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Area forecast discussion...updated aviation 
National Weather Service St Louis MO 
506 PM CST Sat Nov 21 2009 


Discussion... 
/330 PM CST Sat Nov 21 2009/ 


Rather benign weather is expected across the region through Monday 
with weak surface ridging dominating and nuetral to weak warm air advection forecast. 
The upper low currently in vicinity of extreme eastern Texas is forecast to remain 
well south of the area as it moves eastward into the lower Tennessee Valley on 
Sunday in response to the upstream kicker shortwv moving through the 
Great Basin at this time. With little real overall change in the air 
mass...temperatures will to a degree be dependent on the amount of 
clouds. Making this a little more challenging is the fact the 
model relative humidity fields have been rather poor over the last few days in 
depicting the clouds. I think for the most part the clouds tonight 
will decrease and move north-northeastward and the greatest cloud cover on 
Sunday will reside in the western portion of MO touching portions of 
central and NE MO as well. The forecast temperature are a blend of 
persistence and the warmer side of the MOS guidance suite. 


The main emphasis this forecast period will be on the middle/upper 
low forecast to develop in the northern rockies on Sunday night 
and move into the middle MS valley on Tuesday. There is some 
spread on the low position by 00z Wednesday...but it is not exceedingly 
great. A consensus position would be in vicinity of southeast Iowa...with the low 
exiting into the lower Great Lakes by Wednesday morning in 
response to the next digging upstream shortwv trough/upper low. In 
terms of sensible weather...will need to keep an eye on a lead 
impulse moving through the area on Sunday night/early Monday...which has a 
very low chance of bringing a sprinkle or light shower to far northern 
sections. Otherwise precipitation chances will ramp up Monday night into 
Tuesday with the strong middle/upper low-trof. Low level moisture will 
be rather dismal ahead of this feature and the attendant 
cold front...however large scale ascent will be quite robust along with 
plentiful middle level moisture...supportive of a band of rain with 
cloud bases in the middle levels. Accordingly I have boosted probability of precipitation in 
the western County Warning Area on Monday night...and in the eastern County Warning Area on Tuesday. 
Some good cold air advection will occur in the wake of the front along with 
probably a decent amount of stratus. 


A chilly period with below average temperatures is expected 
Wednesday into Thursday with the aforementioned secondary shortwave 
trough/upper low digging into the region and bring a colder 
reinforcing shot of cold air. There had been some differences in 
the extended range models with respect to this scenario...but with 
the new 12z European model (ecmwf) run...they have all come onboard. Expecting 
moderation of the temperatures to above normal next weekend as the 
trough begins exiting the eastern Seaboard and heights rise aloft. 


Glass 


&& 


Aviation... 
/455 PM CST Sat Nov 21 2009/ 


For the 00z tafs...little if any changes to the previous taf 
forecasts. Clouds currently over taf sites will continue to move 
northward and break up during the evening hours. With clear skies 
overnight...expect light fog to develop and persist through middle morning 
hours. Am a little uncertain as to exactly when fog will become 
MVFR...but have pushed it back a couple of hours as clouds persist 
over the area. If clouds fail to break up...fog development may not 
occur until much later...near 12z. Otherwise...Sunday should be sky clear 
with light southeast to east-southeast winds. 


Tilly 


&& 


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


$$ 


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