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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
646 am CDT Monday Oct 13 2008 


Discussion... 
/353 am CDT Monday Oct 13 2008/ 


Primary forecast concern continues to be onset of precipitation associated 
with strong but slow moving system over the Great Plains. The 
fastest-moving portion of the cold front over Nebraska and South 
Dakota is only moving at about 12kts according to AWIPS speed tool. 
This speed would keep the cold front out of even the northwest 
portions of the County warning forecast area like Knox County until midnight tonight...or 
even later. Given that the front will be moving away from its 
upper level support...it seems unlikely that it will speed up at 
all...and indeed it should slow down. Models agree with this 
idea...and both the GFS and NAM are very similar with frontal 
positions through the end of the week. Radar over the Great Plains 
is showing a large area of precipitation extending from western Illinois 
through much of central and western Kansas...central and eastern 
Nebraska...and northeastward through the Dakotas into Minnesota. 
While there is an area of pre-frontal precipitation in Nebraska and 
Kansas...much of the rest of the precipitation is Post-frontal. I think 
that as the front moves away from the upper level dynamical 
support we'll see much of the pre-frontal precipitation dry up. Can't 
rule out isolated showers ahead of the front...but I think the 
main precipitation threat will be behind it. Guidance is pretty stingy 
with instability parameters...so thunder threat will likely be 
minimal at least through Tuesday and Tuesday night. As I mentioned 
before...the front will be moving away from its upper level 
support...so it also looks likely that the Post-frontal precipitation 
will become more scattered/lighter than it is now as it moves into 
our area. 


On Wednesday the secondary shortwave...which is currently cutting 
off over the Utah/Arizona border...gets kicked out into the 
plains and will finally push the front through the remainder of 
the County warning forecast area. The wave will bring in cooler air aloft allowing 
lapserates to steepen and therefore greater instability to 
develop. Therefore I think Wednesday will be our wettest day 
associated with this week's system...and will have the greatest 
chance for thunderstorms. However i'm not sure if this instability 
will be realized through extensive cloud cover and low level cold 
advection. Regardless...as the front moves and the cold advection 
ramps up...we should see frontogenetical processes increase the 
coverage and intensity of precipitation over the area...much like 
what is occurring over northwest Kansas and central Nebraska at 
this time. 


No major changes to the remainder of the forecast. We should see a 
clearing sky and cooler temperatures for Thursday as high pressure 
builds into the region. A weak clipper will ripple through northern 
Missouri/Iowa on Thursday night and Friday. This system might bring 
northern portions of the area some mid-clouds...but it appears that 
the precipitation threat...if any at all...will remain further north over 
Iowa and northern Illinois. 


Carney 


&& 


Aviation... 
/646 am CDT Monday Oct 13 2008/ 


For the 12z tafs...large area of cloud cover over the central Continental U.S. 
Between 3 and 6kft bases...with patches of 2 to 3kft bases...will 
continue to pour into the forecast area from the S through this morning and 
early afternoon as the flow at cloud level gradually veers more to 
the south-southwest by 18z. Pilot weather reports from Arkansas last evening indicated that the 
cloud layer was rather thin...just about 1kft...but this is also 
middle October where the sun just isn/T what it used to be when it 
comes to vaporizing cloud layers. Should see clouds hang tough 
through early afternoon before seeing significant breaks...plus diurnal 
component will slow down any decreasing trend. Otherwise...next 
round of low clouds will hold off until frontal passage...which will be late 
tonight for KUIN and kcou. Have bought into near IFR ceilings for KUIN 
with more northerly location and greater likelihood of precipitation...but backed 
off slightly from these values for kcou. May see lower visibilities late 
tonight as well...depending on how low ceilings get and how much rain 
actually falls. 


Tes 


&& 


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


$$ 


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