Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
1017 am EST Thursday Jan 29 2015

Synopsis...
a cold front will cross the forecast area later tonight with high
pressure building in its wake on Friday. The high will linger over
the region into Sunday before a moist low pressure system develops
to our southwest and brings widespread precipitation to the area
late Sunday and Monday. Another Canadian high moves back into the
area late Monday and lingers into midweek.

&&

Near term /through tonight/...
1000 EST Thursday update...latest surrounding regional radars show a
band of light returns was approaching the southern Appalachians from
the Tennessee Valley as of this morning. Latest hrrr and other mesoscale
models indicate that this band of light precipitation associated with an
upper wave will move east across the NC mountain zones between 16-18z and
across the NC foothills and portions of the northern NC Piedmont
18-20z. Therefore...probability of precipitation were updated to account for this through this
afternoon. However...precipitation amounts appear to be very light given
relatively dry airmass and shallow moist layer. No wintry p-type is
expected through this afternoon given good warming trend.

1115 UTC update...winds were updated with a two to one blend of the
NAM and adjmav...favoring the former for Stringer winds and gusts in
the mountains...both ahead of and behind the approaching cold front.
Gusts still appear to reach advisory criteria only at the very
highest mountain peaks late tonight...so no advisory is planed at
this time. Sky cover was updated from infrared satellite imagery.

At 230 am EST Thursday...an upper ridge of modest amplitude will
cross the eastern Seaboard this morning...while and upper trough
moves from the MS River Valley to the Ohio and Tennessee River valleys. A
shortwave moving through this trough will reach the central
Appalachians by midday. The ridge moves to the Gulf Stream this
afternoon...while the shortwave moves off the middle Atlantic coast.
Another and more robust shortwave moving through the trough reaches
the central and southern Appalachians in the middle of Thursday
night...moving east of our area by morning....while the trough axis
reaches the East Coast.

A surface ridge along the East Coast will move offshore today...
while a cold front moves from the MS River Valley to the
Appalachians. Precipitation ahead of the front will reach the
mountains sometime this morning...perhaps early enough to start as
light snow. With warming...snow levels rise above the mountain tops
today. No appreciable snow accumulations are expected this morning.
The models do not show much depth to the moist layer east of the
Blue Ridge...and precipitation chances are quite limited there.

Frontal passage takes place overnight...with downslope drying in the
Lee of the mountains...and moist northwest flow into the NC mountains
bordering Tennessee. Snow level are expected to fall to the valley floors
this evening...while coverage retreats closer to the Tennessee border.
Right now it appears that snowfall accumulations will run just under
two inches overnight...but advisory criteria will only be approached
on the very highest peaks...so no Snow Advisory is planned. Winds
will increase in the tight pressure gradient behind the front...but
Wind Advisory criteria will only be reached late tonight on peaks
over 5000 feet...so no Wind Advisory is planned at this time.

High temperatures will run near normal today as warm advection under
the upper ridge is offset by increasing cloud cover...and low
temperatures tonight will run slightly above normal as winds keep
the atmosphere mixed.

&&

Short term /Friday through Saturday night/...
as of 115 am Thursday...the short term begins with an increasing
subsidence zone and a waning nwfs event. Dry cp air will begin
filtering into the mountains and limit the additional snowfall potential.
Likely around another half to one inch through 21z across Western Mountain
spine. Good 850 mb cold air advection along with deep subsidence will continue the
strong winds/gusts across the NC mountains...with advisory or even
warning criteria being met above 4000 feet as model soundings
indicate an upward trend in velocities atop the mixed layer through
middle afternoon. Will keep the severe weather potential statement mention for now and the day shift
will likely issue the npw as the event moves into the 2nd period.
Winds will relax slowly through the evening with a good 850 mb tgrad
eventually loosening after 00z. Maximum temperatures will about 15 degrees below
normal mountain valleys and 3-5 degrees below normal non/mtns. Heights
begin to rise Friday night and flat ridging will be in place by Sat
afternoon. This will enable the cp surface high to cross the forecast area
through the period becoming situated off the Carolina coast by 12z
sun. Thus...skies will clear and temperatures will remain a few degrees
below normal as a low level Theta/east trough mixes in.

&&

Long term /Sunday through Wednesday/...
as of 255 am Thursday morning...the medium range forecast picks up at
12z on Sunday with relatively zonal upper flow over the Carolinas
and a deepening upper trough to our west. Over the next 24 to 36
hours...the trough axis is expected to pass overhead with more zonal
flow in its wake. During this period...a southern stream 500 mb low
spins up over the Baja California peninsula and gradually drifts NE towards our
area. By the end of the medium range...this low will likely get
absorbed by another deepening upper trough.

At the surface...high pressure will be moving off the southeast coast early
Sunday as another low spins up to our west-southwest over the lower miss River
Valley. The associated broad area of deep layer moisture will spread
over the region through the day with overcast conditions and likely
probability of precipitation by late sun/early Monday for pretty much all zones. The GFS
remains about 12 hours faster with the development of this system
compared to the European model (ecmwf) and CMC. In the lows wake...another Canadian
high pushes back into the region from the northwest on Monday and dries things
out. The center of the high will pass over the area on Tuesday and move
offshore by early Wednesday. Beyond this point the model guidance remains
fairly ambiguous with the synoptic pattern. The GFS brings another
reinforcing Canadian high down over the region on Thursday and keeps
things dry. The European model (ecmwf) tries to spin up another Miller a type low
over the northern Gulf of mex on Wednesday and move the system across the
Carolina coast late Wednesday/early Thursday and after that...it dries things
out like the GFS. The previous runs of the GFS hinted more towards
the Miller a type low developing...so the current dry scenario looks
questionable. As for the sensible forecast...I trended a bit cooler for
the Monday/Tuesday period with lows early Tuesday in the low to middle 20s
across the the lower elevations and teens across the higher terrain.
Pop trends were not changed much for the first half of the period
with best chances late sun through early Monday. For tues/Wed...I kept
probability of precipitation at slight to solid chance due to the continuing uncertainty
with the Gulf low.

&&

Aviation /15z Thursday through Monday/...
at kclt...VFR. A high level cirrus ceiling will spread east over the
field this today ahead of a cold front. Guidance has backed off
support for a low VFR ceiling this evening...instead preferring a middle
level ceiling...before erosion late tonight. Some low clouds will be
carried beneath however. Evening precipitation chances still appear
too low to mention at this time. S winds will come up this morning
veering SW this afternoon...with gusts increasing. Frontal
passage and northwest winds will late tonight.

Elsewhere...high level cirrus ceilings will set up this morning in
advance of a cold front. Ceilings fall to low VFR this evening at kavl
and kand...but guidance is less certain at khky...kgsp and
kgmu...where a middle level ceiling is indicated. At the last three
sites...low clouds will be carried beneath the ceiling. The ceiling falls to
MVFR at kavl late tonight in moist northwest flow behind the front.
Precipitation chances ahead of the front will too low to
mention...although kavl will be a close call. If some snow can fall
at kavl behind the front late tonight...visibility might be reduced...but
confidence is too low for a mention at this time. S winds are
expected after dawn...then SW by midday...with gusts increasing.
Kavl winds veer northwest this evening as the front moves through...and
foothills winds follow suit late tonight.

Outlook...a pair of clipper systems will bring chances for rain or
snow showers to the NC mountains Friday. Most of these showers will remain
west and north of kavl...but brief periods of restrictions cannot be
ruled out. Otherwise...mainly VFR conditions are expected across the
area until moisture begins to return Sunday. Drying is expected on
Monday.

Confidence table...

15-21z 21-03z 03-09z 09-12z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 92% medium 75%
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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...jpt
near term...jat/joh
short term...sbk
long term...jpt
aviation...jat

National Weather Service Glossary of Abbreviations