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fxus63 klsx 231750 
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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1150 am CST Monday Nov 23 2009 


Discussion... 
/444 am CST Monday Nov 23 2009/ 


Still watching the trough of low pressure now digging into the Great 
Plains this morning as our first forecast challenge of this 
package. The 23/00z GFS/Gem/ECMWF were in very good agreement in 
handling this storm system this morning...while the NAM was the 
outlier. Not only was the NAM the outlier...it was substantially 
slower than the going forecast...while the other three were actually 
very consistent with previous package. Therefore I ignored the NAM 
this morning and went with the consensus. That being said...I only 
made small cosmetic changes to the forecast this morning. The 
biggest change was to split tonight's probability of precipitation to start the precipitation after 
midnight over central Missouri. Remainder of the forecast is 
virtually the same as the last package. Still expecting showers to 
overspread the area late tonight/Tuesday morning. Not expecting any 
heavy rain this time...with most areas probably seeing around 1/4 
inch ahead of the cold front. A few counties up in northeast 
MO/west central Illinois could get around 1/2 inch as they'll be closer to 
the low center and have a longer period of rain. 


The trough quickly takes on a negative tilt Tuesday night...lifting 
out as another shortwave drops in behind it on Wednesday. This 
second wave is still looking pretty cold...with 850mb temperatures falling 
to around -5 degrees by 06-12z Thursday morning. This would almost 
certainly be cold enough for snow...if there were any moisture left 
over to lift in the wake of tuesday's wave. I certainly wouldn't 
rule out some sprinkles or flurries on Thanksgiving...but I think 
the chance for measurable precipitation is pretty low. Temperatures will be 
substantially colder toward the end of the week behind the second 
low. With clouds and northwest flow we'll be lucky to make it to 
the low 40s on Thursday. And while it looks like we'll see some sun 
on Friday it'll stay in the 40s since we'll be in The Heart of the 
cold airmass at that time. 


Carney 


&& 


Aviation... 
/1041 am CST Monday Nov 23 2009/ 


For the 18z tafs...low level clouds across southern portions of MO Illinois 
advecting slowly northward towards cou...stl and sus at this time. The ceiling 
height of this cloud cover should rise some during the afternoon hours 
with daytime heating and mixing but will need to introduce ceilings of 
only 1000-2000 feet in stl and sus this afternoon...with slightly higher 
ceilings in cou by late this afternoon. This low level cloud cover should 
eventually make it northward into uin by late afternoon or early evening...but 
should be higher around 3000-4000 feet in height. The ceilings 
should drop to 1000-1500 feet late tgt with light fog as well at all 
the taf locations. Rain will move into the taf sites shortly after 
12z Tuesday as low pressure moves eastward through northern MO on Tuesday with a 
trailing cold front moving eastward through cou during the late morning and through 
stl/sus during the afternoon. Southeasterly surface winds will veer around to a southwesterly 
direction Tuesday afternoon after the passage of the cold front. 


Gks 


&& 


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


$$ 


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