As I mentioned yesterday, Illinois state climatologist Jim Angel has certified that March 1 to May 31 was the warmest spring in Illinois history:
This year the statewide average temperature for spring in Illinois was 15.1°C. That makes it the warmest spring on record for Illinois. The statewide records go back to 1895. The [three] warmest springs in Illinois were:
2012 with 15.1°C
1977 with 14.1°C
1921 with 13.3°C
It was also Illinois' fifth-warmest May ever. And so far, it's the warmest year ever in Illinois (since January 1). Interestingly, Angel points out, "of the top five warmest January-May periods, three have occurred in the last 15 years."