Scientific Forecaster Discussion
fxus62 kgsp 090823
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
323 am EST Fri Dec 9 2016
cold high pressure will remain in place across the region, bringing
the coldest temperatures so far this season through this weekend. A
cold front will approach the western Carolinas late Sunday and move
through the area on Monday, raising rain chances next week.
Near term /through tonight/...
broad/low amplitude troughing will remain in place across the
eastern conus through the period, with cold/dry low level high
pressure building into the region. Moderately strong low level northwest
flow will continue, supporting shallow moisture in the upslope
region along the Tennessee border. Per radar, satellite, and short term
model trends, moisture depth and Thermo profiles favorable for snow
showers should remain north of the forecast area, but a few flurries
and/or areas of rime icing may be seen along the immediate Tennessee
border, especially over the northern mtns. Otherwise, skies will
become sunny in all areas by early afternoon. The main short term
story will be the cold temperatures, as temps are expected to
average a solid 10-15 degrees below climo through the period.
Short term /Saturday through Monday night/...
at 250 am friday: short range guidance indicates that the center of
a large portion of a Canadian air mass will slide over the County Warning Area on
Saturday. A h5, a broad longwave trof axis will ripple east,
resulting in zonal flow by Saturday afternoon. This pattern should
provide dry and cool conditions across the forecast area. Using a
blend of guidance, high temperatures are forecast to range from the
low 40s within the mtn valleys to mid to upper 40s east of the mtns.
Afternoon relative humidity values may dip into the upper 20s to low 30s.
Saturday night through Sunday, the pressure gradient across the
western Carolinas and northeast Georgia will begin to increase
between high pressure off the mid Atlantic coast and low pressure
over the Great Plains. However, winds should remain light early
Sunday morning under partly clear sky cover. Low temperatures are
forecast to range from the low 20s across the mtns to upper 20s
across the foothills and Piedmont. Develop return flow should
support high temperatures 2-3 degrees warmer than Saturday, with a
15 percent increase in sfc relative humidity. Moisture should arrive late in the
day, supporting increase sky cover across the region and schc pops
for -ra across the east facing mtn slopes.
Sunday night, S-SW 850 mb winds are forecast to strengthen across the
southern Appalachians as a cold front approaches from the west. Winds
across the NC mtn ridges may strengthen to 25-35 mph with higher
gusts during the late night hours. Otherwise, the strong llvl flow
should support expanding upslope rain across the high terrain during
the evening hours. Later in the night, a band of pre-frontal rain
should sweep across the region. Forecast sounding across the mtns
indicate a very strong warm nose between 3-5 kft during the
overnight. Most areas should see a cold rain, however, areas across
the ridges of the northern NC mtns may see periods of -fzra. Low
temperatures will favor the 30s across the region.
Monday through Monday night, short range guidance is mixed with the
placement and timing of the cold front. The GFS indicates that the
front will slip south of the region late Monday and Monday night,
resulting in dry air to build across the mtns. The European model (ecmwf) is slower
and stalls the front over the upper Savannah River valley on Monday,
then lifting north as a warm front Monday night. I will use a blend
of both solutions, keeping schc pops across the County Warning Area with chance pops
over the SC and Georgia Piedmont, increasing during the late night hours.
Temps on Monday are forecast to rise a couple of degrees above
normal during the afternoon, remaining above freezing across the
mtns Monday night.
Long term /Tuesday through Thursday/...
as of 230 am EST friday: picking up at 12z Tuesday morning, the
medium range begins at a Point of substantial global model
disagreement on the handling of a surface frontal system and
associated precip draped over our region. The drier GFS allows the
front to make a clear passage by Tuesday morning, and is therefore
mainly dry during the day on Tuesday before another low and
associated surface front raise pops once again on Wednesday. The
European model (ecmwf) disagrees...with the first surface front getting hung up over
the southern Appalachians and Little Break in precip between the
first and second lows/fronts. Both models have remained stubborn and
displayed decent individual run-to-run consistency...so as wpc has
done, I have gone with a blend of the two solutions, which seems
reasonable both as far as the progressiveness of the overall pattern
and depth of the second low pressure system.
As far as sensible weather GOES, Tuesday through early Thursday
morning feature almost constant high-end chance pops. Quantitative precipitation forecast looks
generally light through the period, regardless of whether there is a
break in precip between surface fronts. Though weak cold air advection will likely
be ongoing, Tuesday and Wednesday should remain sufficiently warm
that all areas except the high terrain will see liquid precip, with
a brief period of mixed precip/freezing rain possible before
daybreak Wednesday in the mountains. Though more substantial cold air advection
will set up overnight Wednesday into Thursday morning, the deep
surface low and impressive longwave troughing on the European model (ecmwf) seem a
bit too amplified in the context of other guidance and previous
runs. P-type was therefore kept liquid in the majority of the
Piedmont/foothills, with the mountains receiving light snow as
precip tapers off Thursday morning. The period will begin with
temperatures just above normal...dropping to average mid-week and
then well below average by Thursday as a cold surface high sets up
over the Great Plains.
Aviation /09z Friday through Tuesday/...
at kclt and elsewhere: VFR conditions the next 24 hours. High clouds
will also clear out in short order, leaving mostly sky clear skies through
the end of the period. Light northwest winds (except more like 10-13 kts at
kavl) will continue through much of the morning, but a slight
increase is anticipated by 15z before winds diminish again around
sunset. For the time being, a forecast of late morning gusts is
limited to kavl, but would not rule these out at other sites, esp
Outlook: drier and much colder conditions will continue through most
of the weekend. Low level moisture is expected to surge into the
area ahead of a front on Sunday, possibly resulting in cig
restrictions and light precip, with the potential continuing until
the front clears the area late Mon/early Tue.
08-14z 14-20z 20-02z 02-06z
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 100%
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: