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fxus63 klsx 231156 
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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
556 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... 
/541 am CST Monday Nov 23 2009/ 


For the 12z tafs...the NAM low level relative humidity forecasts and forecast 
soundings appear to be doing the best job emulating the gradual 
northward expansion of low clouds into southeast MO from Arkansas and west 
Kentucky during the predawn hours. Judging by the 11-3.9 satellite 
imagery the GFS appears to be too aggressive/too fast with the northward 
expansion of the low VFR/MVFR ceilings...with the latest RUC ouput 
also giving more credence to the slower northward expansion as depicted 
by the NAM. General trend should be for the SC to our S to make a 
slow northward expansion during the day and into the evening...with the 
advection of deeper moisture gradually overcoming the erosion of 
the north edge of the cloud mass due to diurnal mixing. GFS and NAM do 
agree that increasing moisture overnight should cause ceilings to 
lower to at least the low end of the MVFR range...and have trended 
forecasts this direction in the 06-09z time frame. 


Truett 


&& 


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


$$ 


Weather forecast office lsx