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fxus63 klsx 071626 
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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
1126 am CDT sun Sep 7 2008 


Discussion... 
/436 am CDT sun Sep 7 2008/ 


Longwave middle to upper level trough which has dominated the north central 
Continental U.S. And much of noam for that matter over the past few days is 
expected to persist through the next few days...although its grip 
will loosen a bit. A series of disturbances are still expected to 
move around the base of this trough during this time. The first is 
in the process of exiting the forecast area now and should complete its 
departure by middle morning. The next will be a Union of northern and southern 
stream waves...and be coincident with a moderately strong surface 
cold front...and will pass through Monday afternoon and evening. A third 
disturbance will approach the forecast area during middle week...although its 
timing and location are much more questionable than the first two. 
At the surface...weak hi pressure over the area today will give way to a 
cold front Monday and Monday night with a decently strong 1025mb hi 
pressure dominating the forecast area on Tuesday and Tuesday night. 


First disturbance is exiting the forecast area now and the remnants of the 
rain showers that it leaves behind should completely exit or dissipate by 
15z. While the bulk of the cloud cover slowly edges southward and will 
settle into the central/southern forecast area this morning...this leaves the far 
northern forecast area approaching an area of clearing. The clearing area is 
largely being converted into fog and will need to be monitored 
closely for the need for an advisory should it push further S into 
the northern forecast area. 


As the clouds sink slowly southward...they will eventually regroup and 
thin out or break up over most areas by afternoon...but will be most 
persistent in central MO. Favor the cooler met MOS temperatures most areas. 


The approach of a set of disturbances on Monday which will 
eventually act in concert will lead to decent warm air advection ahead of the 
system by Sunday night...and have continued the chance category 
probability of precipitation centered over areas north and west of kstl. The deeper and more 
organized lift will arrive by midday Monday and persist through 
Monday evening...especially over the northern half of the forecast area...and will 
see likely probabilities for precipitation. It is appearing more likely 
that there will be considerable Post-frontal precipitation with this event 
although should also see surface front itself ignite new thunderstorms and rain late 
Monday afternoon and Monday evening for areas further S and east from 
the better and deeper lift to the north. All of the activity Monday 
night is expected to dissipate late as much drier air works in. 
With +17c h850 air pushing in over the southeastern half of the forecast area on 
Monday afternoon...was tempted to go middle 80s over a large 
swath...but cloud cover more uncertain over the stl metropolitan area due 
to proximity of front and precipitation and so only went low 80s for now 
with middle 80s further S. Temperature bust potential also exists to a 
larger degree for areas further north of kstl...with clouds...precipitation and 
frontal posn all key. 


Have played down impacts from middle week disturbance for now due to 
enough uncertainty from models on track and timing. 


Tes 


&& 


Aviation... 
/1123 am CDT sun Sep 7 2008/ 
for the 18z tafs... 
morning stratus and fog lifting to MVFR and should continue to 
scattered this afternoon as middle/high level clouds continue to move west 
to east across the region. Winds remain light with surface high 
pressure centered over Missouri. Could see some fog/stratus 
development again overnight and will hint at that in tempo groups. 
Otherwise...next disturbance should bring at least a chance of 
showers to the region Monday morning in warm advection regime. 


Cvking 


&& 


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


$$ 


Weather forecast office lsx