714 
fxus63 klot 191816 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
116 PM CDT Friday Mar 19 2010 


Discussion... 
336 am CDT 


Snow will fall tonight. Some accumulations may reach an inch of 
wet snow in north central Illinois...northeast Illinois and 
Northwest Indiana by Saturday evening. 


A cold front will move into north central Illinois this evening. 
This front will bring rain showers into north central 
Illinois...northeast Illinois and Northwest Indiana tonight. The 
forecast soundings show mostly rain at first. Then rain mixed with 
snow after midnight. The snow will probably be a wet snow. We used 
a 9 to 1 snow ratio for the snow forecast. A lot of the moist 
layer above the surface was forecast to be warmer than minus 10 
degrees celsius on the forecast sounding valid at 3 am CDT. 


The 500 mb wave trough is forecast to extend south to Texas by 
Saturday evening. All the models show a cyclone intensifying over 
Arkansas by Saturday evening. The cyclone will move into Kentucky 
by midnight Monday morning. Cold air will return to the our 
forecast area by Saturday morning. For most of our forecast area 
we will forecast snow Saturday morning. We will forecast rain south 
of the Kankakee River. By Saturday afternoon the rain and snow 
mix will be south of the Kankakee River. 


The surface low is forecast to move into West Virginia by Monday 
afternoon. Rain may continue Sunday and Sunday night in our far 
south east counties. Some 850 mb wrap around warm air will move 
into central Illinois by Monday morning. This entire upper air 
trough system will become closed circulation Monday as it moves 
into North Carolina. A weak ridge will move into Illinois late 
Tuesday night. The temperature will climb into the 50s by Tuesday. 


We used a combination of the GFS and the European model (ecmwf) models for the long 
range forecasts and the NAM and European model (ecmwf) for the short range 
forecasts. This was based on the cold air advection at 850 mb and 
the 500 mb trough. 


&& 


Aviation... 
115 PM CDT 


1800 UTC tafs...primary aviation concerns will be timing wind 
shift with approaching cold front later this afternoon/early this 
evening and resolving precipitation type and associated impacts on 
ceilings/visibilities from tonight into at least Saturday morning. 


Surface cold front at 17z extends from south central Wisconsin just 
south of msn into far west central Illinois. Previous tafs still 
look to be on track with timing this wind shift into the terminals 
late this afternoon...with more of an early afternoon wind shift 
at rfd as winds have already veered more westerly in the vicinity 
of surface trough axis. Also expecting front to make a typical 
acceleration southward down colder Lake Michigan and impact 
Ord/mdw/gyy around the 22-23z time frame. Low cloudiness lags well 
back to the west behind this front and maintaining VFR conditions 
through most of the evening hours still seems prudent given the 
initially dry air mass in place and stronger low level cold 
advection lagging surface front. Main change to tafs was to introduce 
some gustiness later tonight in association with the stronger low 
level cold advection with some gusts to around 20 knots possible 
by late evening. Other concern tonight will be timing Post frontal 
precipitation with regional radar imagery at 18z indicating precipitation 
extending from northeast Iowa back into eastern Nebraska. 
Expecting this area of light precipitation to gradually shift into 
northern Illinois late this evening as middle level baroclinic 
zone/frontogenesis overspread the area. Low level thermal profiles 
still seem to support initial period of light rain late tonight 
before colder air spreads southward likely changing precipitation to 
snow. Given the stronger cold advection overnight and precipitation 
onset...did trend to MVFR ceilings after midnight...and to lower end 
MVFR ceilings around 1500 feet by 12z. IFR visibilities reductions are possible 
with the snow generally in the 10z-17z period. Drier low level air 
will advect southward afternoon which should begin to take focus 
for steadier light snow south of the terminals. Gusty northeast 
winds to continue into tomorrow with some gusts to 20 to 25 knots 
possible Saturday morning before gusts begin to diminish more into 
the 15 to 20 knot range middle to late afternoon. 


Marsili 


&& 


Marine... 
255 am CDT 


As low pressure in southern Ontario moves east into Quebec 
today...a cold front trailing to the southwest will move east 
across Lake Michigan veering winds from SW and west to northwest. 


A large...cold high will build from Alberta south to the central 
rockies and High Plains overnight. This will increase the gradient 
across the upper Great Lakes...causing further veering to the north 
and NE as well as stiffening winds to a strong breeze. 


Winds will start to slacken during Saturday...especially on the 
northern portion of Lake Michigan as a ridge extending 
northeastward from the high centered in the Central Plains drops 
south over the upper Midwest and upper Great Lakes. Low pressure 
moving across the middle MS valley to the lower Ohio Valley Saturday 
night and Sunday will maintain a moderate pressure gradient 
across the south portion of the lake so while winds will slacken 
a bit there will still be fresh breeze out of the NE. 


Trs 


&& 


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


$$