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fxus63 klot 211200 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
700 am CDT sun Mar 21 2010 


Discussion... 
333 am CDT 


The band of snow this morning is a result of some 
frontogenesis in the upper air. This is seen on the 700 mb 
profiler streamline analysis at 07 UTC. This upper air forcing 
will diminish and move south by 18 UTC. The NAM model forecasts 
this feature the best. We will leave likely snow for the early 
morning as it is seen on the radar. Then we will forecast a chance 
of precipitation later this morning. We used the NAM model for the 
temperature forecast after checking the model performance for last 
several days using mean absolute errors. A northeast wind will 
continue today through Monday as a cyclone over Arkansas moves 
into Kentucky. A chance of rain may linger in our southeast 
portion of our forecast area. 


Tuesday through Saturday...the high temperatures will rise to 
the middle 50s by Wednesday. There will be a chance of showers 
Thursday and Thursday night. A cold front will move through 
northern Illinois and Northwest Indiana . A large high is forecast 
to move into the Lake Superior and Ontario region. Cooler weather 
is forecast for Friday and Saturday. 


&& 


Aviation... 
700 am CDT 


1200 UTC tafs...area of MVFR conditions remains across the chi 
area terminals as light precipitation...-sn northwest and -ra southeast...persists. 
Advection of dry air as surface ridge sank further S toward the area 
had already eliminated any clouds below 060 at krfd since late last 
evening. 


Southern end of short wave trough that has moved east-southeast from 
Manitoba to central Ontario during the night is prognosticated to 
continue across the North Woods and into the upper Great Lakes today. 
Between the short wave trough passage and the arklatex close low 
slowly moving east-southeast...the baroclinic zone and associated lower and 
middle level isentropic lift gradually shifts further S to northwest ind 
and west central Illinois. This to finally bring and end to lingering 
light precipitation in the immediate vc of the local terminals by late 
morning/early afternoon. 


With light precipitation band shifting S and low level dry air advection 
pushing deeper into the area expect prevailing ceilings to lift to 
VFR middle to late morning in chi area...and a continued clearing 
trend as edge of cloud shield associated with the southern upper 
low to be just to the southeast of the terminals. 


Only a moderate pressure gradient remains across the local area 
with the high pressure ridge across northern lower Michigan...Central Lake 
Michigan...and east central and SW WI. Ridge only make slight progress 
further S today before becoming near stationary just to the north of 
the terminals so NE winds to remain in 10-13 knots range into the 
afternoon. 


Trs 


&& 


Marine... 
420 am CDT 


A high pressure ridge extending across the upper MS valley and 
the upper Great Lakes to southern Quebec will sink a little 
further S today before becoming near stationary across Central 
Lake Michigan. This ridge will then weaken while low pressure in 
Arkansas moves to the northeast...reaching the upper Ohio Valley by 
late Monday. 


Winds on northern and Central Lake Michigan are light with the 
ridge overhead or nearby. Winds on Southern Lake Michigan have 
slackened a bit as the tighter pressure gradient has shifted S 
to central Illinois and northern in...just clipping the far S end of 
the lake. With winds falling off a tad and waves to also subside 
this morning will allow the Small Craft Advisory expire at 7 am 
CDT. Winds will favor a SW direction and increase a little during 
the day on the north end as the ridge axis sinks further S over 
Central Lake Michigan. 


Between the low moving up the upper Ohio Valley and high pressure 
moving across northern Ontario into Quebec Monday...east to NE winds 
will prevail at up to 20 knots on the S end where gradient will be 
strongest on periphery of upper Ohio Valley low. Looks marginal at 
this time for reaching Small Craft Advisory criteria for Illinois and in 
waters during Monday. 


A cold front will move through the upper Great Lakes late Tuesday 
and night shifting winds to northwesterly...then veering to 
northerly at up to a sustained 20 knots as high pressure builds from 
the northern rockies to the Central Plains Tuesday night and 
Wednesday. 


Trs 


&& 


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


&& 


$$