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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
625 PM CDT Sat Sep 6 2008 


Discussion... 
/233 PM CDT Sat Sep 6 2008/ 
main concern the next few days is precipitation chances. NAM/GFS 
mass fields are in decent agreement until late Monday when the GFS 
becomes more progressive than the NAM with the trough that digs 
out of Canada. Went closer the GFS which is supported by the other 
global models. 


Have gone with chances of rain showers over the northwestern half of the County Warning Area this 
evening and spread the chance areawide overnight with likely probability of precipitation in 
the north as Central Plains shortwave moves eastward tonight. While there 
will be some subsidence tomorrow once the shortwaves passes to the 
east...GFS is showing some modest low level forcing setting up 
across the Ozarks...so have kept chance of rain showers. Only weak instability 
will exist...so have just slight chance of thunderstorms and rain. Better chances will 
come Sunday night as the western half of the County Warning Area will be on the 
eastern periphery of broad 40 knots low level jet. Have kept mention of 
thunderstorms and rain given the GFS MUCAPES of 400-800 j/kg. GFS has MLCAPES 
increasing into the 1000-1500 j/kg range on Monday ahead of decent 
shortwave trough and it's accompanying cold front. Do not believe that 
northern half of the County Warning Area will get warm enough to realize the nearly 
2000 j/kg MLCAPES the the GFS has depicted because of expected 
rainfall. Have likely probability of precipitation going over the northern half of the County Warning Area 
Monday/Monday night with just chance probability of precipitation elsewhere. Deep layer 
shear on Monday will be in the 30-40kt range by late Monday 
indicating the potential for some strong storms if aforementioned 
instability can be realized. 


Went a bit warmer than guidance tonight based on current dewpoints 
and expected cloud cover overnight. MOS guidance is in good 
agreement and looks decent Sunday/Sunday night...gut did cut 
guidance in the north on Monday because of expected clouds/rain. 


Stayed with the GFS solution middle and late week as ecwmf has shown 
some significant problems with continuity since yesterday. Have 
refined precipitation chances from Wednesday night into early Friday 
with the passage of the next shortwave trough/cold front. 
Temperatures are based on a combination of persistence...HPC 
guidance and the GFS mean MOS. 


Britt 


&& 


Aviation... 
/611 PM CDT Sat Sep 6 2008/ 
for the 00z tafs...area of rain showers is not developing as far S as previous 
tafs indicated. Latest radar trends and model guidance suggest that 
rain showers should stay just north of cou. Cou may still see rain showers...but do not 
think it will lower visbys below 6sm or ceilings below VFR. Have moved 
up timing for precipitation at uin. Models indicate rain showers to continue to 
develop and should move over uin through much of the evening hours...but 
should be ending just after midnight. Stl and sus should remain dry 
tonight. Have added vcsh to account for some light precipitation chances. 
Again...any precipitation should not lower visbys or ceilings significantly. 
Ceilings should gradually lower tonight and remain for much of Sunday 
morning. Around 18z...ceilings should begin to improve to VFR with a broken 
middle layer remaining as another system approaches. 


Tilly 


&& 


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


$$ 


Weather forecast office lsx