The NOAA ranked May 2013 80th out of 119. Which means 39 other Mays were warmer.

Here they are: 1896,1897,1900,1902,1911,1914,1922,1928,1934,1936,1937,1939,1941,1944,1949,1955,1956,1958,1962,1964,1969,1970,1977,1985,1986,1987,1988,1991,1992,1996,1998,2000,2001,2003,2004,2006,2007,2009,2012

May of 2013 was 3.5F colder than 1934 and even 1.24F colder than 1896.

May in the USA has been cooling at -0.55F per decade.