359 
fxus63 klot 231148 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
548 am CST sun Nov 23 2008 


Discussion... 
239 am CST 


Main forecast concern is mixed precipitation late tonight and Monday 
morning. Weakening short wave moved through the Ohio Valley 
overnight. Water vapor satellite image shows nice subsidence 
drying across Illinois in the wake of this feature. Ridging across the 
Midwest today will bring abundant sunshine and southerly surface 
winds...which should allow temperatures to climb into the 40s. 
Brisk south wind will keep it feeling chilly though. 


A pair of short waves...somewhat in phase...were moving east. One 
was moving across the Northern Prairie provinces of 
Canada...with the other plowing through The Rockies. The southern 
short wave looks pretty vigorous and should generate some precipitation 
beginning around midnight. Model forecast soundings from GFS would 
indicate column will be cold enough for snow except surface 
temperatures remain at or just above freezing. NAM sounding shows 
a deeper warm layer in the low levels. Lots of dry air will cause 
cooling of column as precipitation begins...so I expect the precipitation 
will be mostly snow in far north Illinois. Will continue to stay pretty 
close to previous forecast with rain-snow mix over far northern Illinois 
changing to mostly snow by 12z. Might get an accumulation of an 
inch or two along and just north of surface low track...roughly 
north of a Dixon to Libertyville line. South of there...through 
the west suburbs and Chicago there could be some light slushy 
accumulation during peak precipitation intensity...which looks to be just 
before or around Monday morning rush hour. Temperatures should 
remain above freezing all day...so any snow that falls will not 
stick around long. South of I-80 precipitation should be lighter and 
mostly rain. All precipitation should exit into Indiana by midday and we 
might even get some breaks in the clouds Monday afternoon. Then 
upper low dives down out of Canada...an area of light snow or 
flurries is possible Monday night. 


Northwest flow behind this system will drop 850 temperatures to 
about -10c Monday night into Tuesday but low level winds should 
keep lake effect snows into north central in and Michigan. Will just 
have a chance of snow showers in northeast Porter County...with no 
accumulation expected at this time. After a cold and blustery 
Tuesday...medium range models continue to advertise upper low 
wobbling around the eastern Great Lakes for a few days. This keeps 
northern Illinois/northwest in in dry quiet pattern with slight moderation in 
temperatures by Thanksgiving. No real changes beyond Thanksgiving 
at this time. 


Allsopp 


&& 


Aviation... 
548 am CDT 


1200 UTC tafs...fairly quiet start to the taf period today with 
just some high clouds and some gusty south-southwest winds to 
20kts to contend with. Things will get more interesting overnight 
as two systems converge on the region. One low in southern Canada 
will drag a cold front east as another low at the southern edge 
of the front tracks east with it toward Southern Lake Michigan. 
Conditions will rapidly deteriorate early on Monday as the Gulf 
opens up as a moisture source for this system. Precipitation will begin 
to fall around 9z mainly as rain at the chicagoland taf sites and 
mainly snow at Rockford. As low level temperatures continue to 
cool...precipitation will transition from rain to all snow and persist 
into middle morning. Ceilings will quickly drop to MVFR around 9z and 
then to IFR by 12z at Ord/mdw. After the gusts this 
afternoon...winds should remain at or below 12kts through Monday 
morning when the cold air surging in may bring in some more gusts 
for Ord Monday morning. 


Expect VFR conditions until overnight when ceilings/visible will likely 
drop to MVFR and then to IFR by early Monday morning 


Halbach 


&& 


Marine... 
214 am CST 


After a quiet period over the lake as a broad area of high 
pressure moves through the eastern US...things will begin to 
become more active toward Monday as two weather systems merge over 
the Lake. One system will be diving southeast across southern 
Canada while the other will be tracking through the plains along a 
cold front that extends south from the first low. As these lows 
meet up over the central Great Lakes...the subsequent low will 
deepen and pull down much colder air from Canada. This deepening 
will increase wind speeds across the lake to the point where 
frequent northwest gusts to gale force appear likely for Monday 
afternoon into Tuesday night. This low is expected to stall out 
across the eastern lakes through the end of the week...but should 
weaken as it loses most of its upper level support. Will be 
issuing a gale watch for 18z Monday through 00z Wednesday with the 
3am Gulf and may include some of the Indiana nearshore zones in 
that watch. 


Halbach 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...gale watch...lmz080-lmz261-lmz362-lmz364-lmz366-lmz563-lmz565- 
lmz567-lmz669-lmz671-lmz673-lmz675-lmz745-lmz777-lmz779- 
lmz868-lmz870-lmz872-lmz874-lmz876-lmz878...noon Monday to 
6 PM Tuesday. 


&& 


$$