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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
635 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... 
/615 am CDT sun Sep 7 2008/ 
for the 12z tafs... 
primary concern for aviators this morning will be MVFR/IFR ceilings 
over parts of the bi-state region. The region of lowest clouds 
appears to extend from the Jefferson City/Columbia area north and 
northeast into the Kirksville/Hannibal/Quincy areas. VFR ceilings and 
visibilities are being reported in the St. Louis metropolitan area and points 
east and south all the way to Farmington and Sparta Illinois. I 
must admit...I don't have a good feel for how the ceilings are going 
to evolve this morning. Some shreds of IFR cloud are being 
reported at the metropolitan area terminals...and those shreds could 
conceivably build into an IFR/low MVFR ceiling early in the taf 
period. Establishing trends over the last few hours has been next 
to impossible as observations have been bouncing between IFR and 
VFR and all points in between in many locations. Needless to 
say...this is a low confidence forecast this morning. 
Regardless...MVFR/IFR ceilings should generally improve to VFR toward 
mid-day. 


Carney 


&& 


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


$$ 


Weather forecast office lsx