Area Forecast Discussion
Issued by NWS Greer, SC

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006
FXUS62 KGSP 171450
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
950 AM EST Tue Jan 17 2017

.SYNOPSIS...
Above average temperatures will persist through the rest of the
work week. A weak cold front will approach the Carolinas from the
west by late Tuesday bringing increased chances for precipitation. A
warm front will move north over the Southeast on Thursday and
Friday, bringing more rain to the forecast area. A more significant
low pressure system is expected to develop over the region during
the later part of the weekend, bringing plentiful rain to the area.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1000 AM EST Tuesday: Still have quite a bit of dense fog
across the Piedmont (especially the Upstate and NE GA) but starting
to see some signs of improvement. Will let the Dense Fog Advisory
expire at the top of the hour. Precipitation ahead of the cold front
isn`t making much progress so did reduce pops slightly, and with
some adjustments to sky cover came some minor adjustments to hourly
trends. Right now looks like highs could be a little too warm but
with the upper ridging in place, once the low clouds associated with
the damming manage to erode, could see a hefty warmup.

Otherwise, erosion of the remnant cool pool appears to be well
underway, with light southerly winds being reported in some areas of
the Piedmont. Thus the first post-fog problem of the day is max
temps, which should continue to follow the trend of the roller
coaster ride of the past 5 or so days. Clouds and fog should erode
in time to see temps warm as much as 10 degrees above yesterday`s
readings in most locations, with lower-to-mid 60s expected across
the northeast zones, to the upper 60s/lower 70s across the upper
Savannah River Valley and mtn valleys along the TN border.

A southern stream short wave trough currently lifting into the Great
Lakes, and a northern stream wave dropping in behind it will result
in phasing of the two streams across the eastern Conus today, while
a split flow pattern persists across the western 2/3 of the country,
with a massive upper low positioned in the vicinity of the Four
Corners. A cold front associated with the former southern stream
short wave will march across the TN Valley today. Abundant moisture,
weak buoyancy, and a decent frontal circulation should force a nice
precip band along the front throughout the day. However, veered low
level flow, which is expected to only veer more as the front
progresses to the east, along with the expectation that the front
will take on an increasingly zonal orientation in response to
additional short waves ejecting from the Four Corners upper low all
suggest that the frontal circulation will weaken considerably as the
band moves into the southern Appalachians. In fact, most guidance
depicts little in the way of QPF east of the TN border counties this
afternoon through tonight. Pops will be advertised in the likely
range along the TN border by the end of today, continuing into the
evening, but are not expected to break the 30% mark across the
Piedmont. There is a small chance of thunder this evening, esp
across the Smokies and vicinity, but chances appear to be slight at
best. Min temps tonight will be well above climo, in fact, they are
expected to be warmer than the average highs for this time of year.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
As of 255 AM EST Tuesday...Lingering -shra mainly across the NC mtn
zones to begin the short range period as an h5 s/w trof crosses an
existing sfc bndry. Dry air will work in pretty quickly
however...and reduce pops to nil by noon. The bndry remains over the
area...but will become inactive while strong subs and nw/ly llvl
flow increases. This will make for a very nice afternoon with max
temps reaching 70 F non/mtns and L60s mtn valleys. A small scale h5
ridge will develop to the west and bring an associated sfc high east
over the mid-Atl into Thu night. This will maintain the dry conds
thru the early morning Thu...before mid-level clouds push in ahead
of a developing storm system over the MS Valley.

Cloud cover becomes more layered Thu and compressional warming will
wane as llvl winds back ahead of deepening a low pressure
system...thus max temps will only reach arnd 10 degrees above
normal. Only. A strong h5 s/w approaches the CWFA Thu night and mech
lift will be enhanced by increasing ulvl div...which will increase
-shra/shra coverage east from the mtns through the day Fri. Bulk
shear values increase quite a bit...esp across the mtns where
soundings are showing arnd 80+ kts and over 60 kts non/mtns. Due to
the cloud cover...afternoon instability shud remain quite low while the
cold front loses structure as it/s h5 trof gets elongated to the
northwest. Thus there wont be much llvl convg either. However...a
few strong or even severe tstms are possible as the strong dynamics
interact with a couple llvl theta/e ridges...mainly across the wrn
NC mtns and the srn zones. Max temps will remain pleasant Fri
despite cloudy conds...about 10 degrees above normal across the srn
zones as sfc winds remain s/ly even with the front moving in by mid
day. The nrn zones will be a bit cooler with persistent e/ly winds
due to a sfc ridge slowly breaking down.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
As of 230 AM EST Tuesday: The medium range forecast still
looks generally wet/highly amplified...and with little end to this
pattern in sight, it`s beginning to sound like a broken record. The
forecast picks up at 00Z Saturday with a brief lull in pops with the
area in the wake of a cold frontal passage. A small, de-amplified
upper level ridge will cross the area Saturday during the day,
allowing for a potentially dry day before an upper shortwave rides
over the ridge ahead of an approaching upper low, increasing pops
again Saturday night. A surface high centered over Quebec may allow
a weak wedge to briefly develop along the spine of the Appalachians
overnight Saturday, right around the time the warm front associated
with a large, deep upper low begins to lift northward through Gulf
coast states and the Carolinas. An interesting note is that there is
almost no discontinuity in warm advection and moisture return off
the Gulf of Mexico into the southeast up until the arrival of the
pre-frontal precip ahead of this low, even after the cold front
passage late Friday. All this is to say there will likely be decent
low-level support and plentiful moisture for precip to spread into
our CWA late Saturday night from southwest to northeast.

This large upper low induces rapid surface cyclogenesis over the
southern plains on Sunday, and a progressive trough sweeps the low
down through the southeast Monday. Precipitation on Sunday will be
pre-frontal and generally associated with the warm front lifting
north from the Gulf coast. The question of convection is raised with
this pre-frontal precip on sunday afternoon, though as per usual
this time of year, instability is likely a limiting factor in seeing
anything severe develop. There is good agreement that the
approaching low will be quite dynamic with very good upper synoptic
support, though, so the system will bear some watching. Lapse rates
are also not impressive on Sunday, though PWs will be quite high for
this time of year (approaching daily maxes). The good news is that
this system is quite transient, so widespread hydro issues don`t
appear to be a concern at the current time. Much is still uncertain
with this system, and as is often the case, mesoscale details will
need to be fleshed out before confident conclusions can be made.
Some wrap-around moisture off the back of the occluding surface low
will keep rain chances elevated on Monday before another small upper
ridge noses into the southeast, providing some brief drying Tuesday.

Temperatures will remain 10-15 degrees above average through next
weekend, though there is a slight cooling trend possible at the
beginning of next week that would bring temps back down to just
about 5 degrees above normal.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
At KCLT and Elsewhere: Only minor changes to the KCLT TAF for the
15z AMD this morning, improving vsby to MVFR and lifting cigs ever
so slightly. Still see a slow improving trend through the day as
cold air damming gradually continues to erode.  Otherwise, as the
cold pool continues to erode, S/SW winds at around 10 kts should
develop at most terminals by early afternoon, which should clear out
the fog and lift cigs to IFR, and eventually VFR (by late
afternoon).

A cold front will approach the area this evening, possibly bringing
some showers near KAVL. However, showers chances diminish greatly
east of the mtns. Guidance brings cigs and visby crashing down again
late tonight, but the gradient in the vicinity of the front should
allow light surface winds to continue, which would tend to hinder
fog/low stratus development. For the time being, opted to limit
restrictions to MVFR cigs developing after midnight.

Outlook: A moist airmass and sfc boundary will remain close to the
fcst area through Friday, with flight restrictions possible each
day. Surface low pressure approaching from the southwest will bring
chances for widespread precip to the area Thu and Fri.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Low   49%     High  96%     High  87%     Med   73%
KGSP       Med   60%     High  96%     Med   79%     Med   77%
KAVL       Low   55%     High  83%     Med   67%     Med   77%
KHKY       Med   71%     Med   75%     Low   46%     High  82%
KGMU       Low   49%     High  96%     Med   65%     Med   77%
KAND       Low   56%     Med   68%     Med   60%     High  81%

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...Dense Fog Advisory until 10 AM EST this morning for GAZ029.
NC...Dense Fog Advisory until 10 AM EST this morning for NCZ071-072-
     082.
SC...Dense Fog Advisory until 10 AM EST this morning for SCZ009-
     011>014-019.

&&

$$
SYNOPSIS...LEV
NEAR TERM...JDL/TDP
SHORT TERM...SBK
LONG TERM...LEV
AVIATION...JDL/TDP



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