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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
518 am CST Friday Nov 21 2008 


Discussion... 
/254 am CST Friday Nov 21 2008/ 
main question the next few days is precipitation chances both 
Saturday and then again Sunday night. NAM/GFS are in relatively 
good agreement with each other and also with the European model (ecmwf)/UKMET/ 
Canadian with the timing and amplitude of troughs that will move 
through the area through early next week. 


Strong subsidence through early tonight will keep the area dry and 
relatively cloud free before the clouds stream in later this evening 
and overnight ahead of trough currently racing eastward across the 
northern rockies. Large scale ascent will increase over the area on 
Saturday...but enough dry air will remain in place below 800mb to 
keep measurable precipitation out of the forecast. Did mention 
chance of flurries/sprinkles over the northern County Warning Area on Saturday where 
forcing will be the best and dry air with be the shallowest. 
Subsidence then sets in behind the trough bringing dry conditions 
through Sunday. Ascent will be increasing late Sunday/Sunday night 
in response to warm air advection ahead of deepening trough digging 
southeastward out of the northern plains. Precipitation with the trough will be 
developing as it moves through the area...so have just slight 
chances over the northwestern third of County Warning Area but increased probability of precipitation to high chance 
over the southeastern counties. Forecast soundings indicate precipitation 
should be rain expect for the far northern County Warning Area late Sunday night 
which may have some mixing with snow before the precipitation 
ends. Subsidence will once again set in behind trough and have 
kept forecast dry through next week. 


Went slightly below temperature guidance today given cold morning 
start and continued slight cold air advection through midday. Used temperatures 
closer to the cooler NAM/Eta model MOS...though went with steady temperatures 
after midnight as clouds move in ahead of saturdays system. Also went 
cooler than guidance on Saturday with clouds over most of the 
area. 


Britt 


&& 


Aviation... 
/515 am CST Friday Nov 21 2008/ 
for the 12z tafs... 


High pressure building across the County Warning Area will insure VFR conditions 
and diminishing winds through this evening. High and perhaps middle 
clouds will begin overspreading the area and winds will become 
southerly late tonight/early tomorrow morning as a middle-level 
shortwave dives southeast toward the area. 


Cvking 


&& 


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


$$ 


Weather forecast office lsx