882 
fxus63 klot 201800 
afdlot 


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


Discussion... 
958 am CDT 


Acars and forecast soundings show warm layer just above freezing 
around 800-900 mb over areas S of Chicago. Mix of rain...sleet and 
snow was occurring in the southern fringe of the precipitation. Precipitation 
making very slow progress southward. Updated to add sleet to narrow 
ribbon of precipitation S of I-80 corridor. Also cut probability of precipitation to chance or 
slight chance far south over Ford...Iroquois counties into 
Newton...Jasper and Benton in. As precipitation winds down this afternoon 
temperatures may recover a couple degrees into middle 30s...so tweaked maximum 
temperatures a degree or two up. 


Allsopp 


Previous discussion... 
334 am CDT 


Colder air and snow will move into the forecast area of north 
central Illinois... northeast Illinois and Northwest Indiana 
today. 


Today and tonight...the radar shows the band of snow and rain 
moving into north central Illinois at 08 UTC this morning. The 
precipitation will spread across the forecast area today. Most of 
the area north of a line from Valparaiso Indiana to Pontiac 
Illinois will get snow today. There may be snow mixed with rain 
early this morning...but the precipitation will be all snow this 
afternoon. South of the line rain mixed with snow is expected to 
occur. We used the NAM and GFS models...after looking at the 850 
mb temperature advection...forecast soundings and acars soundings 
and the 500 mb height pattern with the water vapor imagery loops. 
For the temperature forecast we used the NAM for the maximum 
temperature today because of its lowest mean absolute error for 
the last 7 days. The temperature is expected to be near steady 
today as the cold air moves into Illinois and Northwest Indiana. 
We adjusted the NAM model forecast. The accumulation of snow today 
through Sunday is forecast to be 1 to 3 inches. Most of the snow 
will fall in the central portion of the forecast area. South of 
the Valparaiso Pontiac line will be rain and snow. The forecast 
soundings show layers above freezing below 850 mb for that area 
through today and tonight. 


Sunday through Tuesday...a strong surface low will develop in 
Arkansas and will move into Kentucky and West Virginia by Monday 
morning. The upper air will have a strong cyclonic circulation 
with this low and the 850 mb wind will be east to northeast over 
northern Illinois and Northwest Indiana. This may bring some 
warmer air as the circulation brings the air from the southeast 
and wraps it around to Illinois. We expect the Sunday high 
temperature to be in the 40s and the precipitation will be in the 
southern half of the forecast area. We expect this precipitation 
to be rain. The rain will move east Monday and Monday night. We 
forecast the high temperature to be in the 50s Tuesday. 


&& 


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... 
315 am CDT 


Low pressure in OK will move east to Arkansas...and a broad high extending 
from The Rockies to Ontario will settle S over the southern and 
Central High plains...the upper Midwest...and the upper great 
by this evening. This will maintain the moderately strong pressure 
gradient across the S half of Lake Michigan resulting in NE winds to 25 
knots. Conditions will warrant the continuation of the Small Craft 
Advisory for the Illinois and ind nearshore waters into tonight. On 
the north portion north winds to 20 knots will diminish and eventually become 
variable 10 knots or less by evening as the ridge axis across Ontario 
drops S into the northern Great Lakes region. 


The ridge axis slowly settles across Middle Lake overnight then 
remains there as low pressure moves from the Ozarks across the Tennessee 
Valley Sunday night. The low will maintain 15 to 20 knots NE winds on 
the S end of the lake Sunday and Sunday night while north of the ridge 
axis SW winds of 15 to 20 knots will be found on Northern Lake Michigan 
during this time. 


Monday and Monday night a high will move southeast across northern 
Ontario to central Quebec with winds east to NE winds across the 
entire lake before veering to the southeast and S ahead of a low 
moving east from the Dakotas into the upper Midwest. 


Trs 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...nearshore waters until 1 am Sunday. 


&& 


$$