Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181752

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1252 PM EST Sat Feb 18 2017

A weak upper low moves east today across the southeast states,
reaching the Carolinas tonight then off the Carolina coast Sunday. A
strong upper ridge moves in to start the week, with much above
normal temperatures. Another period of light rainfall is possible
mid week as a weak upper trough moves through the region. A cold
front approaches at the end of the week with possible showers and


As of 1235 PM EST: high- and mid-level clouds continue to invade
from the SW, while the leading edge of the light precip had crossed
the Savannah R. Some of the precip was not reaching the ground, and
where it was, the character was a very light rain. See no reason to
raise precip chances right now as many places will still only get a
trace. Temp trends doing much better at midday, so no changes.

Otherwise...a closed upper level low pressure center moving over
Arkansas this morning will open up over the mid MS River Valley
today and cross the Appalachians tonight. The deepest moisture ahead
of the lifting system will cross the forecast area mainly through
mid- to late-afternoon, but with the precipitation band encountering
increasingly dry air as it progresses northeast and outpaces the
upper forcing. This will lead to a fairly sharp PoP gradient ranging
from low end likely across the far southwest NC mountains to sub
slight chance east of I-77. A similar maximum temperature gradient
will result, with the driest eastern piedmont locations possibly
approaching 70 again for highs.

W to NW upslope flow moisture will continue along the spine of the
southern Appalachians tonight, possibly enhanced by the passing
upper trough axis. PoP should shut off quickly east of the mountains
this evening, but low end shower chances will only gradually pare
back toward the TN line overnight. Patchy fog will likely develop
overnight as mid level drying wraps in atop a slightly moist
boundary layer, with temperatures in the 40s everywhere but the
higher mountain ridges.


At 200 AM EST Saturday...No major changes needed the going fcst. The
models continue to show good agreement with the synoptic pattern
which features a strong ridge overtaking a departing s/w trof
Sun...then slowly crossing the area thru the period. There will
likely be some lingering upslope -shra across the nrn NC mtns Sun
morning with little additional precip...but after that the column
dries thru a deep layer as a strong subs inver lowers to h85. This
inversion along with warming mid levels will shunt any chance of
deep convection thereby allowing good insol with compressional
warming to the sfc. Thus...two very nice days are in store to end
the holiday weekend. Max temps will have no problem reaching arnd 15
degrees above normal each day with only fair-wx Cu and increasing Ci
ahead of an approaching frontal system by Mon. A cP sfc high ridges
down the east coast early Mon and will veer the prevailing w/ly
winds to se/ly through the afternoon. Little airmass mix will be had
however as theta/e adv remains weak within the bndry layer.


As of 140 AM EST Saturday:  The huge upper ridge over the east coast
begins to break down as a weak upper trough moves in across the
Mississippi Valley on Tuesday.  A surface high pressure centered
over northern New England and eastern Canada on Tuesday noses down
into the Carolinas Tuesday channeling cooler maritime air into the
Carolinas and for a while have a light low level upslope flow
against the SE facing mountains. The low level flow becomes more
southerly through mid week as surface high pressure gets pushed out
toward Bermuda.  The GFS and ECMWF take a closed 500mb low across
the Gulf Tuesday and Wednesday reaching Florida Wednesday night. The
Canadian model is completely different with the weak trough moving
rapidly off the east coast with 500mb flow becoming west to east.
The result of the weak trough or closed low will be some patchy
light rain or showers in mid week. At the end of our current
forecast medium range on Friday we see on the GFS and EC a strong
500mb closed low headed toward the Great Lakes with an associated
surface cold front crossing the Mississippi Valley heading east.
This front is forecast to cross our area early Saturday. GFS model
runs are producing 400 to nearly 700 CAPE values across NE GA and
the Upstate late Friday while EC is much weaker Directional shear
combined with the instability may result in strong storms but
currently this is mainly illustrated on one model.  The best rain
event of the medium range should come on the last day with the
prefrontal convection.

Temperatures will be 10 to 15 degrees above normal through the
medium range.


At KCLT and elsewhere: expect clouds to thicken with ceiling
steadily dropping as deeper moisture plume and forcing ahead of an
upper low move steadily across the region through the early evening.
For the most part, ceiling will stay above 060, especially over the
east as a precip band is expected to weaken. Cannot rule out enough
light rain bringing the ceiling down into the MVFR category for a
period in the late afternoon, especially at KAND. The model guidance
suggests that conditions will drop to MVFR this evening after the
precip band moves through. Chances are best over the wrn Upstate of
SC but diminish from there. Have included a brief TEMPO at KCLT for
some light fog in the early morning hours, but confidence barely
enuf to mention. Wind should be mainly SW through late tonight, with
a change to NW in the pre-dawn hours with the passage of a boundary.
Downslope developing E of the mtns should take care of any
restrictions before sunrise.

Outlook: Expect VFR conditions from Sunday afternoon through early
next week. Another system may bring chances of precipitation and
restrictions to the area Tuesday into Wednesday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High  95%     High  95%     High 100%
KAVL       High 100%     High  96%     High  82%     High  90%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High  91%     High  93%     High 100%
KAND       High  90%     Med   78%     High  86%     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 2014     16 2015     54 1951      3 2015
   KCLT      76 2014     26 2015     55 1961      8 1958
   KGSP      75 2014     30 2015     55 1961      9 1958
                1916        1900                    1900


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      75 1939     26 1908     50 1951      4 2015
   KCLT      77 2014     31 2015     59 1939      7 2015
   KGSP      75 2014     33 2015     57 1939      9 2015




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