Weather

149 
fxus63 klot 071740 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1240 PM CDT sun Sep 7 2008 


Discussion... 
334 am CDT 


Today...showers will move east this morning. A weak surface trough 
will move east from north central Illinois to Michigan and Indiana 
later this morning. This surface trough movement was forecast 
using our local WRF model. We made a slight change in the 
temperature forecast and will keep the forecast of lower 70s for 
the high temperature today. 


A stronger cold front will move across northern Illinois and 
northwest Iowa Monday. There is a 500 mb short wave trough that 
will increase its amplitude as it moves through Wisconsin and 
northern Illinois. There is some large vertical wind shear from 
700 to 500 mb over Montana and western Canada at 00 UTC. This shows 
that the short wave trough will begin its growth today in the 
Great Plains. We expect a large thickness gradient over Illinois 
Monday and wind shear from surface to 500 mb. There will be a lot 
of lift for precipitation. We expect mostly showers at this time. 
Forecast soundings show level of free convection at 1,100 feet and the equilibrium 
level at 2,700 feet Monday afternoon. Strong moisture transport 
is forecast for Monday afternoon. The GFS model brings a cyclone 
through Northwest Indiana Monday evening. 


Tuesday through Friday. . high pressure moves into Iowa by 
Tuesday afternoon. This high moves across Northwest Indiana by 
midnight Wednesday morning. We expect lower high temperatures 
Tuesday and Wednesday. When the high moves east into New York early 
Thursday the wind is forecast to become more from the south. We 
expect warmer weather Thursday and Friday. There will be a chance 
of thunderstorms Wednesday night through Thursday night. We used the 
European model (ecmwf) and GFS models for our forecast this period, 


&& 


Aviation... 


18z tafs...quiet weather through Monday morning. Scattered/broken cumulus field 
this afternoon will dissipate with sunset with increasing high/middle 
clouds through the night. West-southwest winds 10-12kts will turn more southwest 
this evening and diminish to 5kts. A weak surface trough will move into 
northern Illinois overnight toward daybreak Monday. Light SW winds will 
shift northerly as this trough/front moves through. However...wind 
speeds will be light and for now have continued previous forecast 
of vrb winds during this time. Eventually winds become easterly 
during Monday morning. 


As for precipitation chances...models differ in timing by 6 hours or so 
with the GFS slightly faster...with precipitation arriving/developing 
over northeast Illinois by 18z. From this distance have opted to stay 
dry...except at rfd. Next forecast can then better pin down a 
start time for rain. However...conditions will deteriorate 
Monday afternoon...perhaps rapidly with MVFR ceilings/visible in light to 
moderate rain into Monday evening. Included a tempo at rfd for the 
last 4 hours...though prevailing rain likely to begin during this 
time period. Cms 


&& 


Marine... 


210 PM...have made some significant changes to the going forecast 
and grids from Monday afternoon through Tuesday afternoon. 


In the short term...weak high pressure over the Central Plains 
slowly moves east across the Ohio Valley tonight while a weak low 
moves across Ontario. By Sunday afternoon/evening...a trough/cold 
front from this low will form/extend across the western lakes. As 
a result...the gradient will tighten and expect west/southwest 
winds to increase to 15-25kts...especially over Northern Lake 
Michigan. This trough shifts east Monday leaving a weak ridge of 
high pressure in its wake. 


By Monday afternoon...weak low pressure over the Central Plains 
will lift northeast reaching Lake Michigan early Tuesday morning. 
An upper trough will begin to deepen over this system while a tight 
and rather impressive thermal gradient for this time of year 
develops over the western lakes. Most of the models show this 
area of low pressure deepening Monday night and Tuesday morning 
over Michigan and it continues to deepen as it lifts northeast 
across the eastern lakes Tuesday afternoon/evening. While the 
models differ on the exact track of this low...the latest trends 
of the models warrant increasing wind speeds across Lake Michigan 
from Monday evening into Tuesday. While gales are possible... 
confidence is not high enough for mention especially since the 
track of the low is not certain. However...plan to bump speeds to 
30kts across Southern Lake Michigan by Tuesday morning. Winds 
quickly diminish Tuesday evening as high pressure moves across 
the lakes Tuesday night. This high quickly shifts east with a 
southerly gradient increasing across the western lakes by 
Wednesday night and have increased speeds during this time as 
well. Cms 


&& 


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


$$