According to the NOAA, May 2016 was ranked 49th coldest out of 122.
That is essentially a tie with May 2015. The warmest May Tmax’s were 1934, 1936, 2012 and and 1939.
The following map shows state rankings. 122 = warmest. 1 would be coldest.
Note the states ranked 7th and 8th coldest. Wow. Not much El Nino effects …