Weather

257 
fxus63 klot 282009 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
309 PM CDT Thursday Aug 28 2008 


Discussion... 
309 PM CDT 


Toughest portion of the forecast remains with our chances for 
precipitation through tonight and how potent they will be. Cloud cover 
from an mesoscale convective system that came through earlier today has really held back 
temperatures from rebounding this afternoon. Main hope for any 
convective initiation will be out ahead of the cold front 
currently propagating through eastern Iowa...but at this point it 
is not looking all that great. A few showers have already popped 
up along the Mississippi...but a rapid thickening of the cloud 
deck out in that region has once again stunted the growth of the 
cells. So...am not expecting much in the way of widespread 
convection...let alone severe convection. Conditions are a bit 
better further to the west where dew points are up near 70 and 
skies cleared out quicker than they have around here. Have dropped 
probability of precipitation down below likely category and have added scattered wording 
with just some isolated thunderstorms. After the front comes 
through tonight...skies are expected to clear out and in comes the 
dry conditions through the middle of next week. 


In the upper levels of the atmosphere...an amplifying ridge is 
expected to lift northward from the Gulf Coast on up into the 
Great Lakes by early next week. Most of the Midwest will be in 
southerly flow from the surface on up into the upper portions of the 
atmosphere...which will lead to a surge of warmer air into the 
region. 850mb temperatures are expected to quickly climb into the 17c to 
19c range on Sunday as the upper level ridge axis propagates into 
the Central Lakes. This should lead to high temperatures reaching the middle 
to high 80s from the weekend on into the middle of next week. This 
could be one of the better "heat waves" of the Summer as there 
wont be any stalled out frontal boundaries near the region that a 
wayward mesoscale convective system can track along and hold back temperatures. 
Instead...we should be in the core of the ridge away from any 
other influences or cloud cover that would inhibit any lowering of 
temperatures. Will refrain from tossing in any 90 degree temperatures up 
near rfd yet...but at this point would not be surprised to see 
them get their first 90 of the year. 


Next chance at any precipitation will be toward Thursday as a cold front 
comes through the region in association with an upper level 
shortwave. There still are some timing differences between the 
European model (ecmwf) and GFS...but it looks like their trending toward the European model (ecmwf) 
solution of bringing it through later rather than earlier. The 
remnants of Gustav may also pump some moisture up in ahead of the 
front and add more coverage of precipitation. So...added some low chance 
probability of precipitation Wednesday night into Thursday. 


Halbach 


&& 


Aviation... 
1230 PM 


Thunderstorms are the forecast problem this afternoon and evening. 


The cold front is forecast to move through north central Illinois 
between 23 UTC and 02 UTC. Showers and thunderstorms are forecast to 
move through Rockford at that time. The front is expected to move 
through northeast Illinois and Northwest Indiana by 03 UTC. We 
expect showers and thunderstorms between 00 UTC and 03 UTC for the 
Chicago area airports and Gary. Lingering showers are expected after 
the front passes and we will forecast MVFR ceilings. Some ground fog 
may be around after 06 UTC. A temperature inversion is forecast 
between 1000 to 1200 feet Friday morning. The wind is expected to be 
from the north Friday and we expect a decrease in moisture in the 
air. We will forecast VFR conditions on Friday. 


&& 


Marine... 


245 PM CDT 


A cold front will approach Lake Michigan tonight. This will give a 
wind shift from the south to northwest by Friday morning. By 
Saturday morning a high will move into the region from Iowa to 
Michigan. We expect the wind will become light Saturday. However 
on Friday there will be a wind of 15 to 20 knots. This may produce 
some significant waves of 3 to 5 feet on the south half of Lake 
Michigan Friday. 


&& 


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


$$