250 
fxus63 klot 211215 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
615 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... 
613 am CST 


1200 UTC tafs...aviation forecast concerns will be fog both early 
this morning and Sunday morning. 


Surface ridge axis extends from the West Virginia into northern 
Illinois early this morning. Subsidence in the wake of departing 
upper level short wave across western Great Lakes has allowed for 
mainly clear skies across far northern Illinois...although some 
patchy stratocu noted on satellite imagery this morning across 
central Illinois and across eastern Iowa. Based on current 
satellite trends and expectation of continued synoptic scale 
subsidence...not expecting these low clouds to affect terminals. 
Clear skies/light winds have led to conducive radiational cooling 
conditions and patchy fog is common across the area. Ord/mdw 
generally holding at or above 5 miles for visibilities with temporary 
visibility reductions to IFR at dpa. Did include tempo IFR visibilities for 
patchy denser shallow fog for a few hours this morning through 14z 
at dpa/rfd closer to the surface ridge axis. After morning fog 
dissipates...quiet weather expected through the day with perhaps 
some low coverage cumulus developing around 3k feet...but not expecting 
ceilings at this time. Surface ridge axis beginning to depart to the 
east should result in light southeast winds through the day of 5 
to 10 knots. Good radiational cooling conditions in store again 
tonight with winds expected to become light once again under mostly 
clear skies. At this time have included MVFR visibilities across all 
terminals tonight...but later forecasts may need to go a bit more 
pessimistic at dpa/rfd with possibility of some short lived IFR 
visibilities not out of the question...but did not have confidence to 
insert with the 12z tafs at this forecast distance. 


Marsili 


&& 


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-ilz021-ilz023-ilz032- 
ilz039 until 9 am Saturday. 


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


$$