Weather

863 
fxus63 klot 292332 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
632 PM CDT Friday Aug 29 2008 


Discussion... 
232 PM CDT 


This will be a dry Labor Day weekend for north central Illinois... 
northeast Illinois and Northwest Indiana. 


We expect no precipitation for the next 12 hours or Saturday. The 
forecast low temperature has been changed a little since the last 
forecast. After examining relationships between forecast high 
temperatures and the observed high temperature for past several 
days...we found a lower temperature bias in the low temperature 
forecasts. We adjusted the forecast low temperature for tonight to 
a little higher value to compensate for the bias. 


The high temperature for Saturday was adjusted for a higher bias. 
We followed the same methods that were used to elucidate the 
relationship between observed and forecast low temperature. 


For Sunday through Monday...a large high will move across the 
lower Great Lakes region and the upper Ohio Valley to 
Pennsylvania by Monday morning. A low is expected to develop over 
Montana and will move South Dakota by Monday morning. These two 
synoptic features will give the forecast area a south wind Monday 
and Tuesday. Warmer air will return to the region. We expect high 
temperatures in the upper 80s to lower 90s Tuesday. We used the 
GFS model for our guidance. 


For Wednesday through Friday... we used the European model (ecmwf) model for 
guidance for days 4 through 7. The large high is forecast to move 
into the southern appalachian region by Wednesday evening. The low 
that was over South Dakota moves to Ontario by Wednesday morning. The 
low will drag a front into the western Great Lakes. This model is 
slower with the eastward movement of these features because it 
brings Gustav into Oklahoma by Wednesday evening. We will follow 
that trend for our chance of precipitation forecasts. We will 
forecast increasing chances of showers and thunderstorms Wednesday 
through Friday. The highest probability of precipitation will be on Friday. 


&& 


Aviation... 
630 PM CDT 


00z tafors...ridge of high pressure to be over area through the 
period. Current remnants of cumulus near kord and kmdw to be gone by 
beginning of forecast...and clear through the night and Sat with 
core of low dew point air over the terminals. Momentum keeping 
portion of lake breeze across southern Cook and eastern will 
counties pushing westward but with loss of surface heating 
temperatures will quickly drop as air is dry so winds to go calm 
inland by middle evening and with a weak land breeze near the lake 
developing. Northeast to southwest oriented surface ridge axis 
to extend across Southern Lower Michigan...northwest in and west 
central Illinois during by midday Sat. Under weak pressure gradient 
lake breeze will develop again. 


Trs 


&& 


Marine... 


1230 PM CDT 


A weak cold front extends from Hudson Bay...southward 
across Lake Huron and the lower Ohio Valley. This front will 
continue to push eastward this afternoon as a sprawling high 
pressure inches builds across the middle and upper Mississippi 
Valley tonight...and settles across the southern Great Lakes on 
Saturday. As a result...winds across Lake Michigan will remain 
generally light. Given weak pressure gradient...expect that 
prevailing northerly wind flow today will become variable 
tonight...dominated more by local land/sea breezes. 


The high will continue on across the lower lakes on Sunday as 
a low pressure develops over the northern rockies and moves out 
across the northern plains on Sunday. The high will remain 
nearly stationary over the eastern lakes through Monday as the 
plains low tracks northeast into southern Ontario. This will 
gradually build south to southwest flow across the western Great 
Lakes heading into early next week. 


Merzlock 


&& 


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


&& 


$$