Weather

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fxus62 kgsp 060548 
afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
1248 am EST Tuesday Jan 6 2009 


Synopsis... 
a stationary front will remain across the western Carolinas down to 
Louisiana through Wednesday. Areas of low pressure will move 
northeast across the front today and tonight. The front will move 
east of the region Wednesday...followed by cool high pressure until 
the next front reaches our area in the weekend. 


&& 


Near term /through today/... 
the large rain shield associated with surface wave moving along the 
stationary front has made its way into southeast tenn/north central 
Georgia as of 02z. This activity will begin overspreading the County warning forecast area 
over the next couple of hours...with categorical coverage expected 
in most areas by 08z or so. Will continue with 100 probability of precipitation area-wide. 
Quantitative precipitation forecast through daybreak should range from 3/4 to 1 inch across the 
Southwest Mountains...to less than 1/4 inch across the lower Piedmont. We 
actually saw a couple of isolated ts develop across the NC foothills 
this evening...and these have moved into the northwest Piedmont. 
However...looking upstream...have seen very little lightning 
activity in the rain shield...and with the lower atmosphere gradually 
stabilizing...will resist the temptation to add thunder to the 
overnight forecast. 


The main change to the 1st period will be to increase min temperatures 
about a category. As of 02z...temperatures were hovering around 60 across 
much of the area. With widespread cloud cover/increasing rainfall 
activity...the 10 degrees of cooling advertised by the current 
forecast appears a bit much. 


Tuesday...amplifying 500 mb trough will ripple east toward the southeast 
during the day. Low level forcing along the front will deepen within a 
very moist precipitable water airmass...precipitable waters  ranging from 1-1.3 inches. The greatest 
lift will likely remain over the mountains and I-40 corridor through the 
day. Mesoscale models indicate a surge in high pressure across the 
NC foothills and Piedmont associated with cad. Temperatures may 
slowly cool across the I-40 corridor east of the mountains after sunrise. 
Elsewhere...widespread cloud cover and coverage of moderate rain showers 
will limit diurnal heating to within 5-8 degrees of morning 
lows...with near 50 around khky to near 60 around kand. 


&& 


Short term /tonight through Friday/... 
as of 245 PM Monday...low level isentropic upglide and moisture transport 
increase again Tuesday evening. The best upper divergence and 700 mb 
frontogenesis is located along and just to the west of the 
mountains. However...the strongest low level warm advection and 
frontogenesis at or below 800 mb will be along and just east of the 
mountains...and heavy rainfall is likely to continue overnight over 
the mountains and foothills. I will not expand the Flash Flood Watch 
farther to the northeast yet...but later shifts will probably have 
to expand it to cover the rest of the NC mountains and foothills. 
Despite the better forcing being off to the west...the weak wedge 
over the NC Piedmont still has ME concerned. With the strong SW flow 
oriented nearly orthogonal to the boundary...there could be enough lift 
to generate heavy rain over the Piedmont and even into the Charlotte 
metropolitan area. 


A surface wave will move up the front Tuesday nght as well. The GFS is a 
little faster with the translation of the wave and it is several mb 
deeper with the low than the NAM. Considering the overall positive 
tilt to the upper trough...a blend of the two is a good bet. Warm 
sector instability will be minimal though there will be very strong 
shear over the region. At this time I think the severe threat is very 
small...but if some moisture can pool along the weak wedge...then 
there could still be a chance of an isolated severe storm...or even 
tornado over the northern upstate or southern Piedmont. 


The strong low level jet will move east of the forecast area after 12 UTC Wednesday...but 
the trailing southern stream short wave will result in strong divergence-q on 
Wednesday and a possible continuation of lighter precipitation at least through the 
morning hours. 800 mb temperatures are looking warmer than on past runs...and I 
Don/T have much snow over the mountains during the day. There will be 
strong wind gust potential immediately behind the front on Wednesday owing 
to deep mixing and strong pressure rises. 


The NAM is not as cold or as tight with the gradient as the GFS 
behind the front Wednesday night. I went with a blend of the 
two...populating my grids with the sref where I could. This keeps 
probability of precipitation generally in the chance range Wednesday nght and into early Thursday 
over the Tennessee border counties with very light northwest flow 
snowfall amounts as model soundings Don/T show and amount of 
saturate air in the good snow Crystal growth area. 


&& 


Long term /Friday night through Monday/... 
as of 230 PM Monday...the pressure gradient and associated winds 
will relax Thursday night as a flat upper ridge builds in and 
persists through Friday. 


The next clipper system will dig through the Midwest Friday...with 
the resulting broad upper trough axis likely crossing the region on 
Saturday. The ecm is quite a bit slower than the operational GFS in 
bringing the moisture along the frontal zone into the area on 
Saturday. This slower ecm solution is closer to the ensemble means 
and is preferred. This will mean less of a chance of freezing rain ahead of 
the system Sat morning...with lower quantitative precipitation forecast. And...some degree of probability of precipitation 
will be carried through Sat night with the slower departure of the 
frontal zone. 


Upper heights then briefly rise on Sunday...with strong model 
differences developing in the handling of low pressure upstream in 
the upper Midwest. Any moisture return in the SW flow ahead of this 
system should hold off until Monday afternoon. 


&& 


Aviation /06z Tuesday through Saturday/... 
at kclt...IFR conditions are expected west/in the next hour or two...as 
a heavier steady rain develops across the area. Precipitation shield will 
shift north Tuesday...as a warm front lifts north of the area. 
Guidance IFR/MVFR through the period. However...as rain becomes more 
showery and intermittent...visibility and ceilings should improve (and could become 
VFR by middle/late afternoon. Winds will be generally 5-10 kts out of the 
S/SW. 


Elsewhere...rain shield has moved in...and visibilities/ceilings are gradually 
lowering west/ the precipitation. All sites should be LIFR/IFR west/in the next 
couple of hours (if they are not already). The bulk of rain will 
shift northward across the Tennessee Valley/NC mountains/middle Atlantic...leaving the upstate 
will just some scattered/showery activity. So it is expected that conds 
will improve to at least MVFR by middle/late afternoon. Winds will be 
generally light out of the S/SW. 


Outlook...showers and perhaps a thunderstorm will accompany a cold 
front...which will push into the area early Wednesday. Precipitation will 
end...and conds improve behind the fnt later on Wednesday...W/ gusty 
westerly winds expected. 


&& 


Gsp watches/warnings/advisories... 
Georgia...Flash Flood Watch through Wednesday evening for gaz010-017-018. 
NC...Flash Flood Watch through Wednesday evening for ncz048-051>053- 
058-059-062>065. 
SC...Flash Flood Watch through Wednesday evening for scz001>006. 


&& 


$$ 
Synopsis...McAvoy 
near term...jdl/Ned 
short term...McAvoy 
long term...hg 
aviation...Arkansas 

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