Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus62 kgsp 122006 
afdgsp

Area forecast discussion
National Weather Service Greenville-Spartanburg SC
306 PM EST Thu Dec 12 2019

Synopsis...
lingering cold air combined with a developing coastal cyclone will
a period of freezing rain across northern areas late tonight into
Friday morning before becoming all rain. A brief break in the action
is expected later this weekend before a slow moving cold front
brings rain back into the region early next week.

&&

Near term /through Friday/...
as of 2:45 PM thursday: scattered mid-lvl clouds are just beginning
to gradually move into our NE Georgia counties, as the rest of the
fcst area remains mostly clear. Temps have climbed into the mid to
upper 40s across most of the County warning forecast area with the lower terrain as cool,
or cooler than the mountain valleys. With the quasi-wedge now in
place, this was, of course, expected. The Wedge will persist over
the region thru tonight and into the overnight.

Otherwise, phased northern and southern stream shortwaves will cross
the plains this evening and then lift over the fcst area early Friday
morning. As the shortwaves dissipate, broader upper trofing will amplify
over the central Continental U.S. Thru the day on Friday. As the near-term period
ends late Fri, the upper trof axis will still be centered over the
Mississippi River valley. At the surface, strong high pressure will
steadily shift to the northeast today and tonight and is expected to
be centered over the New England coast by 00z Friday. The high will
continue to drift farther NE thru Friday night and off the coast of
Nova Scotia by the end of the period. As the high moves farther from
our area, deep-lyr moisture will overspread the fcst area from the
south beginning this evening. On Friday, low-lvl winds will remain
northeasterly as The Wedge hangs on thru most of the day and moist upglide
continues thru the day and into the evening. By late Friday evening,
disorganized low pressure will lift out of the northern Gulf of Mexico
and track up and over the Carolinas late Fri into early Sat.

The main concern for the near-term period continues to be the likely
Hood of accumulating freezing rain and possibly some sleet over much
of western NC from tonight thru tomorrow morning. Pops reach likely
range over the GA, western SC, and NC Escarpment zones around midnight.
The cooling of temps remains tricky, with increasing cloud cover but
also evaporative cooling as precip gets underway. The latest run of
the NAM appears a bit quicker to warm temps tomorrow morning as the
parent high moves offshore. The latest model profiles still generate
enough of a warm nose to produce freezing rain where sfc temps are
below 0 degrees c. As previously mentioned, even if the warm nose is
not as strong as fcst, the layer under the inversion will be shallow
and it will be hard to get much refreezing. Overall, the majority of
of our western NC zones between the Blue Ridge Escarpment and I-85,
excluding the Charlotte Metro heat island, will be at or below frz
for at least some of tonight's precip. Accumulation looks unlikely
to exceed a tenth of an inch in most areas, and unlikely to exceed
a quarter inch throughout. As such, the current Winter Weather Advisory
from 02z to 17z Friday looks good. Precip will likely continue into
Friday afternoon, but should be all rain. It will be a cold and
wedge-like day with breezy NE winds. High temps will be near normal
over the far west, but as much as 15 degrees below normal over the
eastern zones.

&&

Short term /Friday night through Sunday/...
as of 250 PM EST thursday: with upper trough axis just west of the
forecast area and plenty of moisture in place given west-southwest flow, an unsettled
weather pattern is expected to persist Friday night through into
Saturday, with all precipitation expected to fall as rain as
temperatures remain just above freezing. Additional rainfall of 0.25-
0.35" will be possible, with higher amounts expected mainly across
the upslope areas. Latest guidance continues to depict the southern
stream trough axis swinging through overhead early Saturday morning,
as the coastal low along the NC coast ejects off to the northeast.
Precipitation looks to linger along the Tennessee border, while elsewhere,
precipitation will taper off by early Saturday afternoon, with
lingering clouds throughout the rest of the day. With colder air
infiltrating in Saturday night, precipitation will likely taper off
as rain/snow showers across the NC mountains. However, given lower
quantitative precipitation forecast amounts with this fcst package, do not expect much in the way of
snow accumulations. Expect a drying trend and gradually clearing
skies on Sunday with nearly zonal flow aloft and a brief period of
sfc high pressure, with an approaching cold front well to the northwest.
Max temperatures will climb just a few degrees above normal Saturday
and Sunday, with min temperatures well above normal.

&&

Long term /Sunday night through Thursday/...
as of 235 PM thursday: not too much in the overall thinking for the
extended period. Zonal flow with weak high pressure over the area
Monday will give way to a slowly developing and moving positively
tilted low wave trough. An attendant cold front is expected to
slowly press through the area tuesdayish. Strong southwest flow and
an increasing moisture tap from the Gulf of Mexico may support a
period of heavier rain in advance of the cold front, focused across
the Southwest Mountains. There also continues to be a non-zero Low
Cape/ high shear severe weather potential focused southeast of I-85.
However, the best upper level forcing remaining well to the north
and orientation of the front gradually becoming more southwest to
northeast may limit these threats. Above normal temperatures well
into the 60s will be possible in advance of the front. Northwest
flow and moisture seems to be limited behind the front so any
turnover to snow across higher elevations would likely be brief with
minimal impacts. Seasonably cold and drier air filters in behind the
front by Wednesday. A long-wave trough centered over the Hudson
Valley may support continued northwest flow atop the region through
later in the week with fair conditions prevailing for most. However,
this pattern may be conducive for northwest flow induced snow
showers across favored mountain locations at some point.

&&

Aviation /20z Thursday through Tuesday/...
strong dry high pressure north of the region will keep skies
mostly clear thru the afternoon. Moist isentropic lift over a
cold sfc high will introduce low VFR cigs this evening. With
the onset of steady precip, most sites will drop to MVFR and
then IFR to possibly LIFR before daybreak. The other complicating
factor will be development of freezing rain as warm precip falls
into the cold boundary layer air. Still thinking that this should
only be an issue at kavl and possibly khky. The sfc high will
remain transient unlike in a true cad event. Furthermore, the
warm air advection is expected to have some impact at the ground, raising temps
during the late overnight. Thus, the freezing rain should transition
back to rain by sunrise. At the other sites, confidence is high
enough on temps staying just above freezing, and on the strength
of the "warm nose" aloft, to keep all precip as rain. However, a
slight downward trend in temps above the boundary layer could
result in pl reaching the sfc. The IFR that develops overnight
is likely to persist thru the end of the taf period. Winds will
remain southerly to southeasterly at kavl and northerly to northeasterly at all the other sites,
reflecting The Wedge-like pattern. A few low-end gusts are possible
especially across the upstate.

Outlook: though rainfall rates will taper off Friday evening,
periodic restrictions are likely until the system departs the
region on Saturday. Dry/VFR conditions will then return until
the next front arrives Monday night or Tuesday.

Confidence table...

20-02z 02-08z 08-14z 14-18z
kclt high 100% Med 77% Med 78% high 83%
kgsp high 100% high 86% low 57% Med 75%
kavl high 100% high 100% low 54% low 58%
khky high 100% high 100% high 80% Med 73%
kgmu high 100% high 84% low 59% Med 68%
kand high 100% high 89% low 59% low 50%

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:

Www.Weather.Gov/gsp/aviation

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...Winter Weather Advisory from 9 PM this evening to noon EST
Friday for ncz033-035>037-049-050-053-056-057-063>065-
068>070-501>509.
SC...none.

&&

$$

National Weather Service Glossary of Abbreviations