I think it's mostly CC, warmer autumns, and wetter times. The 60s-80s were pretty dry and cold. CON had a lot of October 1st 32Fs back in the yore days so maybe we're just getting back to those levels after the cool baby boomer period. Even for last freezes...there were lots of Junes in that cool period, but only one before 1964.
DateLastFirst
1869May-07 (1869)Oct-19 (1869)
1870May-23 (1870)Sep-13 (1870)
1871May-16 (1871)Sep-19 (1871)
1872Apr-24 (1872)Oct-12 (1872)
1873May-16 (1873)Oct-01 (1873)
1874May-12 (1874)Oct-04 (1874)
1875Apr-29 (1875)Sep-23 (1875)
1876May-16 (1876)Oct-03 (1876)
1877May-05 (1877)Oct-06 (1877)
1878May-15 (1878)Sep-29 (1878)
1879May-04 (1879)Sep-26 (1879)
1880May-15 (1880)Oct-08 (1880)
1881May-05 (1881)Oct-05 (1881)
1882May-16 (1882)Oct-22 (1882)
1883May-14 (1883)Oct-02 (1883)
1884May-31 (1884)Oct-10 (1884)
1885May-04 (1885)Oct-08 (1885)
1886May-01 (1886)Oct-02 (1886)
1887Apr-22 (1887)Sep-24 (1887)
1888May-03 (1888)Sep-29 (1888)
1889Apr-23 (1889)Oct-11 (1889)
1890Apr-29 (1890)Sep-25 (1890)
1891May-06 (1891)Oct-12 (1891)
1892May-01 (1892)Oct-03 (1892)
1893May-08 (1893)Oct-17 (1893)
1894Apr-30 (1894)Sep-26 (1894)
1895May-17 (1895)Sep-15 (1895)
1896May-01 (1896)Oct-10 (1896)
1897Apr-29 (1897)Sep-19 (1897)
1898May-10 (1898)Sep-13 (1898)
1899May-16 (1899)Sep-14 (1899)
1900May-29 (1900)Sep-19 (1900)
1901May-06 (1901)Sep-26 (1901)
1902May-21 (1902)Sep-06 (1902)
1903May-24 (1903)Sep-30 (1903)
1904Apr-23 (1904)Sep-22 (1904)
1905May-05 (1905)Oct-07 (1905)
1906May-21 (1906)Sep-25 (1906)
1907May-23 (1907)Oct-02 (1907)
1908Apr-23 (1908)Oct-05 (1908)
1909May-01 (1909)Oct-18 (1909)
1910May-12 (1910)Oct-08 (1910)
1911May-06 (1911)Sep-14 (1911)
1912May-05 (1912)Oct-03 (1912)
1913May-12 (1913)Sep-15 (1913)
1914May-16 (1914)Sep-28 (1914)
1915Jun-03 (1915)Oct-10 (1915)
1916Apr-25 (1916)Oct-02 (1916)
1917May-04 (1917)Sep-11 (1917)
1918Apr-27 (1918)Sep-23 (1918)
1919May-01 (1919)Oct-08 (1919)
1920May-06 (1920)Oct-07 (1920)
1921May-12 (1921)Oct-25 (1921)
1922May-01 (1922)Sep-26 (1922)
1923Apr-20 (1923)Sep-15 (1923)
1924Apr-29 (1924)Sep-25 (1924)
1925Apr-22 (1925)Sep-23 (1925)
1926May-05 (1926)Oct-12 (1926)
1927May-03 (1927)Oct-10 (1927)
1928May-10 (1928)Sep-29 (1928)
1929May-14 (1929)Sep-20 (1929)
1930Apr-29 (1930)Oct-01 (1930)
1931May-18 (1931)Oct-10 (1931)
1932May-24 (1932)Oct-13 (1932)
1933May-06 (1933)Oct-14 (1933)
1934May-09 (1934)Oct-02 (1934)
1935May-24 (1935)Oct-03 (1935)
1936May-22 (1936)Sep-26 (1936)
1937Apr-30 (1937)Oct-03 (1937)
1938May-13 (1938)Oct-05 (1938)
1939May-17 (1939)Oct-15 (1939)
1940May-10 (1940)Sep-27 (1940)
1941May-15 (1941)Sep-19 (1941)
1942May-12 (1942)Sep-29 (1942)
1943May-15 (1943)Sep-13 (1943)
1944May-21 (1944)Sep-23 (1944)
1945May-12 (1945)Sep-30 (1945)
1946May-11 (1946)Sep-13 (1946)
1947May-16 (1947)Sep-20 (1947)
1948May-16 (1948)Sep-16 (1948)
1949May-30 (1949)Sep-26 (1949)
1950May-09 (1950)Sep-18 (1950)
1951May-19 (1951)Sep-27 (1951)
1952May-17 (1952)Oct-08 (1952)
1953May-04 (1953)Sep-23 (1953)
1954Apr-29 (1954)Oct-07 (1954)
1955May-20 (1955)Sep-23 (1955)
1956May-26 (1956)Sep-18 (1956)
1957May-19 (1957)Sep-25 (1957)
1958May-15 (1958)Sep-30 (1958)
1959May-18 (1959)Sep-17 (1959)
1960May-03 (1960)Oct-02 (1960)
1961May-31 (1961)Oct-16 (1961)
1962May-13 (1962)Sep-21 (1962)
1963May-24 (1963)Sep-14 (1963)
1964Jun-06 (1964)Sep-15 (1964)
1965May-24 (1965)Aug-30 (1965)
1966May-16 (1966)Sep-26 (1966)
1967Jun-01 (1967)Sep-11 (1967)
1968May-29 (1968)Sep-30 (1968)
1969May-27 (1969)Sep-29 (1969)
1970May-30 (1970)Oct-05 (1970)
1971Jun-06 (1971)Sep-25 (1971)
1972Jun-12 (1972)Sep-29 (1972)
1973May-01 (1973)Sep-21 (1973)
1974May-21 (1974)Sep-24 (1974)
1975Jun-10 (1975)Sep-14 (1975)
1976May-21 (1976)Aug-31 (1976)
1977May-16 (1977)Sep-12 (1977)
1978May-22 (1978)Sep-08 (1978)
1979Jun-26 (1979)Sep-20 (1979)
1980May-29 (1980)Sep-09 (1980)
1981May-20 (1981)Oct-10 (1981)
1982May-05 (1982)Oct-10 (1982)
1983May-19 (1983)Sep-15 (1983)
1984May-19 (1984)Oct-05 (1984)
1985May-15 (1985)Sep-12 (1985)
1986May-14 (1986)Aug-29 (1986)
1987May-14 (1987)Sep-25 (1987)
1988May-09 (1988)Sep-16 (1988)
1989May-09 (1989)Sep-25 (1989)
1990May-31 (1990)Sep-19 (1990)
1991Apr-26 (1991)Sep-22 (1991)
1992May-26 (1992)Sep-24 (1992)
1993Apr-30 (1993)Sep-20 (1993)
1994May-28 (1994)Oct-02 (1994)
1995May-09 (1995)Sep-11 (1995)
1996May-26 (1996)Oct-04 (1996)
1997May-11 (1997)Sep-22 (1997)
1998May-14 (1998)Sep-24 (1998)
1999May-14 (1999)Oct-06 (1999)
2000May-20 (2000)Sep-28 (2000)
2001May-14 (2001)Sep-30 (2001)
2002May-23 (2002)Oct-08 (2002)
2003May-18 (2003)Oct-02 (2003)
2004May-05 (2004)Oct-05 (2004)
2005May-13 (2005)Oct-20 (2005)
2006May-08 (2006)Sep-30 (2006)
2007May-14 (2007)Oct-14 (2007)
2008May-02 (2008)Oct-04 (2008)
2009May-26 (2009)Sep-20 (2009)
2010May-13 (2010)Oct-03 (2010)
2011May-06 (2011)Sep-17 (2011)
2012May-12 (2012)Sep-25 (2012)
2013May-27 (2013)Sep-17 (2013)
2014May-08 (2014)Sep-19 (2014)
2015May-14 (2015)Oct-05 (2015)
2016May-10 (2016)Sep-26 (2016)
2017May-09 (2017)Oct-13 (2017)
2018Apr-24 (2018)Oct-13 (2018)
2019May-09 (2019)-
MinimumApr-20 (1923)Aug-29 (1986)
MeanMay-13Sep-27
MaximumJun-26 (1979)Oct-25 (1921)