898 
fxus63 klot 211843 
afdlot 


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


Discussion... 


315 am...upper low over Arkansas/MO early this morning beginning its March 
north. Large area of showers over eastern MO and southwest Illinois will 
move across eastern Iowa and western Illinois later today into this evening. 
Have bumped probability of precipitation up across the western County Warning Area. Further east across 
eastern Illinois and northwest in...much drier air being advected west 
with dewpoints in the lower 50s across northern in which is now 
spreading into east/central Illinois. Think it will take much of today 
for dewpoints to recover back into the lower 60s over these areas. 
Models continue to show a weak shortwave lifting north across 
eastern Illinois by this evening. Chances for a few showers that far east 
small...despite limited instability...but Worth mentioning by late 
afternoon. Middle/high cloud cover keeping lows from dropping too far 
this morning but highs across the western areas will struggle into 
the middle/upper 70s under overcast skies with precipitation today. 


After this first wave lifts north by early/middle evening...difficult 
to find any one feature that will be responsible for additional 
precipitation overnight. Appears another wave moves across northeast Illinois 
toward sunrise Friday. Maybe several dry hours tonight but will 
maintain chance probability of precipitation with no timing for now. Likewise...maybe some 
activity Friday morning...perhaps some isolated diurnal showers in 
the afternoon/early evening. Certainly not enough confidence to 
pull probability of precipitation...but again several dry hours Friday and Friday evening. 
High temperatures on Friday problematic. Should enough sunshine develop... 
mav temperatures of upper 80s reachable but confidence not high enough to 
go that warm yet. 


Cold front pushes through the area Saturday afternoon and Saturday 
evening. Models continue to gradually speed up frontal passage timing and any 
lingering precipitation should be pushing south of the County Warning Area Sunday morning. 
Large Canadian high pressure builds across the lakes Sunday night 
and Monday...as seen on the extend models for several runs now...so 
dry weather expected through at least Tuesday. Cms 


&& 


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. 
&& 


$$