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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC 
911 PM EST Friday Nov 21 2008 


Synopsis... 
cool high pressure will continue over our region through the 
weekend. As the high moves off the Carolina coast Sunday and 
Monday...a more southerly flow will warm temperatures a bit ahead of 
the next cold front. This front should cross our area Monday 
night...followed by high pressure again through Thanksgiving. 


&& 


Near term /through Saturday/... 
upper trough axis will continue moving off the East Coast tonight while 
cold surface high builds in from the west. With light winds and clear 
skies...temperatures will drop into the upper teens to around 20...with 
10-15 degree readings expected in mountain valleys. Tweaked clouds over 
mountains...hourly T/dp/rh and winds this update. 


Upper ridge will cross the region Sat as cool...dry...high pressure 
builds over the area. Maximum temperatures will top out in the 40s...with a few 
30s in the mountains some high clouds will move in from the northwest in the 
afternoon. 


&& 


Short term /Saturday night through Tuesday night/... 
as of 200 PM Friday...the models show a shortwave trough in the Ohio 
Valley Saturday evening...that dives southeast to the Carolina coast 
as it moves into a western Atlantic longwave. Heights rise in the 
wake of this feature on Sunday...as a flat upper ridge approaches 
the mountains. The Flat Ridge crosses the area on Sunday night... 
then a well defined shortwave moves around the base of a developing 
upper low over the Great Lakes on Monday...before lifting out to the 
northeast on Monday night. Another shortwave moves around the south 
side of the upper low as it reaches the coastal northeast on 
Tuesday. The upper low remains nearly stationary into Tuesday 
night....with heights quite low over our area. 


Middle and upper level clouds move across the northern portion of our 
area Saturday night in association with the first shortwave...with 
with very dry low levels...no precipitation is expected to reach the 
ground. Sunday features a very dry surface Ridge Crossing the area. 
Late on Sunday night...moisture on the back side of the ridge ahead 
of an advancing cold front will reach the mountains. Although 
increasing cloud cover should hold minimum temperatures up...GFS 
soundings along the Tennessee border show a warm nose aloft...and sub 
freezing surface temperatures...suggesting the possibility of 
freezing rain at the start of the event. Any quantitative precipitation forecast would be quite 
limited at that time...and only minor ice accumulations would be 
expected. The front moves rapidly east on Monday...accompanied by 
scattered showers and modest quantitative precipitation forecast. 


By Monday evening...the Lee of the mountains will dry out..and a 
northwest flow snow event will begin...lasting into Tuesday night as 
moisture remains along the Tennessee line in cold upslope flow. Arguing 
against big accumulations are shallow depth of the moisture... 
limited moisture in the ice nucleation zone...and maximum 
temperatures above freezing Tuesday afternoon. In fact...valley 
floors should mix with or change to rain showers late Tuesday. Probability of precipitation 
will decrease as moisture wanes Tuesday night. 


Temperatures warm from near middle winter values Saturday...to just 
below normal Monday under upper ridging ahead of the front...then 
crash back to middle winter values on Tuesday as heights fall with the 
development of the upper low to our north. 


&& 


Long term /Wednesday through Friday/... 
as of 130 PM Friday...moderating hi pressure will slowly weaken...and 
shift S and east for the period from Wednesday into Friday. Meanwhile...upper 
levels become blocked over the eastern Pacific...as Omega/Rex block tries to 
set up in vicinity of west U.S. Coast. Both the 12z GFS and 00z/21 European model (ecmwf) are 
in rather good agreement on a piece of energy ejecting from upper low 
across the southwest states Thursday...and tracking into the Southern 
Plains Friday. Both models show plenty of low level moisture advecting 
northward ahead of the system...with synoptic lift producing moderate-heavy precipitation 
across the south-central states. Per the models...moisture and lift 
reaches the western third of our forecast area by late Friday morning...with 
thicknesses supporting all rain p-type from onset. However...GFS 
soundings in the mountain valleys did show enough sub-freezing air to 
support at least some early morning -fzra/fzdz. Forecast min temperatures 
Friday morning do dip below freezing in the mountains of course...this 
is new day 7...so there is little confidence in timing of onset of 
rain...and what the temperatures will be at onset. For now...I went with 
mex/HPC blend on pop and temperatures for days 6-8...resulting in slight chance 
to low-end chance for rain on Friday (w/ a very brief period of freezing rain in 
some valleys Friday morning). 


&& 


Aviation /02z Saturday through Wednesday/... 
VFR with mclr through the taf period. Winds will continue to taper off 
this evening as pressure gradient relaxes. Light SW winds expected Sat. 


Outlook...VFR conditions are expected across the region Sat night 
through sun. As high pressure moves off the coast...a cold front will 
move through the area Monday with a chance of showers and associated 
ceiling/visibility restrictions. Gusty winds likely behind the front...with 
possible MVFR conditions at kavl into early Tuesday...before dry high 
pressure builds in from the west. 


&& 


Fire weather... 
relative humidity will fall into the lower 20s Sat to warrant another red flag 
warning in NE Georgia. Marginal event sun with red flag relative humidity reached for 
just the minimum four hours. A Fire Weather Watch remians in effect 
for NE Georgia Sun afternoon. 


&& 


Gsp watches/warnings/advisories... 
Georgia...red flag warning from 9 am to 7 PM EST Saturday for gaz010-017- 
018-026-028-029. 
Fire Weather Watch Sunday afternoon for gaz010-017-018-026-028- 
029. 
NC...none. 
SC...none. 


&& 


$$ 
Synopsis...deo 
near term...rb 
short term...jat 
long term...Arkansas 
aviation...rb 
fire weather...