Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191820

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
120 PM EST Fri Jan 19 2018

Cool high pressure will gradually move offshore this weekend giving
way to a warming southerly flow of air. A cold front arriving from
the west will bring precipitation to the region early next week,
with drying following through mid-week.


As of 1245 PM EST: A weak, but dry, upper trough axis will approach
from the west late today, while the southern part of the upstream
split flow pattern continues to close off over eastern Texas and
western Louisiana. Then, the 500 mb closed system will slowly
migrate across the Deep South tonight through Saturday. Any deeper
layer moisture should remain shunted to our southwest through the
period, however 950 to 850 mb moisture in westerly upslope flow
could return to the NC mountains late tonight through Saturday. The
latest NAM and SREF are spitting out some light QPF along the
western mountains overnight through Saturday. This is troublesome
because if it occurs early enough, western mountain temps will still
be below freezing. However, the GFS and ECMWF remain dry, and the
SREF has onset a touch later, so will confine any daybreak PoPs to
slight chance of freezing rain or drizzle, at worst.

Additionally, another SPS for daybreak black ice will likely be
needed for the northwest NC piedmont with visible imagery showing
some lingering snow cover and temps expected to dip well into the
20s overnight. Maxes should rebound nicely into the 50s most
everywhere Saturday afternoon, with upper 40s along the mountain
ridge tops.


As of 200 AM EST Friday: Dry surface high pressure will remain
centered over the Gulf Coast states Saturday with broad upper
ridging in place over the eastern 2/3 of the CONUS and a weak
embedded upper low propagating through the mean flow over the
southeast. The surface high will shift off the east coast Sunday,
with the upper level low pinching off and generally drifting over
the Carolinas. The low is too weak to have developed a surface
reflection by this point...and surface flow will be so weak and
variable Saturday that very little in the form of moisture return,
and therefore QPF, materializes near our area. The system appears
unimpressive at the moment, though an occasional model run that
develops a small amount of QPF  in the NC mountains keeps popping
from time to time. Pops were kept dry for now, but there remains
some uncertainty surrounding the system. Warm southeasterly
return flow sets up over the Carolinas Sunday and Sunday night
and transient upper ridging will propagate off the east coast by
Monday morning ahead of an approaching cold front. For the past
few model cycles, guidance has been trending later on precip onset
time associated with the cold front, so pops were held out of the
forecast through 12Z Monday morning.

Despite numerous small disturbances propagating through the mean
flow, in general, our area will be under the influence of upper
ridging through the weekend, resulting in temperatures climbing
to almost 10 degrees above average across the forecast area.


As of 230 AM EST Friday: The medium range begins with a strong
upper/surface low swinging across the central CONUS with a cold
front trailing off of it. The system has slowed down even more with
the new model guidance, and agreement on precip onset time is
generally better, closer to 18Z Monday or possibly even later. The
onset of pops were therefore delayed slightly from the last forecast
package. though dramatic changes weren`t necessary. The bulk of the
pre-frontal precipitation will move in overnight Monday, resulting
in a quick inch or so of QPF across much of the area. Precip end
time is proving a bit more tricky for the models to resolve, though
there is agreement that the system will be progressive, and the
front will have passed by 18Z Tuesday. Above normal temperatures
will keep precip types all liquid to start the event Monday, with a
transition to snow expected across the high terrain of the NC
mountains very early Tuesday morning just before the system departs
to the east. QPF does not appear to be a problem at the moment,
though any slowing of the system could make it possible for a few
isolated hydro issues to develop.

The system is also quite dynamic, and shear profiles (particularly
in the low levels) are very impressive ahead of the surface cold
front. The main precip shield and system will be moving through the
area overnight though, a diurnally unfavorable time for instability.
Guidance continues to show virtually no surface-based instability
and extremely meager elevated instability, so a severe threat looks
unlikely at the moment given current guidance. Shear is so
impressive, however, that this system will need to be watched
closely over the next couple of days.

After the front passes, much drier air will arrive in the forecast
area, with a sprawling surface high moving through the OH River
Valley through the end of the work week. Temperatures will return to
near normal as the progressive upper flow moderates to a more zonal
pattern through the end of the week.


At KCLT and Elsewhere: Most terminals will see little more than
slowly increasing high clouds through the period. The main
exceptions will be increasing W to NW upslope moisture into the
western mountains, and spotty low clouds and fog in the northwest NC
piedmont where the melting snow is making for a somewhat moist near-
surface layer. The current feeling is that stratocumulus ceilings in
the western mountains will generally stay down-valley from KAVL, and
that lower restrictions will not affect the airfield at KCLT.
Conditions will need to be closely monitored at these two sites
overnight. Otherwise, expect dry and VFR conditions through the
period, with light SW winds east of the mountains and light NW flow
at KAVL. Some low end gusts will be possibly at KAVL from time to
time late today and again with mixing on Saturday.

Outlook: Dry/VFR conditions are expected to continue through the
late weekend. Moisture will return along and ahead of an approaching
cold front Monday into Tuesday, with associated restrictions
possible. Dry weather returns by the middle of next week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High  96%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     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:




SHORT TERM...Carroll
LONG TERM...Carroll
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