Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
229 PM EST Monday Dec 22 2014
a moist airmass will remain atop the region before a cold front
crosses by Wednesday night. Dry high pressure will arrive for
Christmas day and linger into Friday. Another cold front will cross
the area on Saturday...followed by high pressure on Sunday.
Near term /through Tuesday/...
as of 225 PM...cold temperatures...low clouds and patchy drizzle will
continue through the afternoon...except all but the Tennessee border counties
where breaks in the clouds have lead to relatively warm temperatures.
Patchy rain beginning to spread back into the southern tier of the County warning forecast area
and this trend will continue through the afternoon...with some slow
northerly movement of the precipitation.
A series of short waves will move across the area in the southwesterly flow
ahead of a deepening central Continental U.S. Trough tonight and Tuesday. At the
surface...cad regime will remain in place through the period as well
keeping copious amounts of low level moisture across the area and
northeasterly surface flow...except across the mountains where southerly flow prevails. The
low level flow will remain southerly as well...and strengthen tonight
increasing isentropic upglide across the area. Therefore...rain should
return along with the continuing drizzle through the night. Expect low
clouds and areas of fog to continue or redevelop as well. Cannot
rule out some patchy dense fog toward daybreak...but location not
certain enough to include in the forecast at this time. Lows will be in the 30s
for all but the higher elevations of the southwestern mountains and Tennessee border
counties where they are outside of the cad dome and tap into the
warming 850 mb temperatures. This will also help keep precipitation all liquid.
There may be a brief break in the better lift Tuesday morning...but it
returns through the day...along with deeper moisture and some upper
divergence as a jet streak moves north into the area. Expect rain
and drizzle to become widespread by afternoon...although areas of
drizzle will likely continue even where rain diminishes. Expect
another cool day under the cad. However...would not be surprised if
temperatures warm a few degrees above the Monday readings as the parent
high is shunted eastward and cad takes on more of an in-situ
pattern. The Tennessee border counties will once again be much warmer than
areas under the cad.
Short term /Tuesday night through Thursday/...
at 200 PM EST Monday..12z model runs indicate that some semblance of
a wedge will remain through most if not all of Tuesday night. The NAM
does suggest that perhaps the southern upstate may get uncovered by
12z Wednesday. The upshot for Tuesday night is that isentropic lift will
continue over The Wedge with widespread light to even moderate rain.
Best rainfall rates are expected to be over the western upstate...NE
Georgia mountains and the western Escarpment where best forcing/upslope
flow will become established. Rainfall in those areas is likely to
be upwards of an inch. On Wednesday...the consensus is that the remainder
of The Wedge will gradually be uncovered...but as usual it could
linger well into the afternoon from the central upstate eastward.
Rain/drizzle will become showery as wedge dissipates. The next issue
is the cold front that will translate across the area from the west
as low pressure moves from central Tennessee to the lower Great Lakes on
Wednesday. The 12z models are trending slower with the frontal passage...
with the NAM the slowest of the operational runs....a good 6 hours
slower than the global models. The 09z sref supports the slower NAM.
In regard to timing will use a consensus approach which will move
the front into western sections during Wednesday afternoon and then across
the remainder of the area late Wednesday afternoon/evening. The main
concern with the frontal passage will be the threat for some sort of
qlcs event. All the models show some low end cape upwards of 200
j/kg ahead of the front. Shear/helicity is adequate although not
through the roof but forecast sherb values generally <1. Also...the
upper forcing is not coincident with the front. The upper trough is
forecast to trail the surface front with the main upper forcing
going well north of the region. Hence...chances for severe
convection appear on the low end. The cips analog does give make ME
pause as it paints a 30% area of top analogs producing at least 1
severe event across the piedmonts of NE Georgia...SC and NC. Storm Prediction Center keeps
the mention of severe east of the area...so bottom line is that we
will not not mention the threat of severe in the severe weather potential statement. Dryer air and
downsloping winds will finally scour clouds out east of the
mountains Wednesday night with perhaps a period of some northwest flow snow/rain
showers early Christmas in the mountains.
In regard to Hydro...I have hefty quantitative precipitation forecast in my forecast grids with
storm total rainfall around 2.5 inches over NE Georgia...western upstate
and the western Escarpment. However...antecedent conditions are
benign so do not anticipate any significant Hydro concerns.
Temperatures will be a challenge as a non-diurnal trend will occur
Tuesday night where The Wedge erodes. This would most likely be across
the southern peripherey of the forecast area. Temperatures on Wednesday should
surge well into the 60s as The Wedge dissipates. Cooler temperatures will
then occur Wednesday night with maximum temperatures Thursday near climatology.
Long term /Thursday night through Sunday/...
as of 200 PM Monday...high pressure over the southern Atlantic region will
make for settled conditions in our area Thursday night through early
Sat...before the high is forced offshore by a longwave trough digging
into the central Continental U.S.. global models have delayed arrival of the
cold front on the leading edge of the trough...now suggesting the
front will cross the mountains during the day Sat at the earliest.
The GFS has maintained consistency with earlier runs...deamplifying
the trough over the weekend and allowing dry high pressure to fill in
across the southeast in its wake. Ec and now the opnl CMC-Gem show a
similar evolution albeit markedly different from the GFS. They depict
a shortwave shearing off the trough over the Southern Plains Saturday
which induces Gulf cyclogenesis and makes for a wet Sunday over our
County warning forecast area. This is not indicated by any 22/06z gefs members and only a
couple of the 22/12z Gem ensemble members. Noting how different the
ec/CMC solution is from earlier runs it appears wise to discount it
to some degree. Looking a bit further forward...GFS does feature its
own differences by late Monday as it spins up a new Gulf low at that
time and begins to spread precipitation northward through GA/SC. Confidence in the
forecast is not very high beyond Saturday.
Temperatures should trend upward through Sunday...with mins ending up a couple
degrees above climatology and maxes 5-7 above climatology. Slightly cooler temperatures
return for Monday with high pressure just north of the region.
Aviation /20z Monday through Saturday/...
at kclt...even though rain ended...low level moisture remains high and
continued upglide over the surface cad layer has kept IFR visibility in
drizzle and low clouds across the area. This will continue through the
afternoon...so have gone pessimistic with the taf and keep current
conditions through the early evening. Conditions will be at least LIFR
overnight in ceilings and possibly visibility as drizzle continues and rain
develops. Improvement of IFR is possible Tuesday morning...but LIFR is most
likely to continue along with rain and drizzle. Northeasterly wind 10 kts or
less continues through the period.
Elsewhere...similar conditions to kclt. The biggest difference is
the VFR visibility and IFR ceiling at kand. These may continue through the
afternoon...but deterioration is expected there by evening as well.
Kavl also different with southerly winds through the period...but LIFR
conditions as well.
Outlook...deep moisture continues through Tuesday night ahead of the
next approaching cold front. Low restrictions and widespread rain
showers are expected. A cold front will cross the region from the
west on Wednesday. Dry high pressure will return through late week.
19-01z 01-07z 07-13z 13-18z
kclt medium 71% medium 68% medium 76% medium 78%
kgsp low 59% medium 68% medium 75% medium 78%
kavl medium 64% medium 79% high 83% medium 76%
khky medium 70% medium 68% medium 78% high 84%
kgmu medium 63% medium 62% medium 75% medium 78%
kand medium 68% medium 66% medium 66% medium 76%
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: (must be lower case)