929 
fxus62 kgsp 082006 
afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
306 PM EST Thursday Jan 8 2009 


Synopsis... 
a ridge of high pressure will build over the region tonight and then 
move off the Atlantic coast late Friday. A low pressure system and 
its associated cold front will cross the area Saturday evening 
followed by another surface ridge thats builds in early next week. 


&& 


Near term /through Friday/... 
northwest flow precipitation along the Tennessee border will decrease late 
this afternoon as moisture becomes more shallow. The Winter Storm 
Warning and Winter Weather Advisory will be cancelled early. 


The upper trough axis over the southern Appalachians will move 
offshore tonight...and a flat upper ridge will move over the 
southeast on Friday. Northwest flow snow showers will continue to 
taper off this evening as moisture decreases further...despite 
increasing low level winds that veer to become more perpendicular to 
the mountains. 


Winds will diminish in the Lee of the mountains...but 850 winds and 
downward vertical motion will support a lake Wind Advisory in the 
mountains...which will be issued as the winter weather products are 
cancelled. Minimum temperatures will be slightly below climatology 
tonight... and any melt water on roads in the mountains could freeze 
to create black ice. 


Friday features a nearly clear sky...expected perhaps north of 
Interstate 40 where some middle cloud associated with a warm front west 
of the mountains will extend. Maximum temperatures will be near 
climatology...as downslope flow will have ended...but southerly flow 
on the back side of the ridge will be slow to set up. 


&& 


Short term /Friday night through Sunday night/... 
as of 245 PM Thursday...the trend in the model guidance is to delay 
the approach and arrival of the next cold front Friday night and 
Saturday. The GFS is an outlier with its depiction of precipitation 
developing over the mountains by sunrise on Saturday. Instead...prefer to 
follow a consensus of the other models...mainly NAM/ECMWF...that 
keep precipitation to our west tomorrow night. Precipitation chance has been 
eliminated as a result...which avoids any messy situation late at 
night across the mountains warm advection ahead of the approaching front 
should help to keep temperatures a bit warmer...especially at the higher 
elevations which should be a few degrees warmer than the valleys. 


All signs point to having the ingredients in place for another 
decent round of precipitation Saturday and Saturday evening...with deep 
moisture along the front and deep upper forcing. Mosguide has 
already jumped to categorical across parts of the forecast area but 
prefer at this point to merely trend the forecast in that direction... 
thus everyone gets a likely pop by afternoon. Not anticipating any 
precipitation type issues as the front moves through late in the day or in 
the evening. Kept the likely pop going over the eastern zones before 
midnight and over the western mountains colder air should start moving in 
after midnight with thickness falling rapidly. This should bring 
snow levels down quickly to where any leftover precipitation would be snow 
after midnight. Not quite sure how the temperature trend will work but it 
might stay warm well into the evening if the front is any more 
delayed...as it is...some parts of the forecast area mainly east have 
been bumped up as much as a category. 


The NAM supports a northwest flow snow event through Sunday afternoon which 
seems reasonable at this point...so precipitation chances have been raised 
along the Tennessee border. Potential is for a few inches before it winds 
down late in the day...so this will have to be watched. Maximum temperatures 
have been nudged down a degree or two. Sunday night should be quiet 
with high pressure ridging in from the west. 


&& 


Long term /Monday through Thursday/... 
as of 300 PM Thursday...models agree that long wave trough will remain 
over the east through the period. Significant disagreement remains on 
the timing...strength and position of the short waves that will move 
through the trough through the period. Previous thoughts were to lean toward 
the HPC solution which trended toward the European model (ecmwf) and its ensembles. 
However...the 12z European model (ecmwf) has made a significant break in continuity 
as it now phases short waves during the Tuesday time frame and has a 
strong coastal low in addition to a northern stream clipper. The GFS has 
only a clipper. That said...the GFS has broken continuity as 
well...but during the Wednesday time frame. The GFS flattens the 
trough and allows a significant warm up with rising thicknesses. Later 
in the period...the previous European model (ecmwf) was already significantly slower 
than the GFS with a potential developing winter storm. The GFS still 
rapidly swings this system through Thursday and Thursday night...while the European model (ecmwf) 
has slowed the system further. Ensemble spaghetti plots show quite a 
spread of solutions through the period indicating low confidence. 


Therefore...given all the new model differences...have trended the 
forecast toward the HPC consensus and ensemble guidance. This means 
a dry forecast Monday...then mountain chance pop late Monday night and Tuesday 
morning. Temperatures are cold enough that snow would fall over the 
mountains...with sprinkles or flurries elsewhere. That said...have 
removed any significant quantitative precipitation forecast or snow amounts given the range in model 
solutions. Kept the forecast dry through the rest of the period. This 
seems the prudent approach for now given the run to run and model to 
model inconsistency. If any probability of precipitation were added for this period...they 
would be slight chance at best...and this keeps US from having period 
after period of slight chance pop for three days. 


For temperatures...again followed the consensus...and kept away from 
the very warm middle week mex temperatures. These are way out of line 
given the ensemble 850 mb temperatures which are significantly colder. That 
said...ensembles show 850 mb temperatures only one Standard deviation below 
normal. Therefore...near normal temperatures falling below normal still 
look good. However...Arctic outbreak not yet in the forecast. 


&& 


Aviation /20z Thursday through Tuesday/... 
at kclt...winds are expected to veer to the northwest and become 
gusty late this afternoon as a cold front crosses the area. Clouds 
cover will diminish in a drying downslope flow. Northwest winds will 
diminish this evening...and by middle morning light southerly winds are 
expected as the surface ridge axis moves by. 


Elsewhere...GFS cloud heights were used...as they were least 
pessimistic in the mountains. Low VFR ceilings will persist at kavl 
into the evening in a moist upvalley flow...while an afternoon 
frontal passage followed by drying downslope winds decreases sky 
cover at other sites. Winds will become gusty from the northwest 
this afternoon...diminishing this evening. By middle morning a 
southerly or even southeasterly flow will develop around the 
southwest side of the surface ridge. 


Outlook...dry high pressure begins to leave the area Friday night. On 
Saturday...a cold front will bring showers into the area...with the 
potential of restrictions. Drying high pressure builds back in late 
on Sunday and into Monday. 


&& 


Gsp watches/warnings/advisories... 
Georgia...Lake Wind Advisory until 6 am EST Friday for gaz010-017. 
Lake Wind Advisory until 7 PM EST this evening for gaz018-026- 
028-029. 
NC...lake Wind Advisory until 6 am EST Friday for ncz033-048>053- 
058-059-062>065-501-503-505-507-509. 
Lake Wind Advisory until 7 PM EST this evening for ncz035>037- 
056-057-068>072-082-502-504-506-508-510. 
SC...lake Wind Advisory until 6 am EST Friday for scz001>003. 
Lake Wind Advisory until 7 PM EST this evening for scz004>014- 
019. 


&& 


$$ 
Synopsis...jpt 
near term...jat 
short term...PM 
long term...rwh 
aviation...jat 

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