801 
fxus63 klot 080040 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
740 PM CDT Tuesday Oct 7 2008 


Discussion... 
312 PM CDT 


The rain will end Wednesday morning. 


Tonight and Wednesday...the large surface trough that extends 
from Wisconsin to western Kentucky this afternoon. This trough 
will move east to Michigan to western Kentucky by Wednesday 
afternoon. The rain will move east with this surface feature. The 
local WRF arw shows the 500 mb wave trough moving into Indiana 
with the positive vorticity advection in Michigan and Northern 
Ohio. Negative vorticity advection is forecast for Wednesday 
afternoon and evening. The sinking air with this advection will 
give a clearing sky. 


Thursday through Sunday...a large high will spread into the Great 
Lakes and Ohio Valley. By Friday morning the high will be over 
Pennsylvania. A low will develop over eastern Colorado. By Sunday 
this low will move into the Great Plains. This will bring warm air 
back to our forecast area of northern Illinois and Northwest 
Indiana by Sunday. There will be increasing of surface moisture 
and a another large 500 mb wave trough moving into the Great 
Plains Monday. We will add a chance of thunderstorms Monday and 
Tuesday. We used the European model (ecmwf) and GFS models for this forecast. 


&& 


Aviation... 
737 PM CDT 


00z tafors...strong short wave lifting north-northeast along the MS river into 
far SW WI at 23z. This resulting in widespread -ra with embedded 
rain/local +ra lifting through north central and NE Illinois...and northwest in. 
Precipitation does not end as upper trough axis still to the west of MS valley 
but coverage drops off with -shra activity. 


Surface low vc quadrant cities with warm front extending east then east-southeast across northern 
Illinois and northern in. As it continues to lift NE across S central will during 
this evening higher middle 60f dewpoints and accompanying IFR ceilings will continue 
to spread across the local terminals. 


Surface trough/cold front to continue to push east across the MS river this evening and 
reach eastern Illinois during the predawn. Drier air advects in behind the 
cold front with ceilings and visibilities expected to improve as a result. 


Middle/upper trough prognosticated to the Illinois-in border by 18z Wednesday with strong 
drying spreading across area clearing out any remaining stratocu ceilings. 


Trs 


&& 


Marine... 


218 PM...southeasterly gradient will continue across Lake Michigan through 
this evening as high pressure over New England and the Middle 
Atlantic States slowly moves east tonight...and low pressure over 
northern Missouri moves northeast. This low will move across 
Central Lake Michigan tonight. As this low approaches wind speeds 
are expected to increase across the southern half of the lake with 
30kts possible. This low begins to weaken as it moves across 
eastern Ontario Wednesday. Winds will shift westerly behind the 
low and speeds are expected to diminish to 10-20kts. 


A weak ridge of high pressure will extend from the plains to the 
Ohio Valley by Thursday morning. But strong low pressure will move 
from Manitoba to James Bay Wednesday night into Thursday. This 
will keep a slightly stronger gradient across Northern Lake Michigan. 
Eventually the ridge will build northeast across the lakes region 
Thursday night into Friday...and continue to build northeast and 
strengthen Friday night into Saturday. Cms 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...nearshore waters 
until 4 am Wednesday. 


&& 


$$