Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181857

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
257 PM EDT Wed Apr 18 2018

A cold front will move through the area tonight and bring rain
showers across the mountains. On Thursday, dry high pressure will
build in from the northwest and linger, before another moist
frontal system approaches the region late Sunday.


As of 200 PM: Breezy southwesterly winds will continue this evening,
with our area being in the warm sector of low pressure now moving
into the lower Ohio Valley. With locally critical relative humidity
plus gusting, a Fire Danger Statement remains in effect through

Through Thursday morning, the aforementioned sfc low will move
east toward Chesapeake Bay. Accordingly, its cold front will trail
behind, pushing through our area in the early morning hours. Strong
gusts will develop at high elevations of the northern mountains
perhaps as soon as late evening, so we will hoist a Wind Advisory
at 02z. Any moisture associated with the front will be shallow,
and most guidance produces PoPs only over the NC mountains; east
thereof, models seem to dissipate any precip due to downsloping. A
couple WRF-based models do redevelop some very light QPF over
the southern Upstate, but for now we will go in favor of the dry
majority. As cold advection will get underway before dawn over the
NW half of the area, min temps will be near climo there, flirting
with freezing at some high elevations. Contrast that with mins
as much as 10 degrees above in the lower Piedmont. Moisture is
expected to diminish before temperatures cool enough to support
types other than rain. It is not out of the question that a few
very high elevation spots would see snow or perhaps freezing
drizzle before the event comes to an end.

Skies will be mostly sunny Thursday. Wind gusts will continue under
persistent cold advection, though subsidence is progged to limit
mixing and moderate gusts over most of the area. Our northern
mountains however may continue to see some advisory level gusts
so we will maintain that headline until 22z. Dewpoints will fall
once again in the wake of the front. While temperatures will be
notably cooler, peaking 7 to 15 degrees below normal, mixing should
be sufficient to bring dewpoints below 25 percent in localized
areas. With continuing gusts a Fire Danger Statement is warranted
again Thursday for NC.


As of 130 PM EDT Wednesday: Fair and cool will be the rule for
the end of the week as high pressure slowly builds in from the
west thru Saturday. The axis of the upper ridge will remain to
our west, which should spell dry weather across the region. The
main concern will be the possibility of frost/freeze early Friday
morning. Min temps over the mtns will be down into the lower/middle
30s, so a Freeze Watch might be required for those zones. Elsewhere,
cannot rule out some frost, but min temps are expected to stay in
the upper 30s/lower 40s. The center of the surface high over Iowa
Thursday night will keep enough pressure gradient over the region
to maintain a light N/NW wind, so only sheltered locations that
manage the mid 30s, such as the nrn foothills/NW Piedmont of NC,
stand much of a chance of frost. The rest of the period will feature
sunny/clear sky with temps running about five degrees below normal.


As of 240 PM EDT Wednesday: The 12Z model guidance has returned to a
wet and cool forecast for the first part of next week. Have trended
that way as well. The main difference in the models is at the
beginning where the GFS is faster with precip onset than either the
Canadian or ECMWF. Have slowed chance precip onset to late in the
day mainly over the western CWFA. Sunday will be the warmest day of
the period as well with lows near to slightly below normal and
highs around 5 degrees below normal. An upper low crosses the mid MS
Valley late Sunday into Monday. The low then slowly crosses the TN
valley into the southern Appalachians Tuesday before being kicked
out or absorbed by another upper low diving into the area Wednesday
from the Great Lakes. At the surface, cool high pressure builds in
from the north in a cold air damming pattern Sunday. A low pressure
center associated with the upper low moves east along the gulf coast
bringing a moist southerly low level flow across the area and over
the dome of high pressure. This leads to increasing precip chances
Sunday night and Monday. There is some discrepancy about the speed
of the surface low. However, even the faster GFS only slowly moves
the low up the southeastern coast Monday night and Tuesday which
keeps a moist easterly isentropic upglide in place both periods.
Therefore, have at least good chance PoP across the area through the
period, but limited the likely PoP to the western CWFA and south and
southeast upslope flow areas where there is good agreement on precip
continuing. The guidance has also kept the wet trend in place. The
GEFS mean is between 2 and 3 inches across the CWFA for the event
with the ECMWF in the 3 to 4 inch range. While most of this precip
is spread out over time and not particularly concentrated in a short
time windy, will still have to keep an eye on these trends for flood
potential. With the strong damming in place, precip should be in the
form of rain. There will be some cold temps across the higher
elevations, but precip should remain liquid rain even there. Lows
will be near to slightly above normal while highs will be around 10
degrees below normal. Highs could be even colder depending on the
amount and duration of the precip falling into the wedge.

The secondary upper low on Wednesday brings a clipper type system or
cold front across the area. This will bring another round of showers
and possibly thunderstorms as the wedge erodes and weak instability
develops. Lows rise slightly and highs rebound but remain around 5
degrees below normal.


At KCLT and elsewhere: VFR and breezy this evening with an
appreciable pressure gradient over the area in advance of an
approaching shortwave trough and sfc low. These features will
skirt by to our north late tonight into Thu morning, pulling a
cold front across the area. The front may induce some SHRA over
the mountains, but strong downsloping will keep precip chances
too low to mention over all the TAF sites, even KAVL. A band of
low VFR to MVFR cloud bases is expected to move over the region,
though no restrictions are expected at the terminals. On the other
side of the system Thursday, winds will shift to NW and remain
gusty until the end of the period.

Outlook: Broad high pressure will overspread the region on Friday
leading to lighter winds, plus dry and pleasant conditions through
Saturday. Restrictions may redevelop Sunday as a warm front lifts
over the area, bringing plentiful moisture to start the new week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  97%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  84%     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:


Warm temperatures, well above-normal, with a dry airmass in
place will lead to RH values hovering generally in the 25 to 35
percent range this afternoon, with SW winds of 15-20 mph across
the area. This will lead to high fire danger conditions across
NC, and possibly across SC and GA as well. A nearly dry front
will push through tonight, with cooler but much drier conditions
expected through the end of the work week. RH values will slowly
drop Thursday to around 30 percent, with locally drier RH as
low as 20 percent possible. Furthermore, NW winds Thursday will
combine with the drying airmass and will result in elevated fire
danger across the area again. Winds will decrease on Friday below
critical thresholds but will have to be monitored, since RH values
will remain critically low.


NC...Freeze Watch from late Thursday night through Friday morning
     for NCZ048-051>053-058-059-062>065.
     Fire Danger Statement until 7 PM EDT this evening for NCZ033-
     Fire Danger Statement from Thursday afternoon through Thursday
     evening for NCZ033-035>037-048>053-056>059-062>065-
     Wind Advisory from 10 PM this evening to 6 PM EDT Thursday for


NEAR TERM...Wimberley
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