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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
546 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... 
/540 am CST Sat Nov 21 2009/ 


For the 12z tafs... 
primary concern once again this morning is fog...but the fog is 
neither as widespread or as dense this morning as yesterday. Most 
of the fog is MVFR...with intermittent drops below 3 miles. A few 
spots...primarily in river valleys...have reported LIFR conditions. 
I expect this fog to dissipate between 13z and 14z after sunrise. 
Some of the thicker stuff could hang around as long as 15z. After 
15z I expect VFR conditions to prevail with scattered-broken clouds around 
6000ft as an upper level disturbance churns slowly over the 
Texas/Louisiana Gulf Coast today bringing middle-upper level moisture 
streaming up into the middle-Mississippi Valley region. 


Carney 


&& 


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


$$ 


Weather forecast office lsx