Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251140

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

A cold front will move off the Carolina coast late today as cool
high pressure builds into our region from the west. Temperatures
cool to near normal tonight through Sunday night, then warm up again
during the first half of the week. A warm front lifts north through
the area around midweek as a cold front approaches from the west
bringing showers and storms.


As of 630 AM EST Saturday:  As expected, prefrontal convection
continues to work across the NC high terrain at this time, all
the while losing steam as the airmass is relatively more stable
at this hour.  However, a few rumbles of thunder are still likely
as an occasional cg is being reported via ldn.  Not expecting any
of these storms to become severe, but a few strong gusts cannot be
ruled out.  Thus, for the fcst, made a few tweaks to t/td and pops
in the immediate near term to better align with recent radar/ob
trends as well as latest camguide.  Otherwise, no sig changes
needed/made with this update.

As of 245 AM EST Saturday:  An amplified upper pattern prevails
across the East/Central CONUS this morning as a compressed H5
ridge slides eastward atop the western Atlantic while a deep
trough moves across the Midwest.  At the surface, cyclogenesis
continues to support sustainment of low pressure across the
eastern Great Lakes while a sharp cold front extends southward
into the OH/TN Valleys.  As such, convection is ongoing along and
just ahead of said front across middle/east TN at this time with
a few strong storms embedded.  The latest Mesoanalysis from SPC
indicates a region of 4-5 hundred joules mucape across extreme
southeast TN where nearly 50-60kts bulk shear resides as well,
which is where the strongest convection is currently ongoing.
Am expecting this line to progress east into western NC by
around 4AM, with a few isolated cells to lead the way out ahead.
The above mentioned shear maxima will advect into the region in
association with the llj, however instability along and east of
the TN line is sketchy at best.  Therefore, will allow pops to
reach cat/likely levels almost immediately across the SW mtns to
account for shra/tsra, with likely levels stretching northward
up the mtn chain.  Pops will increase to high chances eastward as
the front intrudes into western NC just before or around daybreak,
with any leading convection likely weakened substantially.

Guidance favors little if any support for continued precipitation in
the post dawn hours as instability will be at its lowest levels,
thus only a few linear shra segments are favored with patchy
fog to accompany.  As the front moves into the FtHills/Piedmont
region around midday, some recovery is expected mainly along/east
of I77, therefore with the front still in the vicinity llv
forcing/convergence could support a brief window of enhanced/deeper
convection, although not likely.  Beyond that a much drier/cooler
airmass will advect into the region as quasizonal flow builds aloft
and sfc high pressure moves in from the west.  Thus skies will
clear out late in the day, leading to optimal radiational cooling
conditions and near normal overnight lows.  Lastly, nwly veering
winds will become quite gusty behind the front this afternoon, which
combined with the possibility of little/no precip and low RH values,
will support enhanced fire danger conditions across SC and possibly
GA as well.  Please refer to the Fire Weather section for details.


As of 305 AM Saturday, A low-amplitude/quasi-zonal upper air pattern
will persist through the short term period, with gradually building
heights anticipated across the East. The end of the weekend will be
marked by dry and seasonably cool conditions, with high clouds
expected to increase late Sunday into Sunday night, as a shearing
vorticity max moves rapidly from the southern Great Plains to the
Mid-Atlantic Sunday night through Monday. In response, moist
isentropic lift will develop across the western Carolinas and
northeast GA throughout Monday, with increasing clouds and
precipitation chances expected, peaking by Monday evening.

Some guidance sources are more bullish than others regarding the
overall extent of the precip Monday into Monday night. Some models
(e.g. the ECMWF) indicate a bit more in the way of surface wave
development along the Carolina coast, which tends to disrupt the low
flow across and shut off the upglide more quickly across our area.
The upshot of this would be significant QPF confined to our western
areas, with little to nothing across the eastern zones. With that in
mind, pops range from 60-70 across western areas, to 40-50 along
the I-77 corridor. Monday`s temps will be a bit tricky, depending
upon how quickly precip arrives. As sufficient low level dry air
should be in place to force establishment of in-situ cold air
damming where precip falls. Nevertheless, with current data
indicating that precip is likely to hold off until afternoon, will
advertise maxes near to slightly above climo across the west, and
about 5 degrees above normal across the NC Piedmont.

Pops taper off gradually Monday evening. However, isentropic lift is
expected to increase across the region once again by daybreak
Tuesday, as large scale height falls overspread much of the western
half of the Conus. Another round of precip is therefore possible by
the very end of the period. Temps Monday night will be well above


As of 155 AM EST Saturday: Low level SW flow at the 925mb level will
persist Tuesday and Wednesday ahead of the approaching cold front.
GFS 925mb wind speeds maximize on Wednesday with 30 to 45kts in the
vicinity of I-85 from GA to NC. Moisture will be abundant with GFS
Precipitable water around 1.4 along I-85 Wed afternoon. Instability
is generally restricted to along and south of I-85 Tuesday PM,
especially on the GFS where very low if any CAPE is north of the
corridor. On Wed PM, CAPE of 400 to 600 from NE GA to CLT area along
with the wind shear will give strong to severe storm concern. Of
course, timing of frontal passage coincident with max daytime
heating would make for the max circumstance with dynamics coming
together. The 500mb trough crosses the western states early Tuesday
with axis over the Plains States 12Z to 18Z Wed with low pressure
forming in the eastern trough inflection point and moving NE across
the Appalachians Wed evening. At least the models are in closer
agreement as compared to 24HRs ago. GFS still the fastest model in
taking the front across our area around midnight Wed while the EC
has it passing a few hours later.

After the front passes, there will be lingering moisture along the
TN border with NW Flow combined with shortwave energy crossing just
north of NC on Thursday and early Friday.  Thickness values will be
falling Wed night into Thursday and will be low enough for snow in
the NC Mtns by late Thursday morning. The second shortwave could
bring a brief NW Flow snow shower period around daybreak Friday for
the central and northern NC Mtns.

Temperatures are forecast to be 10 to 15 degrees above normal
Tuesday and Wednesday then fall to near or slightly below normal
behind the cold front for the second half of the week.


At KCLT and Elsewhere:  VFR and possibly MVFR over the next few
hours as convective debris associated with upstream convection
advects eastward across the region.  Frontal convection working
its way across the mtns at this time will likely lose stream
as it pushes into a relatively more stable airmass, therefore
only favoring tsra at KAVL in the immediate near term.  Expecting
redevelopment into late morning further east along the I77 corridor
region, thus did carry a tsra tempo at KCLT for a few hours given
modest diurnal recovery supportive of ts.  Otherwise, carried vcsh
at all other sites through 14z-15z before all tafs go dry with
low clouds scting out to high cirrus then skc for the remainder
of the taf cycle. Swly winds will increase and veer nwly behind
the front with gusts reaching upwards of 20-30kts at times today,
before all winds weaken overnight.

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...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       Med   78%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       Med   66%     High 100%     High 100%     High 100%
KGMU       Med   66%     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:


Gusty W/NW winds and much drier air will overspread the area in
the wake of a cold front Today.  Minimum RH of 25 to 30 percent is
expected across much of the SC Upstate and Northeast GA, and may dip
below 25 percent in some areas.  Sustained winds will increase and
veer nwly behind a departing cold front into the 15-20mph range with
gusts nearing 30-35mph.  These critical conditions could overlap
for an hour or two this afternoon.  Thus a fire danger statement has
been issued for the all of northeast GA minus Rabun County, as well
as all of Upstate SC starting at noon today, running through 6PM.


GA...Fire Danger Statement from noon today to 6 PM EST this evening
     for GAZ017-018-026-028-029.
SC...Fire Danger Statement from noon today to 6 PM EST this evening
     for SCZ001>014-019.


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