906 
fxus63 klsx 211703 
afdlsx 


Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1103 am CST Sat Nov 21 2009 


Discussion... 
/320 am CST Sat Nov 21 2009/ 


Main concerns deal with extent of fog this morning and chances of 
rain early next week. 


Solid band of clouds over the County Warning Area last evening has broken up the 
past few hours. This combined with the light winds caused by our 
close proximity to center of surface high pressure over northern Indiana 
has caused areas of fog. Fewer ob sites are reporting dense 
fog...primarily in low lying areas like cps/sus...than yesterday 
morning. Will include areas of fog in the forecast and the patchy 
dense fog in nowcasts. Like yesterday...fog should dissipate by 
middle morning. Time cross sections are showing high relative humidity values 
through tonight between 5-8kft and a large area of clouds over 
currently over western and southern MO is moving northeastward into the County Warning Area. Have 
mostly cloudy going over most of the County Warning Area today with clearing 
tonight. Dry weather will dominate through tomorrow as area lies 
under neutral vertical motion or subsidence beneath building upper 
ridge. 


Agreement in models begins to diverge by early next week when the 
NAM slightly trails the other models with a weak shortwave trough 
that crosses MO/Illinois on Sunday night. There will not be enough large 
scale ascent or moisture with this system to include a chance of 
rain at this time. There is also some disparity with the timing of 
a much stronger closed low and accompanying cold front with the 
NAM showing a frontal passage about 6-12 hours later on Tuesday than the 
GFS. The Canadian Gem supports the faster GFS and the European model (ecmwf) favors 
the NAM so used a combination of all of them...using a blend of 
the sref mean and the GFS MOS for the mass fields. This calls for 
going dry through Monday with a chance probability of precipitation with frontal passage Monday 
night and Tuesday. 


Went closer to the cooler GFS MOS for temperatures today given expected 
mostly cloudy skies and light southeasterly winds. Went toward the warmer 
side of guidance Sunday Monday as I expect more sun and winds 
veering southerly ahead storm system early next week. 


Britt 


&& 


Aviation... 
/1043 am CST Sat Nov 21 2009/ 


For the 18z tafs...VFR low-middle level clouds...4500-9000 feet should 
continue to push northward through the taf sites this afternoon and into tgt. 
Models depict high relative humidity between 850-700 mb in height this afternoon with 
this layer gradually drying out tgt. Cloud cover may become predominately 
scattered by late tgt and Sun morning. With less cloud cover late tgt and 
early Sun morning there should be light fog during this time period 
with visibilities down to around 3-5sm br. An east-southeasterly surface wind should 
continue through the taf forecast period as the taf sites will be on 
the western periphery of the surface ridge centered over the Great Lakes and 
Ohio Valley regions. 


Gks 


&& 


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


$$ 


Weather forecast office lsx