101 
fxus63 klot 231613 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1013 am CST Monday Nov 23 2009 


Discussion... 
1010 am CST 


While fog is improving...ceilings are not as anxious to clear. 
Thickness is probably a couple thousand feet iaw acars soundings. 
See no breaks forthcoming as higher broken layer over fog area starts 
to clear out to the north. This is going to be a slow burn off and 
extended cloudiness well into the afternoon...makeing more of a cloudy 
forecast and omitting the /becoming partly cloudy/ entirely. 


Rlb 


&& 


Previous discussion... 
326 am CST 


Short term focus this morning is once again on fog across the 
area. Many sites have dipped into the 1-3 sm range so far early 
this morning. At this time...don't think potential is particularly high 
for widespread dense fog at many locations. High cloud cover 
continues to stream north across the area...and many sites have 
also started to develop low stratus deck...suggesting that surface 
visible probably won't drop too much more. Clouds should continue to 
develop and move across the region today as axis of warm air 
advection and weak shortwave move north from middle Missouri Valley. 
All short term model guidance suggests that rain chances are none 
with this first wave...so expecting nothing more than clouds with 
its passage. Tonight...looks as region may be under area of weak 
shortwave ridging/subsidence as aforementioned shortwave trough 
departs. With this in mind...there's certainly the potential for a 
period of partial clearing overnight...and if this is indeed the 
case...patchy fog would once again be possible early Tuesday 
morning. 


During the day Tuesday...band of rain expected to start spreading 
across the area from southwest to northeast as deep trough/upper 
low and associated occluded surface front moves east across 
northern Illinois. GFS/GFS ensemble/European model (ecmwf) and sref in good 
agreement in taking first upper low over or just south of Chicago. 
Recent NAM/WRF runs have been much further south with upper low 
than other guidance...and as a result was essentially discounted 
for this forecast package. 


Showers are expected to continue on Wednesday. Looks as if colder 
air will be a bit slower to arrive...and have delayed changeover 
to rain/snow mix until late Wednesday night. Still expect for 
changeover to all snow by Thursday...with snow showers possible 
throughout the day as secondary cold push/front and second upper 
low move south across area. Of note...recent GFS forecast 
soundings appear to be drier than previous runs...and if this ends 
up being the case...we could struggle to get sufficient ice nuclei 
for widespread snow on Thursday. At this distance though...will 
just maintain going snow showers until details become more clear. 
At the very least...surface temperatures will continue to be above 
freezing...precluding anything in the way of significant snow 
accumulation. 


Snow showers will remain possible on Friday before system departs 
to the east by Saturday morning. At this point...appears that 
upper ridging should give US another nice...albeit 
cooler...weekend. 


Boxell 


&& 


Aviation... 
550 am CST 


1200 UTC tafs...main aviation forecast issue will be timing the 
improvements to current IFR/LIFR conditions across terminals. 


Stratus deck which formed initially along southeast 
Wisconsin/northeast Illinois Lakeshore has advected/developed 
westward through time with shallow southeast low level flow. As of 
1130z westward extent of stratus deck appears to be impinging on 
Interstate 39 corridor. Did go ahead and include IFR conditions at 
rfd for a few hours early this morning. Surface observation and reports 
from southern Wisconsin indicate that fog more dense closer to and 
just north of the Stateline and may need to watch for possibility 
of short lived LIFR conditions at rfd through 14z or 15z. Dpa also 
should continue with LIFR conditions over the next few hours. 
Otherwise...expecting IFR conditions across remainder of terminals 
to gradually improve through the middle morning hours...and may see 
fog/stratus tend to lift into MVFR deck before dissipating late 
this morning. Persistence from past few days would suggest keeping 
idea of previous forecast with MVFR visibilities in haze persisting 
through the late morning hours is a good idea. Otherwise...this 
afternoon should generally just have some middle and high level 
cloudiness with continued light east southeast flow as pressure 
gradient remains fairly weak. 


For tonight...low level southeast winds will begin to veer to 
south southeast as storm system takes shape across Central 
Plains. This more southerly component will likely advect stratus 
deck across lower MS River Valley northward. Expecting these lower 
ceilings with stratus deck to arrive toward end of forecast period and 
for now have indicated transition back to MVFR ceilings after 10z. Fog 
potential is in place once again tonight but wildcard in how low 
to go with visibilities will be dependent on exact timing of stratus. 
There is a potential of getting down to LIFR visibilities again more rural 
locations and at this time have only gone IFR at dpa tonight. 


Marsili 


&& 


Marine... 
326 am CST 


Main marine forecast concern will be evolution of system to affect 
the western Great Lakes region Tuesday into Friday. 


Despite surface high pressure centered well northeast of the 
region...surface ridging continues to lag back into the western Great 
Lakes region which is resulting in only a continued weak to moderate 
surface pressure gradient. With continued eastward progression of 
southeast Canada high today...anticyclonic flow back across Great 
Lakes region will tend to weaken and thus more of an easterly wind 
component is expected to develop this afternoon...with speeds 
generally in the 10 to 15 knots range...although a few slightly higher 
gusts will be possible across far northern part of the lake. 


Attention then turns to fairly strong upper level disturbance moving 
through rockies at this time that will result in surface low pressure 
development across Central Plains tonight...eventually moving into 
southern Wisconsin Wednesday morning...and then across Northern Lake 
Michigan on Thursday. System will be occluding over western Great 
Lakes region so surface low will be slow to exit...and computer model 
guidance has overall been suggesting a slower trend to surface low 
eventually tracking east of the area late in the week. Only change 
made to marine forecast was to slow down onset of stronger north 
northwest winds behind this system more into the late Thursday 
night time period. 


Marsili 


&& 


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


$$ 

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