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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
613 PM EST Wednesday Dec 24 2014

Synopsis...
a cold front will cross our area this evening. High pressure will
arrive on Thursday and linger into Saturday. A pair of cold fronts
will approach the area on Sunday...crossing the area Monday. Gusty
winds will follow the front on Tuesday.

&&

Near term /through Thursday/...
as of 5 PM...regional 88d loops depicting just a smattering of
very light returns across the County warning forecast area with synoptic scale feature of
interest being The Wedge boundary making very slow progress into
the Piedmont...but still just southeast of a kgrd-kuza line. Areas of
dense fog did indeed develop within the immediate cool side of
the The Wedge boundary.

As of 230 PM...still a dynamic situation across the western
Carolinas. The warm conveyor belt is all but east of the forecast area
as of 19z...with the back edge of the steadier precipitation expected to be
east of Union County NC by 20z. That should take care of any
lingering flash flood/severe weather threat. Behind the precipitation
band...continued weak forcing...especially moist low level upglide
above a decaying wedge east of the Blue Ridge...should continue to
force scattered shower development through the rest of the afternoon
east of the mountains the low level upglide should slide east of the forecast area
by early evening...carrying most of the precipitation with it. Precipitation
probabilities reflect this thinking. Another problem will be the
destruction of The Wedge...which appears to be draining more than
eroding from the S because of the precipitation band associated with the
warm conveyor belt. The Wedge boundary has reached eqy but might
have trouble making it much farther northwest as we are going to be losing
daytime heating soon. With The Wedge drainage and the low level
upglide...ceilings are actually dropping and visby is falling as the
stratus layer gets pushed to the ground. This could require a brief
dense fog advisory if visby gets much worse. Temperatures will continue to
be tricky. Many places will not reach the forecast high because The
Wedge will not break in time.

Over the next 24 hours...the upper trough to the west will deamplify
tonight as the bulk of the energy lifts past well to our north...then it
cross the forecast area Christmas morning. The surface front will push
across the region during the late evening or early morning hours.
Behind the front...colder air will filter in. Enough low level
moisture will remain to be pushed up the slope on the Tennessee side of the
mountains...so a chance pop was kept there. Precipitation should change over to
snow showers through daybreak. Amounts will be limited by the
westerly flow and the gradual arrival of colder air...so an advisory
is not planned. The duration will also be limited...as the models
depict the low level moisture pulling out and drying up by middle
morning at the latest. There should be some weak middle/upper forcing
as this moves through on Christmas morning...but moisture will be
very limited...so no precipitation is expected. Instead...the bulk of the
day will be mostly sunny...with temperatures maybe a category above normal.

&&

Short term /Thursday night through Saturday/...
as of 215 PM EST Wednesday...an upper ridge will amplify over the southeast
Thursday night through Friday...with its ridge axis pushing off the East
Coast by Friday night. The upper pattern becomes southwesterly by early Sat as
an upper trough digs into the western Continental U.S. While an upper ridge
builds over the western Atlantic. A series of upper disturbances is
forecast to ripple through the southeast in southwesterly flow aloft Sat afternoon into
Sat night.

At the surface...a modifying high pressure over the southeast states
will gradually shift east through Friday night...yielding dry weather and
warmer temperatures across much of the region Thursday night through Friday night.
As the center of high moves offshore by Sat...we will see increasing
Gulf moisture advection in deep southwesterly flow. Probability of precipitation ramp up from SW to
NE into the chance range mainly across the western half of the County Warning Area
Sat afternoon into the evening in light of deeper moisture
associated with upper disturbances approaching from the SW. Temperatures
will run 3-7 degrees above normal through the period.

&&

Long term /Saturday night through Wednesday/...
as of 215 PM Wednesday...models agree on another round of unsettled
weather across the County warning forecast area for most if not all the medium range...as a
blocky pattern sets up across western North America. A deep persistent
trough will set up across The Rockies/intermountain west...while an
upper ridge lingers along the East Coast. This will place the County warning forecast area
within a deep layer southwesterly flow through the medium range. At the surface...a
large high pressure system will gradually build from west to east across
the Great Lakes...while a surface front stalls in a SW-NE orientation
from the central Gulf to the Middle Atlantic States.

The op models are in disagreement on the timing and potency of a
series of shortwave/impulses that eject from the long wave
trough...and interact with the quasi-stationary front across the
southeast states. The first wave will likely spread a period of
deeper moisture and southwesterly low level upglide flow atop the County warning forecast area Saturday
night into Sunday. The 00z ecwmf had a strong surface wave develop...but
the 12z run has drop that solution...and is in line more with the GFS.
The result is persistent clouds and upglide for Sunday and Monday as
well. Probability of precipitation will be likely to categorical Saturday night and
Sunday...tapering to high-chance on Monday. We will be on the warm side
of the front...with above normal temperatures expected.

On Tuesday...the surface high to our north finally gets far enough east to
allow for cold air damming to set up. This should result in cooler
temperatures Tuesday. However...the 12z European model (ecmwf) still shows a brief break in
the upglide and probability of precipitation...with perhaps some sunshine...weakening the
cad. The GFS is wetter and cooler for Tuesday. Given the likely
cad...I went with the cooler GFS guidance (close to wpc as well). On
Wednesday...the ecwmf ejects the southern stream energy...and develops a
low pressure system in the Gulf...and tracks a Miller-b system along the
Appalachians on Thursday. The GFS holds the southern stream trough/cutoff
low much longer...and has an Ohio Valley low pressure system track for
Friday and Saturday. Given the uncertainty...went with the super
blend for Wednesday...with just a slight chance pop and below normal
temperatures. Despite the unsettled weather...there does not look to be
much potential for wintry precipitation for our region through next Wednesday.

&&

Aviation /23z Wednesday through Monday/...
at kclt...with The Wedge boundary very close to the terminal...enough
mixing seems to have occurred to improve the surface visibility but it still
may take until the arrival of the cold front...at some point later
tonight...raise LIFR ceilings. The good news is that sunshine returns
for Christmas day with westerly winds averaging 5-10 kts.

Elsewhere...a cold is making inroads into the far western forecast area this
evening which will scour low ceilings and visibility from west to east this
evening. VFR on tap all day tomorrow.

Outlook...dry high pressure will build back into the region Thursday
night and Friday with VFR conditions. Another cold front will arrive
from the northwest Saturday night and may stall over the mountains that may
allow restrictions to develop by Sunday morning...perhaps continuing
through the middle of next week.

Confidence table...

23-05z 05-11z 11-17z 17-18z
kclt medium 75% low 55% high 100% high 100%
kgsp high 83% high 83% high 100% high 100%
kavl high 100% high 83% high 100% high 100%
khky low 58% low 48% high 100% high 100%
kgmu high 83% high 83% high 100% high 100%
kand high 100% high 83% 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...dense fog advisory until 8 PM EST this evening for ncz035>037-
056-057-068>072.
SC...dense fog advisory until 8 PM EST this evening for scz001>013-
019.

&&

$$
Synopsis...rwh
near term...csh/PM
short term...joh
long term...Arkansas
aviation...csh/PM

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