Today is a good example of why a tornado alley of higher tornado frequency & higher frequency of strong to violent tornadoes historically exists in our area from southern Benton, northern Warren through Tippecanoe, southern White to Carroll, western northwestern Howard counties.
We are often divided by a strong warm front in spring & early summer in the area.
It is 50s north to 80s south at times. Today saw 55 at Hebron, along the Kankakee River to 80 at Veedersburg. At West Lafayette, we hit 73, but Fowler reached 65.
The lake breeze front is also a big factor this time of year. A north to northwest to northeast wind fetch blowing over still icy-cold waters can send a wave of cold air inland all the way to Pine Village, Lafayette & Monticello.
Both may merge & create a zone of enhanced shear in a severe weather situation. This pattern has seemed to exist since the earliest weather records commenced in our area.
It just isn't in spring & early summer, however, but this line is the site of a large-scale climate zone change.
We see this zone change in the seasonality of everything from first 70s & 80s days to first & last freezes to average cloudiness each month to precipitation frequency & seasonality through the year.
It is also a pronounced transition zone in severe weather climatology over the area.
It shows up well on the vegetation advancement in spring & the progression of fall foliage in the fall.
There is greater influence from Lake Michigan in Newton, Jasper, Pulaski, White, northern Benton to Fulton & northern Cass counties with lake breeze front, lake effect snowfall & lengthy period of lake-induced gloomy, low stratus & stratocumulus cloudiness during the year.
So, we have this zone change from Indianapolis-type climate (typical central Indiana climate) to a Chicago-type climate (northern Illinois, southern Michigan-type). There is even an intersection of more Iowa & northern Missouri-type climate in Newton & Benton to Kankakee counties that is much more like a traditional tallgrass prairie climate (much more wind, rainfall, storm maximum in July, drier winters, more snow than rain in winter, etc.)
Note the transition overall in tornado maximum frequency (does not account for peak in maximum tornado strength in the year) from late May in our southwest (& really southern areas for that matter to around Memorial Day or very early June in Lafayette area to more like mid June in our north, aligning with Chicago area.