Green’s Dictionary of Slang

1. (US gambling) a heavy winner.

1871

Galaxy (N.Y.) July 63: Gamblers have a saying as caustic and more true, that a ‘Stormer is sure to be a piker.’ The first term interpreted into English, means one who has an extraordinary run of good luck by which he has pocketed thousands, while a ‘piker’ is a tolerated collapse who makes a stray bet when he can beg or borrow a ‘check.’.

2. a success.

1929		J.B. Priestley Good Companions 534: They were Hits, Whirlwinds, Riots [...] Stormers.
1986		B. Geldof Is That It? 110: It was a stormer and the audience loved it.
1996		P. Howard The Joy (2015) [ebook] Houghton’s having a stormer.
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 49: JP didn’t put a foot wrong [...] and even Fionn had a stormer.