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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
612 PM CDT Sat Oct 11 2008 


Discussion... 
/347 PM CDT Sat Oct 11 2008/ 
large upper ridge remains parked from central appalacians to lower MS valley this 
afternoon...as strong upper low continues to spin its way toward the central rockies. 
With little change in this pattern surface ftrs should also remain qsntry...and it 
certainly aprs that next 24 hours should be similar to the past 24 hours 
for most parameters. 


Forecast becomes more challenging heading into the early part of the work 
week as the upper air pattern transitions from its current state to a qznl 
flow by Wednesday. During the transition the upper low is forecast to lift into S 
central Canada by Monday evening...with a secondary bit of energy lingering 
near the 4 corners that is prognosticated east into our area by Wednesday. In genl 
terms...the breakdown of the upper ridge will mean the strong frontal zn over the 
plains will be able to drop southeast and into our area...increasing probability of precipitation in the 
Monday night-Tuesday night time frame. Even though air mass ahead of the fnt will be 
fairly moist...precipitation threat with this pattern has a decidedly Post frontal 
feel...as most of the upper level dynamics will remain to the west and north of the 
advancing fnt. 


The real problem during the transition will be temperatures. Entire area will 
remain in the unseasonably warm air through Monday...but there is now a 
difference of opinion on how fast the cold air will arrive heading 
into middle week. Earlier solutions prognosticated a slow east progression of the fnt 
acr the County Warning Area...but the overnight runs and the 12z GFS and European model (ecmwf) are now 
generating a WV on the fnt...associated with the second shtwv...in the 
Tuesday-Wednesday time frame. Given the extremely tight temperature gradient associated with 
the fnt...as can be seen over the plains this afternoon...its exact 
location is going to have a huge impact on temperatures. I'm a bit uncertain 
if this WV development is real or memorex scply since sm upper air data was 
apparently missing over the SW today. Have made a few upward tweeks to 
going temperatures over our southeast areas on Monday night and Tuesday to acct for 
this...but a more dramatic adjacent may be necessary...even into Wednesday...if 
tngts runs continue on this trend. 


It aprs models are also hvg trouble resolving dynamics embedded in 
the zonal flow as forecast heads into the medium range. Primary concern 
is with shtwv that is prognosticated into Pacific northwest on Wednesday...with GFS forecast 
much weaker and quicker with this ftr while European model (ecmwf) is much stronger and 
slower. For now...have gone with consensus that strongly follows European model (ecmwf) 
solutions. Forecast generally reflect dry conds with near normal temperatures in the 
Thursday-Sat time frame. 


Truett 


&& 


Aviation... 
/535 PM CDT Sat Oct 11 2008/ 
for the 00z tafs...not much change from previous forecast. Winds will 
be a little higher compared to last night...which may help prevent 
fog at sus. Still...given current winds...have decided to lower 
visbys for fog potential. Other sites will remain VFR tonight as 
cumulus dissipates after sunset. Tomorrow should only have few-scattered cumulus 
with cirrus over much of the area with southeasterly winds. 


Tilly 


&& 


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


$$ 


Weather forecast office lsx