Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180600

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
100 AM EST Wed Jan 18 2017

A weak cold front will approach the Carolinas from the west later
this evening bringing increased chances for precipitation. A warm
front will lift northward over the region on Thursday and Friday,
bringing more rain to the southeast. A more significant low pressure
system is expected to develop over the region on Sunday, bringing
more unsettled weather to the area.


As of 1245 AM, surface cold front is currently aligned along the
Great Valley in East Tennessee. There is one last narrow band of
showers along the western slopes of the Tennessee mountains, and
this probably supports the current likely pop across our far western
NC zones, but activity east of there is very spotty, and pops will
continue to ramp down to slight chance or less for most areas east
of the Blue Ridge. Warm sector regime will support persistent SW
winds in the Piedmont this morning, with min temps expected to be
about 25-30 degrees(!) above normal, placing record warm minimum
temps for the date in jeopardy.

Otherwise, wind swings around to the NW behind the front in the
morning, and there should be enough moisture for another round of
low clouds, but fog chances are greatly reduced with winds remaining
8-10kt through the period. Will see another night with lows warmer
than seasonal highs, and then skies should clear and should see
another day with well above-normal temperatures tomorrow with highs
around 70 across most of the Piedmont (mid-upper 60s across NW
Piedmont).  Though cooler in the mountains, still far from what we
would expect in January.


As of 230 PM EST Tuesday...The upper pattern will remain highly
amplified and active across the Lower 48 thru the Short Term. An
upper ridge will cross the Southeast Wednesday night thru Thursday,
then a large upper low will lift out as a negatively tilted trough
and cross the area on Friday. At the surface, a weak high will cross
the area Thursday keeping it dry and mild, with lows in the upper
30s to upper 40s and highs in the 50s to lower 60s. Then a low
pressure system associated with the upper wave will drag an occluded
front thru the region late Thursday into Friday. The models have
converged on the timing of highest PoPs to be mainly after midnight
Thursday night to midday Friday. There will be some instability with
the front, but a weak in-situ wedge will limit CAPE to mainly the
mountains and southern piedmont. Shear will be adequate for
organized storms, but the limited instability will keep severe
threat low. Temps will remain above normal with lows in the 40s to
lower 50s and highs in the 50s to upper 60s.


As of 230 PM EST Tuesday:  The medium range fcst period kicks off on
Friday night amidst a brief period of shortwave ridging, while broad
cyclonic flow continues to dominate the pattern out west.  At the
surface, weak high pressure will be in place beneath the
aforementioned upper ridge axis across the Mid Atlantic states,
while moderating southerly waa prevails across the Deep South.
Pattern evolution through the medium range continues to be
highlighted by the western upper trof and a rather robust closed low
that develops through the day Saturday, all the while moving into
the Southern Plains.  In the meantime, an impulse of weak upper
energy combined with moisture convergence along an old stalled
frontal axis and an increasing unstable airmass will lead to robust
convection along the I10 corridor region on Saturday.  Guidance
favors continuation of weak ridging across Northeast GA and the
Western Carolinas, potentially providing focus for upglide forced
stratiform rain Saturday afternoon, before the old frontal boundary
lifts northward as a warm front yielding steepened lapse rates and
thus improving instability promoting a transition to showers and
perhaps isolated thunder for the overnight.

Moving on to Sunday, the primary upper cyclone looks to be stacked
atop a relatively newly formed surface low, centered around
Texarkana by daybreak.  Out ahead in the warm sector, showers and
thunderstorms look increasingly likely along/near the Gulf Coast as
moisture continues to stream in across the Southeast states amongst
improving sswly H85 flow all ahead of a progressive cold front.  As
this entire complex slides east into the TN valley and Southern
Appalachian region late in the day Sunday, copious amounts of
moisture look to advect across the fcst area potentially fueling a
threat of moderate/heavy rainfall given deep convection as well as
upsloping along the southern/eastern escarpment.  Given the strong
swly llj that is progged to pass through ahead/along the front,
strong/severe convection cannot be ruled out.

The sfc front looks to advect east overnight into Monday morning,
likely promoting continued light/moderate rainfall aamidstongoing
convection. Said front should clear the fcst area by around midday
Monday, however pops will remain somewhat elevated into/through
Monday evening across most of the region as the primary upper vort
passes overhead, thus providing ample lift to support continued
showers. The upper low will finally clear the region to the
northeast on Tuesday morning as upper heights look to rise amidst
a brief period of shortwave ridging to last through the remainder
of the fcst period, therefore allowing pops to lower. Temperatures
through the medium range will initialize well above normal on
Saturday, cooling slightly, yet remaining above normal for
Sunday/Monday and Tuesday amidst mostly cloudy skies and area


At KCLT and Elsewhere: Some rather radical changes have been made to
the TAF suite, as observational trends lend little support to the
development of visby or even cig restrictions this morning. This is
due to a 5-10 kt surface wind, which should continue at the Piedmont
and foothills terminals through the morning. (Meanwhile, a NW wind
of around 10 kts will develop at KAVL by around 09Z as a cold front
passes through the area). These winds are expected to maintain the
current 5-10 degree temp/ dewp spread, which won`t be enough to
support fog development at the main TAF sites. Additionally, a
strong westerly downslope flow is developing in the 020-050 layer,
and this will likely prohibit much in the way of sub-VFR cloud
development. Statistical guidance is quickly coming around to this
thinking as well, favoring a much more optimistic forecast than the
current one. Therefore, we have removed all restrictions from the
TAFs for this forecast period. The only exception is at KAND, which
is less impacted by downslope effects in westerly flow, but even
they only see a tempo for MVFR cigs around daybreak.

Otherwise, frontal boundary will cross the area this morning with
shower chances being confined to the mtns, but probabilities only
warrant a VCSH even at KAVL. Winds will turn to the W/NW at around
10 kts at all Piedmont 12Z and 15Z. VFR clouds will clear out by
late morning, with SKC conditions expected through much of the
daylight hours.

Outlook: Surface low pressure approaching from the southwest will
bring chances for widespread precip to the area Thursday and Friday.
Yet another cold front will bring chances of precip and restrictions
on Sunday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  95%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  90%     High  98%     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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      72 1933     20 1977     57 1943      0 1994
   KCLT      71 1937     23 1893     62 1943     10 2003
   KGSP      71 1928     30 1977     60 1943      5 1893




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