376 
fxus63 klot 240003 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
703 PM CDT Wednesday Jul 23 2008 


Discussion... 
234 PM CDT 


Main forecast issues will center around chances of precipitation 
next few days. Generally tranquil weather pattern with no 
temperature extremes and little threat of any significant severe 
weather through the middle of next week. 


Large closed low over Easter Great Lakes will slowly lift east while 
Sharp Ridge pushes into western lakes tonight. A series of minor short 
waves will eventually flatten out high amplitude pattern Thursday and 
Friday. GFS seems to have initialized well. Cluster of convection at 
NE/SD/MN/IA Four Corners area depicted well. As this cuts through 
upper ridge tonight it should drop southeast along thickness lines. The 
low level moisture axis also extends from lower Ohio Valley back 
into Central Plains. Thus expect convection to skirt south of 
forecast area into central and southern Illinois Thursday. 


Another weak perturbation is producing another cluster of showers 
and thunderstorms eastern ND. This will move east into Minnesota and WI 
tonight and Thursday and should weaken as it moves into more stable/dry 
air mass and away from better low level inflow. Thus expect 
increase in cloud cover over northern Illinois and northwest in Thursday but only 
slight chance of precipitation...mainly south portions of forecast area well 
south of I-80. 


There may be a series of thunderstorm clusters that traverse this 
baroclinic boundary from the Central Plains into central and 
southern Illinois Thursday night and Friday. As heights gradually build slightly the 
boundary may creep north closer to northern Illinois and dewpoints will 
gradually creep up. Therefore will continue with small 30 percent probability of precipitation 
for must of the area into Friday...but highest probability of precipitation will be south 
of 1-80. 


Meanwhile more significant short wave in northern stream will 
eventually cause big upper low to redevelop/retrograde into upper 
Great Lakes this weekend...putting northern Illinois into northwesterly flow again 
and suppress boundary further south again. Will include slight 
chance of thunder for Friday night as GFS hinting at a weak 
surface boundary dropping down through the western Great Lakes. May 
be a a chance of precipitation in southern part of area into Sat but will 
maintain dry forecast for now. 


GFS brings another short wave through westerlies into Great Lakes 
about Tuesday but not much consistency with European model (ecmwf)...so will 
maintain dry forecast through middle week which matches well with 
surrounding offices. 


Very pleasant night tonight with clear skies and low dewpoints. 
Dewpoints will eventually creep back into low to middle 60s by Friday 
and Sat but that is certainly tolerable in late July. Then warm 
days and pleasant nights early next week. Light south to southeast 
winds Thursday will allow a just a weak lake breeze to move 
inland...mainly North Shore. Little better southwesterly flow 
Friday and then westerly Sat...so not much lake cooling. Weak 
north flow Sunday into early week should allow lake breezes to 
return. But this time of year not a whole lot of cooling and 
inland temperatures only expected in low to middle 80s. 


Overall very pleasant for late July. 


Allsopp 


&& 


Aviation... 
659 PM CDT 


00z tafors...hi pressure ridge extending from Western Lake Superior S across 
WI and Illinois to move very slowly ewrd during the night. Surface wind already 
light under the ridge axis. With low dewpoint air temperatures to cool rather quickly 
with sunset despite significant hi cloudiness quickly overspreading 
WI and Illinois. Temperature differential between land and lake should disappear by 
middle evening and current gusting NE wind at Ord and mdw drop off to light 
winds. 


Area of rain over northwest Minnesota to continue ewrd ahead of strong short wave moving 
east into far southern Manitoba. With strong north-northwest upper flow east of the MS 
river the second area of rain over northwest and north central Iowa to continue 
dropping southeast across southeast Iowa and into west central Illinois overnight. 


Trs 


&& 


Marine... 


215 PM...large high pressure extends from Ontario south across 
the western lakes. This high will move southeast tonight as it 
slowly weakens...reaching the Ohio Valley Thursday. Low pressure 
near the U.S. And Canadian border will move northeast across 
northern Ontario Thursday night and Friday. Southerly flow will 
develop between this low and the high pressure which will continue 
to slide southeast off the Atlantic coast Friday. A weak trailing 
cold front will turn winds westerly or northwesterly Friday night 
into Saturday across the western lakes. Cms 


&& 


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


&& 


$$