Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171152

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

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.


As of 650 AM, dense fog has actually improved slightly across the
southern tier of forecast zones, with most sites reporting 1/2SM.
However, would not be at all surprised to see things come crashing
down again once the sun gets over the horizon a little more, and we
may yet see it expand. The DFA continues through 10 AM for all of
our southern tier counties, plus Mecklenburg and Cabarrus.

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.

Otherwise, 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.


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.


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.


At KCLT and Elsewhere: Eroding Cold air damming is resulting in
widespread LIFR cigs and IFR visby across the area, with some
pockets of VLIFR. Meanwhile, the mtn valleys, including KAVL have
seen the low clouds and fog dissipate, which is an indication of how
shallow the cold pool has become. In fact, while MVFR conditions are
forecast to develop later this morning at KAVL, and to continue much
of the day, no additional IFR is expected.

At the other terminals, LIFR cigs/IFR fog is expected to persist
through at least late morning. Cannot at all rule out VLIFR cigs
and/or visby in the meantime, but just didn`t have enough confidence
to advertise it at this point, as sites that were reporting 1/4SM
earlier have improved a bit over the past hour. 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...

            12-18Z        18-24Z        00-06Z        06-12Z
KCLT       Med   66%     High  83%     High  96%     High  80%
KGSP       Med   77%     High  91%     High  91%     High  83%
KAVL       Med   60%     Med   78%     Med   69%     Med   71%
KHKY       High  82%     Med   71%     Med   60%     High  94%
KGMU       Med   71%     High  90%     High  80%     High  83%
KAND       Med   69%     Med   77%     Med   61%     High  83%

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:


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-
SC...Dense Fog Advisory until 10 AM EST this morning for SCZ009-


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