430 
fxus63 klot 212007 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
307 PM CDT Thursday Aug 21 2008 


Discussion... 
305 PM CDT 


Upper low lifting north-northeast across eastern Iowa early this afternoon with 
first in series of short waves ejecting NE into southern Manitoba 
ahead of upper low crossing from far southeast b.C. To the Alberta-Montana 
border. As the western upper low center progresses eastward to far 
southern Saskatchewan by 12z Friday the disturbance over Iowa continues 
to open as it continues north-northeast to northwest WI. 


More substantial but still mainly light rain has been confined to 
areas closer to the MS river while the local forecast area has seen only 
spotty light showers and sprinkles as deeper dry air in place this 
far east. The main disturbance will be passing into southeast WI by late 
afternoon resulting in a decrease in precipitation across northern Illinois. 


However...appears to be a couple of minor short waves riding north-northeast 
up the west side of the eastern noam upper ridge with scattered 
thunderstorms and rain noted from central Illinois south-southwest to la and eastern Texas. Much of this 
diurnally driven but will keep chance mention for overnight and 
Friday as these impulses travel across the middle MS valley to the local 
area. 


Probability of precipitation rise for Sat and Sat evening with a cold front slowly 
approaching and eventually dropping through northern Illinois and northwest 
ind in response to the Canadian system progressing further east and 
reaching near James Bay by 06z sun. Low level moisture pooling 
ahead of the front results in increasing instability/cape values 
with some thunderstorms and rain ahead of the front expected with heating during Sat 
and until frontal passage Sat night. 


Low threat of precipitation across the region beyond any lingering rain showers 
in the far southern portion of the forecast area early sun a.M. As high 
pressure building from S central Canada and the northern plains east 
across the upper Midwest and Great Lakes sun through Wednesday dominates. 


Trs 


&& 


Aviation... 


1800 UTC tafs...high pressure moving off to the east will see a 
dynamic wind shift this afternoon through tonight from east to southeast flow 
this afternoon to more southerly advection this evening and eventually 
SW flow Friday morning. An upper low approaching the MS valley has 
captured a rather deep moisture plume from the Gulf and advecting this 
into northern Illinois and Northwest Indiana. Lack of widespread heavy rain over 
terminals is slow to moisten the dryer lower layers. More intense 
rain closer to closed upper low in Iowa which has saturated lower 
layers over Missouri. There ceilings condensing to IFR. Question will 
be with veering of wind to SW Friday morning...will IFR or low MVFR ceilings 
advect into terminals. Have admitted low ceilings into rfd but holding 
back to MVFR for tafs east of I-39. May be a few light showers 
during the afternoon today but not heavy enough to puddle. Will save 
heavier precipitation until closer proximity to upper low starting predawn 
Friday. 


Rlb 


&& 


Marine... 
1245 am CDT 


High pressure off to the east pushing further east as cold front 
approaching from the upper MS valley late tonight and Friday. Will 
maintain a moderate SW flow until frontal passage Saturday night when winds 
shift to the northwest. Winds likely to pick up early Sat to 25 
kts with tighter pressure gradient out ahead of front. Winds 
continuing to veer to north and northeast Sunday and Monday as 
high pressure fills into the lakes. Winds then becoming more south 
Wednesday as high pressure shifts off to the east. No outlook for 
gales. 


Rlb 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...none. 
&& 


$$