022 
fxus63 klot 202027 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
327 PM CDT Sat Mar 20 2010 


Discussion... 
258 PM CDT 


Band of mixed precipitation continues along strong baroclinic 
zone with heaviest now along I-55 corridor. There has been a 
gradual shift southeastward today...with snow diminishing toward 
WI border. Rain-snow line appears to be along I-55. Latest 
surface observation show precipitation has changed to snow at pia but rain at 
pnt. For overnight will go with highest probability of precipitation S and SW part of 
forecast area with lower probability of precipitation near WI border. Expect all snow 
north but a mix of rain and snow and maybe some sleet along I-55 
corridor. Far southeast part of area should just see rain. Some areas...S 
of Chicago to Ottawa...could see up to an inch the rest of this 
afternoon and this evening but mostly on grassy areas with little 
or no travel impacts. Overall trend in models is for the 
baroclinic zone to gradually weaken overnight as upper low spins 
up in Southern Plains/lower MS valley. Low level temperatures gradually 
warm above freezing by Sun morning. At the same time relative humidity in the 
dendridic growth zone of -10c to -15c decreases. Thus only expect 
a chance of light rain/snow in the early morning...then just a 
chance of light rain the rest of the day Sunday. 


As upper low lifts NE into the Appalachians Monday and Tuesday the 
bulk of the precipitation will stay southeast of forecast area. Expect clearing 
north to S across forecast area. Temperatures should recover somewhat Sunday 
without precipitation...and even more Monday and Tuesday with increasing 
isolation. Exception will be along the lake with weak 
northeasterly winds. Winds may shift SW by Tuesday afternoon in 
advance of approaching cold front. GFS now more in line with European model (ecmwf) 
in bringing weak cold front through Tuesday night/Wednesday 
morning. Expect to see some cloudiness but only slight chance of a 
sprinkle. Not a lot of cooling behind this front but lake 
influence will be enhanced with synoptic NE winds. 


Yet another low to pass to our south for the end of the week. At 
this time will include a chance of rain south...otherwise dry. 


Allsopp 


&& 


Aviation... 


18z tafs...multiple forecast concerns and challenges through early 
Sunday morning mainly revolving around the duration of snow. 


Periods of moderate snow have pushed mainly east of the terminals 
early this afternoon but radar imagery shows additional stronger 
returns showing up across parts of central Illinois. Expect this to 
continue northeast and affect the terminals this afternoon. So 
while ceilings have improved to low end MVFR in most location...IFR 
visibilities...likely being affected in part by fog/mist...should 
persist for several more hours with IFR ceilings likely to return in the 
heavier pockets of snow. Good consensus among the models that this 
upcoming wave of precipitation will dissipate late this afternoon into 
this evening and have trended both visible and ceilings into MVFR. Exact 
timing will require some refinement with perhaps an extension of 
IFR at mdw/gyy needed. Overall improvement expected through the 
evening with perhaps some flurries lingering overnight. Further 
northwest at rfd...VFR ceilings just to the northwest. Still some 
light precipitation occurring and confidence as to when this ends and 
when ceilings go VFR is rather low. Opted to hang onto MVFR ceilings through 
early evening and may also need to refine this as well. 


Northeast winds around 15kts or so will continue to gust to 
20-25kts...though gusts will gradually diminish during during the 
late afternoon into the evening hours. Winds remain northeast through 
the period with a few higher gusts possible Sunday. Cms 


&& 


Marine... 


224 PM...a cold front/trough of low pressure extends from New 
England southwest to the western Gulf of Mexico....with a ridge of 
high pressure extending across the northern lakes. New low 
pressure will develop along this cold front over Arkansas tonight 
then slowly move northeast across the Ohio Valley while deepening. 
The ridge across the upper lakes will sag south as another area of 
low pressure moves across northern Ontario and James Bay Sunday. 
As a result of this complex pattern...light and vrb winds over the 
northern parts of Lake Michigan will turn southwesterly tonight. 
Any area of light winds will persist across Central Lake Michigan 
Sunday and Sunday evening until the ridge weakens. Further 
south... in response to a tightening gradient from the low moving 
across the Ohio Valley...expect a persistent northeast flow over 
Southern Lake Michigan through Monday night...when a ridge of high 
pressure moves across the entire lake. This northeast flow could 
be a bit strong a times...15-20kts seems reasonable with higher 
gusts possible. With a short break in winds expected Sunday 
morning... will extend the Small Craft Advisory through 12z Sunday 
mainly for wave action. Cms 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...nearshore waters 
until 7 am Sunday. 


&& 


$$