I am not going to every specific about how a squall line develops around a weak upper level low pressure system. But I will say this; every forcast, TAF, METAR and radar report that day indicated no severe weather of any sort in upper midwest (wi and surrounding states). As far as diverting goes, once the clouds were recognizably dark enough to warrant significant weather in the vicintiy to the north I was base-final with still no wind direction change on the sock. I landed the plane, parked it, and went into the FBO to check on this dark set of clouds. That is when I determined it was a squall line. Then I had the plane buttoned up and in a hangar BEFORE the storm hit. It was not until after I landed that the squall line moved over the airport. After that the weather was all clear and completed the checkride. Perhaps that adds clarity to my previous post.