880 
fxus63 klot 192330 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
530 PM CST Wednesday Nov 19 2008 


Discussion... 
237 PM CST 


Main focus is next 48 hours starting with the approaching cold front 
swinging through the upper Midwest this evening. Massive Post frontal 
SC deck dropping southeast behind this front is highliting the cold 
air advection streaming this way. This surge of cold Canadian air 
may wring out some scattered flurries from this deck overnight but 
otherwise non-incidental except for Northwest Indiana. 


Cold air advection over Warm Lake temperatures again spawning lake effect snows for that 
area. Delta T of 15c over the lake expected to prompt showers by 
midngt overnight...reaching 20c after sunup. Expecting the brunt of 
snow showers to affect Porter County toward noon Thursday and 
accumulating several inches by end of day with 2 to 3 more inches 
overnight Thursday Night. Lake and Jasper counties likewise should 
experience several inches of snow by the end of the event late 
Thursday night. Expecting maybe an inch or two Thursday with 
another inch or two Thursday night. Snowfall affecting mainly the 
eastern half of these counties. 


Winds backing more westerly after midngt Thursday night and heaviest 
snow drifting to the east of Porter County. Northwest surface winds gusting 
to 25 kts or better likely to cause traffic difficulties with visibility 
reduction and difficulty in clearing roads. Have already issued 
lake effect Snow Advisory for Thursday and Thursday night and will 
continue this with little if any change. Clearing skies should move 
into Illinois Thursday night with building high pressure and subsequent 
subsidence. This clearing edging into Northwest Indiana late Friday. 


Models swinging another wave from the Pacific across the plains on 
Friday. This wave begins to dampen out upon reaching the MS valley on 
Saturday. Will likely be increasing cloudiness Saturday with possible 
scattered snow showers or flurries. For now however...will leave dry and 
resolve this with next couple of model runs. 


Warming 850 temperatures in the plains expected to spread above zero 
values into the Dakotas by the weekend. With next wave swinging 
across the plains on Sunday...these temperatures working east ahead of 
it. While 50s not expected to return as yet...40s more likely with 
this overrunning warm advection late in the weekend. May be 
looking for a chance of rain or snow advecting into Northwest Indiana 
from the southwest Sunday night with wraparound flurries into 
Illinois Monday into Tuesday as the main vortex passes north of 
the Great Lakes. This feature is followed by another ridge of 
high pressure working across the MS valley and clearing skies for 
Wednesday. 


Rlb 


&& 


Aviation... 


00z tafs...winds and ceilings are the two primary concerns this 
period. 4kft deck has overspread NE Illinois late this afternoon. Observation 
upstream across WI indicate ceilings generally between 3-4kft. Model 
soundings and guidance would tend to support this trend through 
the night and will maintain overcast ceilings in that range. However... 
leading edge of clouds across northwest Illinois back across Iowa indicate a 
period of high MVFR ceilings is possible...mainly at rfd during the 
evening hours. Ceilings may dip back toward high MVFR Thursday morning 
at/after sunrise...but expect ceilings to slowly lift through the day and 
begin to scatter out Thursday evening. 


Cold front has moved through the tafs with winds now from the 
northwest but gusts have been lacking with only a few gusts to 
18kts or so. Observation further northwest suggest strongest push of winds 
will be southwest of the terminals. Have lowered gusts for this 
evening but am reluctant to remove as soundings show 20-25kts through 
the low layers all night. Trend should be for gusts to increase 
shortly as cold air advects in then diminish somewhat overnight... 
only to increase again middle morning Thursday. Wind direction seems 
very consistent from 330/340. But will need to monitor gusts this 
Evening. 


Lake effect snow showers develop overnight well east of gyy. But 
by late morning...could be a few flurries or possibly a passing 
light snow shower. Have only included mention at gyy for now. Cms 


&& 


Marine... 
230 PM CST 


Low pressure over Lake Superior will move to the eastern Great 
Lakes by tomorrow morning. Strong high pressure will build out of 
southern Saskatchewan and into the northern plains Thursday. The 
high will spread across the Mississippi Valley Thursday night and 
Friday. The combination of the strong high pressure over the 
northern plains and the low to the east will keep northwest to 
north gales over Lake Michigan through Thursday afternoon. As the 
ridge of high pressure pressure builds over Lake Michigan Friday 
night...winds and waves will decrease. 


Krein 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...lake effect Snow Advisory...inz002...9 am Thursday to 9 am 
Friday. 


Lm...Gale Warning...lmz080-lmz261-lmz362-lmz364-lmz366-lmz563-lmz565- 
lmz567-lmz669-lmz671-lmz673-lmz675-lmz740-lmz741-lmz742- 
lmz743-lmz744-lmz745-lmz777-lmz779-lmz868-lmz870-lmz872- 
lmz874-lmz876-lmz878 until 6 PM Thursday. 


&& 


$$