Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS62 KGSP 191855
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
255 PM EDT Mon Jun 19 2017

.SYNOPSIS...
A cold front slowly moves across our area this evening enhancing
shower and thunderstorm coverage before stalling across the Midlands
on Tuesday.  The remnant frontal boundary may lift back north late
Tuesday for another round of showers and storms before dissipating
as drier high pressure builds in for Wednesday.  Rainfall chances
increase yet again late week as tropical moisture spreads across
the Southeast U.S.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 2pm EDT:  Currently one thunderstorm near Greer, with more
activity expected this afternoon and tonight.  Major
northeast-southwest oriented synoptic front lies just northwest of
the TN/SC stateline at this time, with a very moist airmass ahead of
it.  Front and upper wave will gradual pass through the CWA, exiting
Tuesday morning.  Upper flow has improved some with the presence of
the large eastern CONUS upper wave, but with very veered
southwesterly surface flow, overall bulk shear is still weak at less
than 20kts.  CAPE is analyzed to 2000 j/kg ahead of front, and
soundings exhibit high PW values.  These ingredients are not
supportive of supercells, but do support some strong storms with
potentially severe winds in outflows, especially in areas with
higher levels of instability towards the northeast parts of the
CWA.  High PW levels ahead of front also suggest some potential for
flooding in areas receiving training storms.  With improved upper
flow, storm are moving eastward more rapidly than recent storms,
which will mitigate rain accumulation some, but focus provide by
front could lead to some training of storms across the same area.

Heaviest showers will be concentrated along and ahead of the front
with some CAMs showing a possible QLCS just ahead of front tonight
with much reduced precip. over the mountains after dark.

With front to provide focus, scattered showers and thunderstorms
will continue through tonight with some letup early Tuesday morning
as front exits the area and some dry advection begins to stabilize
the airmass.  However, more showers and thunderstorms are expected
Tuesday afternoon in piedmont areas, though at a less severe level, as
forcing from upper wave will continue, and some moisture remains
behind the front.

With cold FROPA and extensive cloud cover, highs Tuesday will be
significantly cooler than recent history, at around 80 degrees in
the piedmont.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 300 PM EDT Monday: The front will be stalled near the area for
the middle of the week as tropical moisture flows northward from the
Gulf of Mexico. Models disagree on the exact location of the
boundary but it does look like most of the area will have rain
chances each period. Right now it looks as though the Piedmont and
blue ridge have the highest chance of rain with the lowest chance
along the Tennessee border. We will have to watch for localized
heavy rainfall with this pattern since the precipitable water values
will be in the 1.5 to 2 inch range.

Due to the increased moisture and cloud cover through the period,
high temperatures both Tuesday and Wednesday will only be in the
lower 80s in the Piedmont with some higher peak remaining in the
60s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
At the beginning of the period, operational model consensus
(ECMWF, GFS, Canadian) place a relatively weak tropical low on
either side of the Sabine River Valley along the TX/LA border.
The associated closed upper-low is to the south of fairly zonal flow
across the CONUS and caught between the weakness of two deep-layer
ridges to its west and east.  Therefore, the overall pattern
places our region within deep southerly to southwesterly flow on
the northwest side of the Atlantic subtropical ridge and to the
east of the tropical low .  This flow will be tropical in origin
with a fetch potentially originating from the western Caribbean
Sea which means the potential is there for very efficient/heavy
rainfall from late Thursday night through Saturday, before the
Atlantic ridge builds west across Florida and the east-west ridge
axis obstructs the tropical moisture fetch.  Guidance has PWs above
2-2.25 inches which would be near record-level values for the region
during this time.  Additionally, pieces of shortwave energy aloft
will eject from the upper-low over the Lower MS River Valley and
propagate across the TN Valley and into the southern Appalachians
Friday as the closed-low weakens and gradually becomes absorbed
into the westerlies late Friday into Saturday, moving across the
area as a shortwave trough late Saturday into early Sunday. While
the pieces are there for periods of heavy rainfall and at least
localized flash flooding, especially as this tropical flow is
forced up the southern Appalachians, guidance isn`t particularly
bullish on widespread heavy rain as the main tropical energy stays
too far west and weakens substantially as the remnants advect east.
We`ll keep a close eye on the evolution of any tropical activity,
and some heavy rainfall is likely, but there`s no need to get overly
concerned at this time.  Therefore, the forecast maintains likely
pops over the mountains through Saturday afternoon, and high-end
chance elsewhere.  High temperatures should be near normal if not
slightly below with dewpoints east of the mountains uncomfortably
in the low-70s keeping low temperatures a few degrees above normal.

Unfortunately, the shortwave trough and remnant tropical moisture is
quickly followed by an approaching cold front and deeper upper-level
trough.  Details on the timing of the frontal passage will have to
be worked out over the next several days but the current forecast is
a blend of the EC and GFS with FroPa overnight Sunday/early Monday.
Quieter, and more importantly, drier weather may follow the front
beyond the period by mid-week next week.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
At KCLT and elsewhere:  Some extended shower and thunderstorm
activity is anticipated through the TAF period with some TSRA now
present, with activity increasing through the afternoon.  Showers
are associated with the slow passage of a synoptic cold front
currently across the area. Progress of cold front across the area
will also veer winds from southwesterly ahead of the front from now
until FROPA Tuesday morning, and northwesterly and lighter behind
the front.  Timing for convective activity relies heavily on CAM
guidance which has activity from VCTS to TSRA from later this
afternoon through early Tuesday morning, followed by a relative lull
until more showers and thunderstorms Tuesday afternoon.  With the
moist surface layer from rain, some morning fog and low ceilings are
possible, though the chances are reduced from recent days due some
reduction in moisture as the front passes.  With extensive precipitation,
BKN to OVC skies will prevail with ceilings typically down to FL040,
and less in areas of active rain.


Outlook: Cold front will stall over the area on Tuesday, maintaining
scattered convection and associated periodic restrictions. Morning
fog/stratus is likely to return Tue-Wed mornings. Drier conditions
may finally return during the latter part of the week if the cold
front can push south of the area. A possible tropical system could
throw all this into disarray late in the week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High  88%     High  82%     High  97%
KGSP       High 100%     High 100%     High  99%     High  91%
KAVL       High 100%     High  84%     Low   56%     High  96%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High  99%     High  91%     High  91%
KAND       High 100%     High  98%     Med   68%     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

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JMP
NEAR TERM...WJM
SHORT TERM...SW
LONG TERM...JMP
AVIATION...WJM



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.