Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240604

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
204 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 145 AM: The near-term models indicate that the stout 850 mb
winds will steadily diminish from southwest to northeast through the
early morning hours. The current Wind Advisory will be allowed to
expire on schedule at 2 AM.

Otherwise, water vapor imagery shows the closed 500 mb system
lifting very slowly northeast into Kentucky early this morning. At
the surface, the existing weak wedge frontal boundary remains draped
just southeast of our forecast area. Upglide forcing continues over
the cold air damming layer, with upslope enhancement still focused
along the eastern escarpment. Minor flood problems will likely start
to creep up from the duration rainfall through the early morning
hours, and the existing Flood Watch will be maintained.

The latest numerical models also feature a surface low traversing
our lower piedmont as the wedge very slowly 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
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.

850 mb winds will turn SW today and then the near-term event will
end with a period of moist northwest flow along the TN border and
the western mountains tonight. PoPs will thus steadily diminish, but
linger the longest along the western mountains. The downsloping flow
will also decay any residual cold air damming through the period.


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.


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: Continued upglide over the wedge layer is
generating plenty of showers around the terminal forecast area, with
IFR cigs either locked in place, or soon to develop. The ENE flow
gradient will keep gusts going for a few more hours, but with
steadily weakening flow 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...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  84%     Med   70%     High  81%     Med   72%
KGSP       High  84%     Med   78%     Med   76%     Med   60%
KAVL       Med   74%     High  83%     Med   64%     Low   59%
KHKY       High  85%     Med   74%     Med   71%     Low   59%
KGMU       Med   78%     Med   71%     Med   76%     Med   61%
KAND       Med   69%     High  81%     Med   79%     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 GAZ010-017-018-026-
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>007-010.


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