168 
fxus62 kgsp 211530 
afdgsp 


Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
1030 am EST Sat Nov 21 2009 


Synopsis... 
high pressure over the region will shift north over the weekend. 
This will allow a low pressure system to move east along the Gulf 
Coast then north along the Carolina coast Sunday through Monday. 
The low is expected to reach the New England coast by Tuesday as 
weak high pressure remains over the southeast states. Another cold 
front or coastal low could move into the area by the middle of next 
week. 


&& 


Near term /through tonight/... 
current forecast is in good shape. I increased the opaque cloud 
cover today based on current trends...but that/S all I needed to 
tweak. Previous discussion follows... 


Current infrared satellite imagery indicates that cirrus shield continues 
to stream in from the southwest...and is expected to continue 
throughout the near term period. Main forecast problem through 
tonight centers around the developing wedge and cyclogenesis over 
the northern Gulf. Current suite of model runs favor much slower 
timing of deep layer moisture spreading over top of the cold dome. 
In fact...even the faster GFS solution supports a dry forecast 
through 06z. Therefore...I have trimmed probability of precipitation significantly featuring 
only low end chance probability of precipitation across the southwestern third of the 
forecast area...similar to the sref. As it stands now only the GFS brings much 
in the way of isentropic lift into the area by 12z so as it stands 
even this pop could be a bit overdone. The surface high slides into 
a more favorable wedge position through the afternoon and should be 
well in place by middle to late morning. 


I have blended high temperatures downward a bit today...more toward MOS 
guidance. With considerable high clouds in place...should help limit 
maximum temperatures. Expect low 60s outside the mountains...and upper 50s in 
the mountain valleys. Lows will dip into the low to middle 40s. 


&& 


Short term /Sunday through Monday night/... 
as of 230 am EST Saturday...a southern stream upper low will move 
from the Gulf states to the Appalachians on Sunday and Sunday night 
while weakening. On Monday and Monday night and upper ridge 
amplifies over the southeast. The GFS continues to be faster to 
spread moisture and isentropic upglide northeast across the area on 
Sunday...and cloud cover and probability of precipitation have been reduced to favor the 
slower NAM/European model (ecmwf) solutions. Probability of precipitation have been increased Sunday night 
and Monday to indicate a slower evolution of cold air damming... 
and a more prolonged event...with less of a decrease of clouds on 
Monday night. Small probability of precipitation will even be carried along the Blue Ridge 
Escarpment Monday night where the European model (ecmwf) indicated some lingering 
moist upslope flow. Maximum temperatures have been raised a bit on 
Sunday...and lowered on Monday...to indicate the delayed onset of 
cold air damming...with minimum raised Sunday night and Monday 
night. None of the models has near the amount of upglide or upslope 
flow...so only modest rainfall amounts wil be carried...with some 
terrain enhancement near the south and southeast facing Blue Ridge 
Escarpment of Georgia and SC. 


&& 


Long term /Tuesday through Friday/... 
as of 315 am Saturday...HPC continues to prefer the European model (ecmwf) for the 
medium range. There are differences between it and the GFS and 
Gem...which have similar solutions. However...the 00z European model (ecmwf) has 
trended toward the GFS and Gem solutions. Given these differences 
the extended forecast remains low confidence. 


The models agree that a closed upper low over the over the plains 
moves east as a strong short wave dives in behind the low creating 
an eastern US trough. This wave then merges with or takes the place of 
the previous low as it then moves off the coast. At the surface...cad 
begins to erode...but there could be some lingering patchy rain Tuesday 
and Tuesday night. Then another occluded fnt approaches the area Wednesday as a 
Gulf Coast low develops. The GFS/Gem keep the forecast dry as the dying 
occluded fnt has little moisture and the coastal low remains very 
far south. The European model (ecmwf) has much more moisture over the area as the low 
rides along the coast. The European model (ecmwf) then dries things out Thursday as the 
low moves up the East Coast and away from the area. Both models show 
some northwest flow moisture Thursday and Friday. However...the GFS is much wetter 
and colder suggesting a p-type of snow. Given all this 
uncertainty...will keep probability of precipitation limited to slight chance. Temperatures start out 
above normal then cool below normal. Kept mountain temperatures above freezing 
during the periods with precipitation...for a rain p-type. 


&& 


Aviation /16z Saturday through Wednesday/... 
at kclt and elsewhere...high clouds will remain across the area 
through the day...leaving all sites with an overcast cirrus layer 
for the early taf period. A surface high will slowly move into a 
better cold air damming position by tonight...and winds will respond 
accordingly going northeasterly...northerly at kavl...by middle morning. Expect a southeasterly 
wind at kavl during the afternoon...returning to northerly during the 
evening. The latter part of the taf period does present some 
challenges mainly because of a change in the timing of low clouds 
and precipitation favored by the various model solutions. Even the fastest 
model solution...the GFS...keeps things dry across the County Warning Area through 
nearly 12z and also keeps any ceiling restrictions to beyond this 
taf period. Therefore...will keep things VFR at all sites...but will 
bring in middle level ceilings and scattered low VFR clouds by 06z. Low VFR ceiling 
is possible at kand by the end of the period...but will likely hold 
off until after. 


Outlook...cloud ceilings will gradually lower Sunday morning with 
MVFR conditions expected by late morning. Rain will develop over the 
region on Sunday with widespread IFR ceilings and MVFR fog expected to 
last through most of Monday. Fair and dry weather is then expected 
by late Tuesday into Wednesday. 


&& 


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


&& 


$$ 
Synopsis...rwh 
near term...bsh/McAvoy 
short term...jat 
long term...rwh 
aviation...bsh/rwh 

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