There is definitely some credence to your grandparents memories. The 1950s winters in this region were mild, overall subpar snow seasons with an unusually high number of spring snowstorms. At Detroit, 1951, 1954, 1955, and 1956 all saw notable snowstorms on or after the spring equinox, and in fact, three consecutive winters (1953-54, 1954-55, 1955-56) saw the seasons largest snowstorm in March.