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fxus63 klsx 231045 
afdlsx 


Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
445 am CST sun Nov 23 2008 


Discussion... 
/234 am CST sun Nov 23 2008/ 
main forecast concern deals with chances of precipitation tonight. 
NAM/GFS match up pretty well with global models through Monday 
before the NAM begins to put more emphasis on a shortwave moving 
down on the west side of Great Lakes low by Tuesday morning. None 
of the other models show this...so have used the GFS as a guide this 
morning. 


Low clouds over the middle south should begin to move northward into 
the southern and western County Warning Area by this afternoon as low level winds back in 
response to low pressure approaching area from the plains. Cirrus 
will also increase as shortwave trough currently over the northern 
rockies moves southeastward into the Central Plains by this 
evening. Large scale ascent will increase over the entire area 
tonight in response to the trough...so have at least a chance of 
rain going over the whole County Warning Area. However...have confined likely probability of precipitation 
to just the southeastern quarter of the County Warning Area which will have the best 
moisture (pwats ~0.75"/850mb dewpoints ~6c) and some 900-800mb 
moisture convergence on the nose of a 50kt low level jet. Forecast 
soundings from both GFS/NAM do not show enough cold air working into 
the northern County Warning Area to support change over to snow...but moisture may be 
shallow enough that drizzle may occur instead of rain. 


Subsidence will then set in behind trough as Great Lakes low moves 
eastward and ridge builds into plains. Have left rest of forecast 
as is including late week with a chance of rain Thursday night 
into Friday night. 00z runs of GFS/ecwmf differ with amount of 
phasing that will occur between upper low moving out of southwestern Continental U.S. 
And northern stream/polar vortex which has implications on precipitation 
type. 


Temperatures today reflect the GFS MOS which has worked out better 
in warm air advection cases recently with a mix of the sref mean and 
GFS MOS tonight and tomorrow as the front moves through. Used GFS 
MOS again Monday night into Tuesday because of model preference. 


Britt 


&& 


Aviation... 
/445 am CST sun Nov 23 2008/ 
for the 12z tafs... 


MVFR stratus will overspread the area this afternoon/early evening 
from the south. Drizzle or very light rain will develop late this 
evening and overnight at stl metropolitan taf sites. 


A south/southwest wind between 10 and 15 kts can be expected until 
a cold front moves across the area early tomorrow morning with a 
wind shift to the west. 


Cvking 


&& 


Lsx watches/warnings/advisories... 
MO...none. 
Illinois...none. 
&& 


$$ 


Weather forecast office lsx