Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
653 PM EST Friday Dec 26 2014
high pressure will move off the Carolina coast tonight and
Saturday as a cold front approaches from the west. Expect
temperatures to be a few degrees above normal this weekend. The
front will cross Monday with a secondary front passing in middle week
dropping temperatures to below normal. The next cold front will
arrive at the end of next week.
Near term /through Saturday/...
as of 650 PM...forecast remains in good shape. Patches of high
clouds will stream NE across the southern appalachian region...as
the 500 mb ridge axis slides east. Surface high pressure is expected to
remain across the region...yielding light to calm winds. I will
update hourly grids to align with observation.
As of 450 PM...latest satellite images indicated a few patches of
high clouds drifting across the western Carolinas late this
afternoon. I will update the forecast to align hourly temperature/dewpoints to
observations and adjust sky cover. Otherwise...the current forecast
remains on track.
As of 200 PM EST Friday...deep moisture associated with a weak upper
disturbance currently over the northern Alabama/Tennessee is expected to lift
neward...passing west of the southern Appalachians through tonight.
This will bring considerable high clouds to the western mountains zones
through the evening hours. Otherwise...we will see continued dry
weather with mostly clear skies as an upper ridge continues to
amplify across the southeast states and a surface high remains in
place. With increasing thicknesses and prevailing weak southerly
winds...morning low temperatures on Sat will be around 2-3 degrees warmer
than this morning/S low.
Sat...moist southwesterly flow develops across the area as the upper ridge
axis moves offshore and an upper trough moves east across The
Rockies. This will allow surface high to migrate off the East Coast
and a surface cold front to move eastward into the Mississippi
Valley through Sat afternoon. Latest models continue to suggest
that a wave of low will develop over the western Gulf and lift NE
along the front through Sat evening. This will draw a deep plume of
Gulf moisture into the southeast. However...models have trended a
bit slower with the onset of precipitation...keeping our County Warning Area dry until 00z
sun. Hence...have gone with dry forecast through Sat evening. Partly
cloudy skies Sat morning will give way to mostly cloudy by Sat
evening in light of increasing moisture. Afternoon temperatures will stay
7-10 degrees above climatology for most locations.
Short term /Saturday night through Monday night/...
as of 230 PM Friday...hope you made the most of the sunshine today and
will make the best of it on Saturday...because it looks like we are
in for another period of cloudy and wet weather starting Saturday
night. The models are in good agreement with spreading forcing and
deep moisture into the region from the southwest starting on
Saturday evening. That said...the models also indicate a delay with
the onset of the precipitation relative to the old forecast. Some middle level
forcing should arrive very late in the day...as a lead wave comes up
from the SW and middle level isentropic upglide commences...but that
will probably not be enough to get the precipitation down to the ground
until after 00z Sunday with an expected dry subcloud layer to
overcome. So...the timing of onset has been narrowed to indicate a
delay then more rapid ramp up of high precipitation probability...
especially over the western Piedmont of NC. The new forecast eventually
cathes up to the old forecast by sunrise on Sunday with a categorical
pop over the west and a likely pop over the east. From that point
Onward...the models keep deep moisture and some manner of forcing
over the region into Monday...be it upper divergence Sunday
morning...middle level DPVA Sunday and Sunday night...or moist upglide
throughout that period. The forecast keeps a fairly high pop...favoring
the mountains a bit through Sunday night. The passage of the main short
wave axis early Monday should bring an end to the better chances of
precipitation. From that point Onward...a slow decrease in coverage on
Monday. We should dry out by sunrise on Tuesday. Fortunately...temperatures
will remain above freezing through the whole period...so no precipitation
type issues are expected at this time. Unlike previous events...the
center of high pressure to our east closer to Bermuda should allow
US to remain un-wedged. Even more fortunately...the models fail to
develop any instability across the region...so severe weather
chances look very low in spite of being free from cold air damming.
Long term /Tuesday through Friday/...
as of 215 PM Friday...models are coming into a little better
agreement on the medium range forecast. Tuesday through Thursday still
looks dry and seasonably cool...as a large modifying Arctic air mass
settles across the middle section of the country under confluent split
flow aloft. A cutoff low in vicinity of California will eventually get kicked
out into the southern stream...at some point late next week. Models
continue to delay the timing of this shortwave trough. Using the
latest consensus...I have delayed probability of precipitation returning until Friday
morning. With a cold air mass still in place when the moisture first
starts advecting back in from the southwest...there may be some p-
type concerns. However...there is still a lot of uncertainty on the
exact timing. Also...regardless of the model solution...it does look
like low level thicknesses will increase as precipitation moves in...such that
precipitation should change over to all rain. Both the GFS and European model (ecmwf) have
only a weak in situ cold air damming wedge across the County warning forecast area by Friday
night. High temperatures will continue to be slightly below normal through
Friday...with near to slightly above normal lows.
Aviation /00z Saturday through Wednesday/...
at kclt and elsewhere...patches of high clouds will stream NE across
the southern appalachian region...as the 500 mb ridge axis slides east.
Surface high pressure is expected to remain across the region...yielding
light to calm winds. The 18z NAM indicates that low...2mb or
less...condensation pressure deficits will develop across the
Piedmont during the pre dawn hours. This appears a little
overdone...but ground fog will likely exist around sunrise. I will
highlight the potential for fog with a tempo of kgsp and kgmu from
10z until 12z. Otherwise...expecting VFR conditions with increasing
middle clouds. Winds will remain light to calm overnight...becoming SW
at 5 kts or less during the daylight hours.
Outlook...moisture will return ahead of the next cold front Saturday
night...which could bring flight restrictions back by Sunday. Some
improvement is expected on Monday...with the next approaching cold
front arriving by Wednesday.
00-06z 06-12z 12-18z 18-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 94% high 88% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 83% high 88% high 100%
kgmu high 100% high 88% high 94% high 100%
kand high 100% high 94% high 94% 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)