Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150901

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
401 AM EST Fri Dec 15 2017

Dry and cool high pressure will dominate our weather through Sunday
morning. Moisture and a gradual warm-up will return to the area
starting Sunday. A series of disturbances will pass across the area
next week, with a prolonged period of unsettled weather.


400 AM EDT Update...Upper level cloud cover hanging tough this
morning and it/s thick enuf to prevent good rad cooling. Hourly
temps are running 3-5 degrees abv the fcst curve and have been adj.
Min temps have also been adj up a degree or two most locales.

Upper level clouds will continue to blanket most of the FA this
morning and thru day tomorrow. There is also some ovc/bkn stcu
upstream across TN which is generally building east. Not expecting
much from the low clouds with perhaps some enhanced cloudiness
within upslope flow over the far wrn NC mtn zones thru mid day. Max
temps will be a little tricky as winds remain n/ly and insol will be
somewhat limited. Still have bumped maxes up a couple degrees based
on recent model biases which still leaves values within the middle
guidance ranges. Min temps are also not so straight-forward with
llvl CAA kicking in aft 00z and persisting thru the morning. Think
most areas will go below freezing tho as a cP airmass mixes in and
cloud cover decreases.

A potential hazard will be strong gusty winds across the higher
terrain this evening and into early Sat. Strong hipres builds in from
the west and will create a rather tight p/grad across the NC mtns as
CAA also aides in downward mixed gustiness. Went with a consensus
of the hires solns which gives winds pushing or occasionally meeting
adv levels abv 4 Kft before weakening arnd 06z.


As of 200 AM EST Friday: A split-flow pattern will dominate our area
through the short term, with a cutoff midlevel low over northern
Mexico and the Desert Southwest combined with a Rockies trough and
an eastern Canada trough. Western Atlantic upper ridging will
retrograde west through the period over the Bahamas and eventually
the Caribbean. Surface high pressure will remain in place over the
Deep South as we start the period, with dry and cool (just a touch
below seasonal normals) conditions for Saturday. The Mexico/Texas
low will lift moisture out of the Gulf and across the South during
the day on Saturday, and we should see an increase in cloud cover
overnight, which leads to some uncertainty in overnight lows.
Surface dry air will slowly begin to moisten from above, and right
now looks like the best moisture will hold off until midday Sunday.
However, it remains possible that some light rain or showers may
encroach extreme western zones just after daybreak, and wetbulbing
*might* allow for some brief freezing rain to form. The
southwest flow aloft should result in a substantial warm nose, so if
any wintry precip mixes in it would be freezing rain.

The moisture will overspread the area on Sunday but sort of splits
as the former low damps and becomes absorbed into the longwave
pattern during the day, with the northern stream moisture staying
north and the southern stream staying south, leaving us in an area
of relatively low moisture but enhanced by isentropic and orographic
upglide. Pops remain capped at high chances on Sunday, but a
secondary wave lifts over the Caribbean upper ridge late in the
period. GFS much more robust with this wave but keeps the best
moisture to our south; likely pops seem warranted but limited mainly
to portions of NE GA and the Upstate.


As of 215 AM EST Friday: Split-flow pattern remains in place as we
start the extended, with a cutoff low developing over the Desert
Southwest from the upper trough over the Rockies during the short
term. The northern portion of this trough gets shunted east toward
the Great Lakes, but refuses to let go of the cutoff and so drags it
east as the low gradually fills. With the upper ridge remaining in
place over the Caribbean, southwest flow aloft will continue to
bring moisture into the area, and with the approach of the weakening
low as well as its little brother (a wave that tries again to cut
off over NW Mexico but fails), the unsettled pattern will continue.
WAA at the surface plus the southwest flow aloft will allow for a
gradual warmup through the period as well. GFS a little faster with
the moisture initially (and clearing it out by the end of the
period) but ECMWF is slower with the "little brother" wave and so
keeps a prolonged period of moist advection across the area. In
either case, best chances of precipitation remain to the south which
may cut off some of the low-level moisture flux. Leaned slightly
toward the GFS which keeps pops in through Tuesday, but has
Wednesday dry and still above normal.

Toward the end of the period, global guidance is in remarkable
agreement with a very strong front marching across the center of the
country in a beautiful classic synoptic system complete with dual
jets and corresponding warm/cold conveyor belts. Southern stream
moisture will again advect into the area, partially lifted around
the western periphery of a surface high pushing off into the
Atlantic. Other than the increasing moisture, we won`t really feel
the effects of the front until after the end of the period.


At KCLT and Elsewhere: VFR conds thru the TAF period at all sites. A
weak sfc bndry will be forced northeast as a strong ulvl jet core
traverses the area. No good chance of precip with this feature as
soundings show dry air thru the column. It will be hard to get clouds
outside of passing Ci...but a few higher based Cu may develop east,
including CLT, closer to the bndry. Winds will remain rather weak
and back ne/ly to nw/ly over the non/mtns and a little  stronger
while channeled nw/ly at KAVL.

Outlook: Potential for short-lived restrictions on Sunday with rain
showers. Otherwise, expect VFR as a series of dry cold fronts move
through the area.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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:




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