Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241804

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
204 PM 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 150 pm: Upper low can be seen in the latest water vapor
imagery wobbling E/SE across the Ohio/Tenn Valleys early this
afternoon. A weak surface low persists across the upper Savannah
River Valley, with a boundary extending east across northern SC.
Weak instability has developed in the vicinity of and south of this
boundary, where temps warming into the 70s have combined with
dewpoints in the 60s to yield sbCAPE of 500-1000 J/kg per the latest
SPC mesoanalysis. Scattered deep convection has been developing in
the vicinity of the boundary throughout the day and a gradual
increase in intensity has been noted over the past hour or so. The
trend suggests at least a few thunderstorms should develop during
mid/late afternoon.

Otherwise, precip remains most concentrated near the eastern Blue
Ridge in response to a persistent SE upslope flow, although the
conveyor belt is showing some signs of weakening, probably due to
weakening of the upslope component. Overall coverage of shower
activity is expected to gradually diminish into the evening, as
upper low continues to wobble toward the Southern Appalachians,
allowing the low level flow to continue to weaken while shifting to
more of a downslope regime. Moisture will also shift more to the
north and northeast as well. Some ongoing minor flooding supports
continuation of the Flood Watch across portions of the NC Blue

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: Scattered to numerous showers continue to
fire this afternoon along and north of a surface boundary extending
across northern SC, with activity primarily concentrated across the
NC foothills in response to a SE upslope flow. An area of weak
instability has developed in the vicinity and especially south of
the boundary, and the intensity of showers has been gradually
increasing over the pass hour or so in these areas. Would expect at
least a few of these to develop into TS, with the best chance for TS
existing at KAND and KCLT, where tempos for -TSRA are carried for a
while this afternoon into the evening. Otherwise, IFR to lMVFR
clouds will persist north of the boundary through much of the
afternoon, although we do expect most sites in these areas to lift
above the IFR level by mid-afternoon. As the flow begins to turn
around to more of a downslope regime, cigs may improve to VFR (or
even scatter by late evening), although ample low level moisture
could allow stratus and/or fog to fill back in overnight, but
overall confidence in flt conditions is fairly low tomorrow morning.
generally E/NE winds will gradually weaken through the evening while
turning toward the NW at most terminals.

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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   79%     High  86%     Med   70%     Med   68%
KGSP       High  83%     High  89%     High  84%     High 100%
KAVL       High  94%     High  85%     High  86%     Med   74%
KHKY       Med   68%     High  80%     Med   61%     Med   70%
KGMU       Med   76%     High  93%     High  88%     High  86%
KAND       High  80%     High  99%     High 100%     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:


NC...Flood Watch until 4 PM EDT this afternoon for NCZ033-049-050-


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