098 
fxus63 klot 210906 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
306 am CST Sat Nov 21 2009 


Discussion... 
306 am CST 


Short term concerns center on fog potential early this morning and 
again tonight. Attention then turns to middle week storm system. 


This morning...region remains situated under the control of 
surface ridge extending from Middle Atlantic States west through 
central Illinois. Skies have been able to clear out across much 
of the region in the wake of departing weak shortwave. As a 
result...fog has formed across much of the area...with many 
locations so far dipping into the 2-4 sm range this morning. 
Wouldn't be surprised to see visible lower a little more toward 
sunrise as planetary boundary layer continues to cool...and patches of dense fog will 
remain possible through middle morning. 


Once the fog dissipates later today...should see the beginnings of 
a very nice middle November weekend with afternoon highs warming into 
the middle 50s across the area. Scenario basically repeats itself 
tonight and Sunday...with patchy fog again possible late tonight 
and Sunday morning...and highs on Sunday in the middle to perhaps 
upper 60s. 


On Monday...advertised middle week storm system will be beginning to 
organize over the northern High Plains. Expect to see middle and high 
level cloud cover start to increase as warm air advection takes shape across 
areas just to the west of lot County Warning Area. Numerical guidance continues to 
struggle with evolution of this system...with European model (ecmwf) generally 
slower and more cutoff from main westerlies compared to much more 
progressive operational GFS. Canadian and GFS ensembles have 
generally been splitting the difference...and seem like the 
prudent solution to follow until details become less Muddy. Of 
note...00z Euro that came in early this morning has come in more 
progressive...seemingly at least a trend toward general idea of 
GFS ensemble/Canadian. 


Confidence is starting to increase that rain will overspread the 
area Tuesday into Wednesday as occluding front moves through the 
area. Chances are that dry slot of some sort will move in behind 
this first shot of rain...but attempting to determine specifics of 
such a detail at this distance all but impossible. Additional 
precipitation will be possible Wednesday and Thursday as main 
upper trough axis...and perhaps secondary cold front...pushes 
through. Precipitation type certainly in question Wednesday night Onward 
as GFS is trending colder than Euro. Specifics of rain vs snow 
will be entirely dependent on exact track and strength of upper 
trough. At this time...decided to change little in the way of 
inherited precipitation type to avoid potential flip flops in later 
forecasts. 


Boxell 


&& 


Aviation... 


0600 UTC tafs...main concern for the remainder of the night 
continues to focus on visibility...particularly at rfd/dpa and 
during the early morning hours. 


The latest satellite imagery shows quickly clearing. This will 
enhance radiative cooling...and when combined with saturated 
ground and winds quickly become lgt/vrbl...visibility should drop 
to MVFR at rfd/dpa shortly after midnight. Due to urban 
influences...Ord/mdw should be a little slower to respond and not 
drop to MVFR until a few hours later. Ord/mdw should remain at 
MVFR with rfd/dpa having a better chance to drop to 
IFR...especially during the pre-dawn hours. 


The fog should burn off shortly after sunrise and the remainder of 
the day will be quiet for aviation weather concerns. 


Krein 


&& 


Marine... 


306 am CST 


Overall very few changes made to the nearshore and open waters 
forecast. A broad surface anticyclone stretching from northern 
Illinois to West Virginia will continue to slowly slide east of the 
area setting up a light south wind over Lake Michigan for this 
afternoon. Upper level ridging is expected to persist across 
southeast Canada/northeast United States over next few days which 
will allow some weak surface ridging to hang back across the Great 
Lakes region. Thus...despite main surface anticyclone shifting east 
northeast next few days...gradient will be slow to strengthen...so 
modest south wind speed increases into the 10 to 15 knots range still 
seems reasonable by Sunday. A fairly strong upper level trough will 
move into the Central Plains Monday night/Tuesday with next surface low 
taking shape across the Central Plains. This low will approach the 
western Great Lakes by Wednesday. The expected track of this low 
would support continued south southeast winds through middle of next 
week. While speeds are expected to generally remain in the 10 to 20 
knot range...the extended south southeast fetch could allow wave 
heights to build into the 3 to 5 feet range toward the end of the 
forecast period late Tuesday night into Wednesday. 


Marsili 


&& 


Lot watches/warnings/advisories... 
Illinois...dense fog advisory...ilz008-ilz010-ilz019-ilz032-ilz039 until 9 
am Saturday. 


In...none. 
Lm...none. 
&& 


$$