Stormy sea was formerly thought to date from 1885/1890: a later date, of 1899/1900, is now considered much more likely, based on the technique, paper, and butterfly signature.

Stormy sea, Private Collection

It was included in MacDonald 1995 (cat. rais.) [more] (cat. no. 1048), based on a photograph. The attribution was later (1999) queried, but a recent (2021) full examination of the work suggests that it is indeed by Whistler's hand, and the attribution to Whistler has been re-established.