday night. The EC has been rather dry with very little
if any showers associated with this axis. The GFS initially has a
band of showers over Tennessee then dissipates all rain as it
reaches our region. The trof axis crosses the Carolinas on Saturday.
 Much drier air enters our area from the west Saturday night into
Sunday with high pressure centered near Arkansas early Saturday
reaching Ohio by 18Z Sunday. As the surface high moves slowly off
the East Coast southerly return flow will begin to gradually
increase cloudiness over our area at the very end of the current
forecast. Temperatures near or just below normal Friday and Saturday
then near or just above normal Sunday and Monday.


At KCLT and elsewhere: Weak upglide over the wedge layer will
continue to generate occasional light to moderate showers around the
terminal forecast area, with IFR cigs either locked in place, or
developing. The ENE flow gradient should relax through the later
morning hours. Any thunder chances look generally confined to the
KCLT airport, and mainly during the peak heating hours this
afternoon as warmer air wraps into the region as a surface low
translates through the piedmont. Anticipate flow turning more
northerly behind the passing low for tonight, with NW winds late.
The abundant low-level moisture will allow for continued
restrictions through the day and into tonight despite the gradual
diminution of precipitation coverage.

Outlook: After very brief drying Wednesday, moisture and unsettled
weather may quickly return Wednesday night into Thursday as a trough
of low pressure redevelops across the eastern part of the country. A
drier cold front will likely cross the region by the early weekend.

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       High  85%     Med   76%     Med   79%     Med   70%
KGSP       Med   77%     Med   76%     High  80%     Low   47%
KAVL       Med   78%     Med   75%     Med   65%     Low   53%
KHKY       High  80%     High  80%     Med   63%     Low   44%
KGMU       Med   79%     Med   72%     Med   74%     Low   51%
KAND       Med   74%     High  83%     High  82%     High 100%

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...Flood Watch until 8 AM EDT this morning for GAZ010-017.
NC...Flood Watch until 8 AM EDT this morning for NCZ033-049-050-053-
SC...Flood Watch until 8 AM EDT this morning for SCZ001>003.


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