A cold or rainy spell, usu in spring or early summer and associated with the flowering of wild blackberry bushes.
1857 Sun (Baltimore MD) 20 May 1/6, The rain continued during the entire day yesterday—a cold, raw, northeast rain. . . Some of the old folks call it the blackberry storm—what is meant by this we scarcely know, only that they look for something like it about the time that blackberries are in blossom, and infer that it will be the close of the bad weather. 1906 DN 3.126 nwAR, Blackberry storm. . . A storm said to occur when blackberries are in blossom. “The Easter snap has passed; we now await the blackberry storm.” 1952 Brown NC Folkl. 1.520, Blackberry storm. . . The cold season that sometimes comes when blackberries are in bloom. 1953 Randolph–Wilson Down in Holler 227 Ozarks, Blackberry winter. . . A late cold spell in May or early June. . . Blackberry squall is used in the same meaning. 1976 Stilwell Democrat–Jrl. (OK) 29 Apr 8/2, The Easter storm is past so what we are experiencing now could be the blackberry storm. Let us hope if there is another to come it will be in a milder form.