Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 161944

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
244 PM EST Sat Dec 16 2017

Dry and cool high pressure will dominate our weather through Sunday
morning. Moisture and a gradual warm-up will return to the area
starting Sunday. A series of disturbances will pass across the area
next week, with a prolonged period of unsettled weather.


As of 230 PM EST Saturday: Temperatures continue to slowly
increase this afternoon as latest obs show low to mid 40`s
across the mountains and slightly warmer in the upper 40`s to
near 50 degrees across the Upstate and Piedmont. Capping off
just at/below normal for the day, expect temperatures to climb a
few more degrees over the next few hours.

With weak ridging aloft along the Appalachians and broad sfc high
pressure across the Southeast, quiet conditions will continue
through tonight. A weakening, northeastward propagating shortwave
which will move out of TX overnight through the MS Valley and into
the Ohio Valley on Sunday will allow Gulf moisture to stream in
overhead with southwest flow in place. As a result, high cloud cover
will increase overnight, inhibiting ideal radiational cooling.
Anticipate overnight low temperatures to dip to around normal, with
low to mid 30`s across the Upstate and Piedmont and slightly lower
into the mid to upper 20`s across the mountains.

On Sunday, clouds will continue to increase as latest fcst soundings
illustrate the gradual saturation of the atmospheric column
from the top through the day, though a majority of the moisture
will remain just outside of our forecast area. As the weakened
shortwave passes well to the northwest late Sunday afternoon,
have kept with increasing pops from west to east in the late
afternoon hours, with "higher" pops across the western portions
of the forecast area. Though anticipate light rain showers
across these areas, confidence is low that any precipitation
will reach areas along and east of the I-77 corridor by early
Sunday evening. Overall, little to no QPF is expected. As for
temperatures, Sunday will be warmer than today with afternoon
high`s in the mid to upper 50`s across the Upstate and Piedmont
to low 50`s to upper 40`s northern foothills to mountains.


As of 235 PM EST Saturday: The deterministic models are similar in
flattening a low amplitude upper level ridge across our forecast
area (FA) Sunday night. Meanwhile, an upper level packet of energy
will be passing well to the north, across Pennsylvania and out into
the western Atlantic. This is some precipitation response, in a weak
isentropic lifting pattern Sunday evening. As a result, we will hang
onto some low POPS Sunday evening before ending.

Complicating matters Monday is an old frontal boundary just to our
south, which is co-located with a jet streak from Mississippi to
South Carolina. This is an area where the models are trying to
highlight an increase in precipitation potential (still chance),
mainly in our southern FA. We have followed the roller coaster ride
of bringing chance POPS back Monday and Monday evening, mainly in
northeast GA and upstate SC. Slight chance POPS to the north look
okay. Any forcing helping to generate precipitation will zip away
from the area late Monday night. Therefore POPS will ramp down to
about NIL Tuesday morning.

However the fast flow aloft, as various energy sources rotate around
several transient upper level low pressure systems in the southwest
CONUS, will yield yet another return in the precipitation potential
Tuesday and beyond. The Tuesday system may offer the better chances
of rain.

Overall, the flow aloft is fast and driving systems in and out of
our region every 12 to 24 hours.

The overall thermal profile continues to show a warming trend. We
have stuck close to guide, which is rather well clustered,
considering the clouds and potential precipitation for this cycle.


As of 235 PM EST Saturday: Guidance now in better agreement on the
mid-week system for the early part of the medium range. However,
inconsistencies return for the end of the period. A southern stream
upper low over TX opens up and moves east toward the area Tuesday
night and into the area Wednesday. The open wave moves east of the
area Wednesday night. The ECMWF remains the strongest and slowest
with this system, but it is now faster than previous runs. The
Canadian is weaker and faster with the GFS in between. This
increases confidence that a surface low will track east toward the
area Tuesday night, into the area Wednesday and move east Wednesday
night. With good moisture flux ahead of this system precip looks
likely during this period. Now feel comfortable going with PoP
increasing to likely with the low and tapering off Wednesday night.
Given the track of the low and the amount of moisture and forcing,
precip amounts could approach 2 inches closer to the low track. Not
an excessive amount given the recent dry spell but very welcome.
Instability could develop south of the low track, but will leave
thunder out for now. H85 temps and thickness values show precip
would be all liquid. Above normal temps Wednesday fall to near
normal for Thursday.

Flat upper ridging builds over the SE CONUS behind the departing
low. The GFS and Canadian are stronger and slower to move the ridge
east keeping a northern stream short wave well to our north Friday
and Saturday while allowing a deepening trough to develop just east
of the Rockies. The ECMWF is weaker with the ridge and moves it east
as an upper low over the Southwestern CONUS opens up and crosses the
area Saturday. The guidance agrees on dry high pressure Thursday
moving east Friday allowing moisture to return ahead of a frontal
system. The GFS and Canadian are weaker with the front brining it
slowly into the area and stalling it with moisture moving back in in
a warm frontal scenario. The ECMWF has a stronger more longitudinal
front with waves of low pressure forming along the front and
sweeping into the area Saturday. Since both scenarios show
increasing moisture and precip, allow PoP to increase into the
chance range for Friday and Saturday but no higher given the
uncertainty. Temps warm back above normal Friday then drop a few
degrees for Saturday. Again H85 temps and thicknesses are all warm
enough for only liquid precip.


At KCLT and Elsewhere: Expect VFR through the valid TAF period. With
broad high pressure across the Southeast today, a few cirrus will
continue to stream across the forecast area with southwest winds
ranging from nearly calm to 7 kts. Winds will become light and
variable overnight with a gradual increase of high clouds ahead of
the next approaching system. After daybreak, southwest winds will
increase to 5 to 7 kts as mid level clouds increase from west to
east. Into the afternoon hours, chances begin to increase for VCSH,
especially at all TAF sites with the exception of KCLT. Short-lived
restriction could occur in any heavy downpours, however, given lower
confidence, have not introduced into going TAF attm.

Outlook: Potential for short-lived restrictions Sunday evening with
rain showers. Otherwise, expect VFR as a series of dry cold fronts
move through the area.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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:




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