Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 250535

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1235 AM EST Sat Feb 25 2017

A cold front will move through the area tonight with showers and
thunderstorms mainly for the mountains. Temperatures cool to near
normal late in the weekend then warm up again early next week. A
warm front lifts north through the area around midweek, bringing
increased rain chances.


As of 1230 AM EST Saturday:  Updated discussion for taf issuance.
Did make a few minor tweaks to the immediate near term which
consisted of slowing previous pop trends a bit as the frontal
convection is just now moving across the Cumberland Plateau region
of TN, thus expected arrival into Western NC is now around 4AM.
Also tweaked t/td trends to better align with recent obs.

As of 230 PM: A robust sfc/upper low will move from near Chicago to
the north shore of Lake Huron by daybreak Saturday. We initially
will remain under the influence of offshore high pressure, seeing
only fair-weather cumulus and light southerly winds. Overnight,
a cold front will move through the lower Ohio and Tennessee
valleys. The airmass preceding the front is characterized
by springlike warmth and humidity, and accordingly a line of
thunderstorms is expected to develop in that area this evening
and progress east toward the Appalachians.

The overall expectations from this activity have not changed much
based on the latest model runs. The incumbent airmass over our
area still features poor low to mid level lapse rates and/or a
distinct cap, each of which should hinder deep convection; though
some elevated CAPE is still present it does not look particularly
viable. PoPs begin to ramp up late evening in the far western CWFA,
reaching the likely range along the Tenn border around dawn. As
the line of storms enters the CWFA hi-res models show the cells
weakening and becoming disorganized. The best CAPE comes as the
front erodes the cap. The potent shortwave trough associated
with the system will bring very strong mid-upper level winds and
accordingly eyebrow-raising deep shear parameters (0-3km bulk shear
of 40-50 kt, 0-6km 60-70 kt). However, with the instability being a
significant limiting factor, SPC continues to feature our mountain
zones in a marginal risk only for Day 1, which seems appropriate.

As the front proceeds across the area winds will shift to
westerly, gusts will pick up, and a dramatic drop in dewpoints is
expected. While models depict instability redeveloping downstream
of the mountains, high LCLs and downsloping seem to be the
reason that models are not depicting any convection beyond late
morning. Afternoon max temps should still reach the lower-mid
70s in the Piedmont--still shy of records. On the other hand,
over the high terrain, they should start off in the low 40s in
the morning and fall through the day. The much lower dewpoints
and gusty winds will create some degree of fire danger; please
see Fire Weather discussion below.


As of 215 PM EST Friday: We begin the short term in the wake of the
weekend front, with modified Canadian high pressure pushing across
the Mid-South toward the Southeast, and perhaps some lingering
northwest flow showers (possibly some snow showers) in the mountains
with the upper trough axis still over the Appalachians. Yes, you
read that right - I said snow, because with the deep-layer CAA
behind the front, temperatures Sunday night will drop down to
downright frigid temperatures...which in this regime means about 3-5
degrees below seasonal normals. Might even see some teens at the
higher elevations of the mountains, but most likely remaining in the
20s as what little moisture is associated with the upper trough
quickly exits. We are left with a sunny day on Sunday with highs
right at seasonal normals.  That doesn`t last long though as the
surface high moves off the Southeast coast and we get return flow
from the Atlantic around the western periphery of the high.
Temperatures will gradually warm through the remainder of the short
term as WAA and moisture flux continues across the area, despite
increasing cloud cover and (generally light) precipitation.

A shortwave pushing off the southern Rockies into the Southern
Plains Sunday night will tap into Gulf Moisture lifting north into
the southern states, with precipitation developing across the Lower
Mississippi Valley Monday, spreading east during the day. Weak
ridging along the Appalachians will remain, almost like a residual
cool pool but not very impressive, as precipitation overspreads our
area Monday afternoon. The ECMWF continues to advertise a bit of a
split in the rain as the mountains break up some of the features,
but a secondary (weaker) shortwave lifts out of the Deep South
Tuesday night to bring another round of light precip to the area.
All in all generally less than an inch. Expect that we`ll see some
thunder especially on Tuesday with surface temperatures in the 60s
and lower 70s allowing for at least moderate instability; however
shear is a little on the low side...not bad, but 30-50kt 0-6km.
Severe threat looks minimal.


As of 230 PM EST Friday: Longwave trough deepening over the Rockies
will push into the Plains early Wednesday, providing another kick in
the form of a subtropical jet and upper diffluence to the already-
moist atmosphere across the Southeast. Timing differences between
the operational models are problematic, with the GFS on the faster
side vs. the ECMWF. There is at least some ensemble support for the
faster solution (which frankly doesn`t really help narrow down the
decision), so have continued with a blend but leaning with WPC
slightly more toward the ECMWF. Concern with the GFS is that with
the faster upper trough, while slowly weakening, timing would put
the greatest lift and shear collocated with best instability
Wednesday afternoon, thus with 300-500J/kg sbCAPE with an impressive
70-80kt 0-6km shear, which would be a bit concerning. With the
slower ECMWF, the threat would be slightly later, but still non-
zero. Will have to continue to refine the forecast as new guidance
comes in.

Behind the front, again depending on the model of choice, we could
see a brief period of northwest flow snow in the mountains Wednesday
night as the CAA moves in in the wake of the front. Should see a
couple of days of near-normal temperatures at the end of the
period, so it might actually feel like early March as it should.


At KCLT and KAVL/KHKY:  VFR conditions will give way to increased
low stratocu from the west through the evening as a band of
prefrontal convection moves out of the TN Valley.  With that,
expecting the development of patchy MVFR fog and cigs out ahead,
with cigs possibly lowering to near IFR at times.  Instability looks
best further west to affect KAVL leading to better chances for
tsra, thus did include vcts mention with a prob30 for tsra while at
KCLT/HKY opted for only vcsh and a prob30.  Winds will increase and
veer nwly behind the front with gusts reaching upwards of 20-30kts
at times, especially in the mtn valleys due to channeling affects.
Likewise, guidance does favor some llws at all sites given the
nocturnal and prefrontal llj therefore such was includes at all
sites before/around daybreak.  Beyond that, skies will sct through
mid/late morning leading to high cirrus then eventually skc to
round out the taf cycle.

At the SC Sites:  Trends very similar to the sites above, however
with the primary difference being less chances for precipitation as
the best forcing/instability will remain northwest of the SC Sites.

Outlook: Drier air will advect in behind a cold front Saturday
afternoon as high pressure builds in from the west, maintaining
VFR thru Sunday. Another wave approaches on Monday, once again
increasing chances for precipitation/restrictions which may last
into Tuesday.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     Med   66%     High 100%     High 100%
KGSP       High 100%     Med   61%     High  87%     High 100%
KAVL       High 100%     Low   55%     High 100%     High 100%
KHKY       High 100%     Med   72%     High 100%     High 100%
KGMU       High 100%     Med   72%     High 100%     High 100%
KAND       High 100%     Med   61%     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:


Gusty W/NW winds and much drier air will overspread the area in the
wake of a cold front Saturday. Minimum RH of 25 to 30 percent is
expected across the Piedmont and foothills Sat afternoon, and may
dip below 25 percent in some areas, especially across Upstate SC.
Meanwhile, sustained winds of 10 to 20, with gusts of 20 to 30 mph
are also expected. These critical conditions could overlap for an
hour or two Sat afternoon, especially across the SC Piedmont. If
these trends hold, coordination on a potential Fire Danger
Statement may be required overnight.


SC...Fire Danger Statement from 11 AM EST this morning through this
     afternoon for SCZ001>014-019.


FIRE WEATHER... is the U.S. government's official web portal to all federal, state and local government web resources and services.