Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 182345
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
645 PM EST Wed Jan 18 2017

.SYNOPSIS...
A warm front will lift northward over the region on Thursday and
Friday bringing more rain to the area, particularly on Thursday
night. A more significant low pressure system is expected to track
across the region this weekend, bringing rain and thunderstorms for
Saturday, Sunday, and Monday. High pressure will build into the
region for the middle of next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 640 PM EST: The passing surface "cold" front has settled
southeast of the forecast area this evening, with little more
fanfare than a wind shift toward northerly flow over most of the
region. Winds in the western Upstate and upper Savannah River area
will remain WSW a bit longer with brief lee troughing in place.
Otherwise, the forecast remains on track as water vapor imagery
shows a closed low system moving east over the southern plains early
this evening, with associated moisture trying to top the downstream
ridge near the base of the southern Appalachians. Mid and high
clouds spilling over the ridge could impact overnight min temps, but
only a slight adjustment toward cooler values in the NW NC piedmont
appears warranted - where high clouds will be thinner.

As the surface high center pushes east tonight and over the
Appalachians, winds will swing around to the northeast, so we will
lose our downsloping component tonight and tomorrow. Upper ridging
will build back in over the area tonight and tomorrow as well. While
still nowhere near seasonal normals, overnight lows tonight behind
the front will be much more reasonable for January, with highs
Thursday progged to be 10-15 degrees colder than those observed
today across the Piedmont (with not much change, and actually maybe
a slight warming trend for the mountains).

A strong upper low over the southern High Plains will eject slightly
northeast through the near term, with surface low enhancement along
the Texas Gulf Coast. As the upper low lifts into the Plains, the
surface low will correspondingly lift into the Lower Mississippi
Valley. A warm front will lift into the Southeast and may bring some
initial precip into extreme western counties right at the end of the
period.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 230 PM EST Wednesday...A long wave upper ridge axis will set
up along the East Coast, while a deep and broad mean upper trough
axis will persist across the Rockies and Plains. Two prominent short
waves will eject out of the trough and cross the CWFA within the
upper level southwesterly flow. The first will cross the area late
Thursday night, and the second during the day on Saturday. The first
wave will bring a warm/occluded front across the area, and will be
accompanied by an area of showers. The high-res models that go out
thru 12z Friday, show the shower activity being fast-moving, with
embedded stronger elements. There may be enough elevated CAPE for
isolated thunder, but no severe weather is expected. Temps will be
above normal Thursday night and Friday, with a lull in PoPs Friday
afternoon.

The second short wave will lift NE across the region on Saturday,
and looks to be accompanied by an activating warm front that will be
draped east to west across the Gulf states. As the wave ejects,
upstream energy will be carving out a vigorous upper low over the
Southern Plains. This will induce rapid cyclogenesis over/near the
TX panhandle. A very large warm sector will become unstable
Saturday, as the warm front lifts north. The NAM and GFS agree on
SBCAPE of 1000-2500 J/kg surging into LA/MS/AL thru the day. This
area will also have plenty of shear with 0-6 km bulk shear of 45-65
kts. It`s still a bit too far out to be certain what mode of
convection will likely fire, but whatever activity develops will
propagate NE toward our NE GA and western Upstate zones Saturday
evening. The Day 4 SPC outlook has a broad 15% area to our SW. This
certainly bears watching in the next couple of model runs and severe
storms mention may be needed in the HWO. PoPs were bumped up to
likely for most of the area on Saturday. Temps will remain well
above normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 200pm EST Wednesday:  Saturday night through Wednesday night
will be a period of unsettled weather.  GFS and EC agreement is
quite good through next Wednesday with a southern diffluent longwave
trough bringing moisture, showers, and thunderstorms on Sunday,
followed by a short-lived longwave ridge with clearing on Tuesday,
followed by the next progressive southern longwave trough due in by
early or late Thursday, though it could begin to affect the forecast
by Wednesday evening.  EC is about 12 hours ahead of the GFS with
the Thursday longwave, but the basic pattern of 2 longwave troughs
giving the region waves of weather is fairly consistent.
Precipitation Sunday/Monday looks generous with an inch or more
possible each day.  Precip on Sunday occurs with warm frontal
passage and possible thunderstorms, followed by wrap-around precip
and cool frontal passage on Monday.

Sunday afternoon has the best chance for thunderstorms, though the
best CAPE forecast is only around 700 j/kg, with a LI to -5.  Best
instability currently looks to be in the Piedmont areas at this
time, as low surface pressure associated with incoming longwave
brings abundant moisture in from the Gulf of Mexico.  While
instability is currently forecast to be modest, bulk shear is very
healthy at 50 to 65 kts Sunday afternoon, especially with the more
backed winds ahead of the surface low.

Minimum temperatures Sunday and Monday look to be in the 50s in
Piedmont areas, down to the 40s in the mountains, so there is little
risk of any winter precipitation.  With region far south of any cold
air masses, temperatures generally will be well above normal through
Wednesday, even behind the front that passes through with the
longwave trough on Monday.  Passage of that front will make Tuesday
the coolest day of the extended forecast, but still 10 degrees above
normal.

Surface winds will also be elevated with the passage of the storm
system Sunday and Monday, with 10 to 15 mph of southerly winds on
Sunday/Monday, becoming northwesterly behind the frontal passage on
Tuesday.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
At KCLT and Elsewhere: VFR conditions are expected through the TAF
period as a downstream ridge sharpens over the terminal forecast
area east of the prominent closed low over the plains. This will
lead to increasing mid and high clouds through the period, but with
all VFR cigs expected. Winds should initialize mainly from the north
as surface high pressure moves over to the north of the region, with
flow becoming light NE overnight, except variable at KAVL and KHKY.
Expect an adjustment toward southerly through the day Thursday,
impacting KCLT around 18Z.

Outlook: Surface low pressure approaching from the southwest will
bring chances for widespread rain showers to the area Thursday night
and Friday. Yet another cold front will bring additional chances of
showers and restrictions on Sunday and Monday.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     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 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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...SW
NEAR TERM...HG/TDP
SHORT TERM...ARK
LONG TERM...WJM
AVIATION...HG/TDP


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