Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171951

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
251 PM EST Tue Jan 17 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 230 PM EST Tuesday: Cold air damming has eroded across the
area (as evidenced by pressures and winds), but residual cool pool
remains in place with weak low-level isentropic upglide continuing
across the area resulting in continued low clouds. Upper ridge over
the Caribbean and Gulf remains over the area, but approaching front
associated with upper low lifting toward the Great Lakes will help
to push the ridge to the south somewhat briefly, though the high
will pop back up behind the front tomorrow (so the shortwave is kind
of like pushing down on a balloon). The downstream ridging will also
be enhanced by a deep cutoff low lifting out of the Desert Southwest
and into the Southern/Central Plains tomorrow, inducing surface low
formation along the TX Gulf Coast (to impact us in the short term).

There really isn`t much to this front and it certainly isn`t cold
behind it, as is typically the case in the wake of CAD. Might see
some precip with the frontal passage but most of that should be
limited to the mountains, with really only slight chances elsewhere.
While SPC does have portions of the SW mountains in a General
Thunder area, CAPEs are nonexistent across the area (even MUCAPE)
and have not reflected that in the grids. Front is just not dynamic
enough for mechanical lift to result in best just some
showers but all pops pretty much gone within a couple of hours after
sunrise. 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 admist ongoing
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: Slowly improving cigs from S to N (except for
KAVL which has been VFR for several hours), but generally still
widespread low MVFR to IFR cigs. Might see a brief period of VFR
this afternoon but increasing moisture ahead of a dissipating front
will bring MVFR to potential IFR back in overnight. Winds should be
a little stronger though so vsbys shouldn`t be as bad...for now
expecting IFR at worst. Could see some SHRA at KAVL or KHKY but for
now handled that with VCSH. Other concern is potential for LLWS
across the NC sites and have included this at KCLT from 04-09z,
though timing may need to be adjusted. With the front pushing
through, winds should shift around to NW, with current wind shift
timing at 15z for KCLT.

Outlook: A moist airmass and sfc boundary will remain close to the
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 Thursday and Friday.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High  88%     High 100%     Med   74%     High  90%
KGSP       High 100%     Low   52%     High  80%     High 100%
KAVL       High 100%     Med   66%     Med   79%     High 100%
KHKY       Med   77%     Low   53%     Low   55%     High 100%
KGMU       High 100%     Low   51%     Med   70%     High 100%
KAND       High 100%     Low   58%     Low   48%     High  93%

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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