There doesn't seem to be a passage that shows Victor trapped in any storm. The only person trapped in the story is Captain Walton when his ship becomes stuck in ice while heading towards the north pole. In Letter IV, Walton and his crew watch the creature travel across the ice on a sledge pulled by dogs while they are trapped. Walton describes the scene to his sister as follows:
"We watched the rapid progress of the traveller with our telescopes until he was lost among the distant inequalities of the ice. This appearance excited our unqualified wonder. We were, as we believed, many hundred miles from any land; but this apparition seemed to denote that it was not, in reality, so distant as we had supposed" (8).
In this passage, Walton describes the creature as though he were an apparition that moves very quickly. It doesn't use the words "supernatural speed," though.
There is another time when Victor is in a storm and he sees the creature for just a moment, but he is not necessarily trapped. In Chapter 7, Victor heads home to Geneva to be with his family after hearing about William's murder. He is abruptly caught in a storm while going to visit the spot where his brother was killed. The thunder and lightning herald the storm that rapidly falls upon him. It is through lightning that Victor catches glimpses of his monster, which he created two years previous to this encounter. He describes the scene as follows:
"I perceived in the gloom a figure which stole from behind a clump of trees near me; I stood fixed, gazing intently; I could not be mistaken. A flash of lightning illuminated the object, and discovered its shape plainly to me; its gigantic stature, and the deformity of its aspect, more hideous than belongs to humanity, instantly informed me that it was the wretch, the filthy daemon to whom I had given life . . . The figure passed me quickly, and I lost it in the gloom . . . another flash discovered him to be hanging among the rocks of the nearly perpendicular ascent of Mont Saleve. . . He soon reached the summit, and disappeared" (50).
In this passage, Victor doesn't use the words "supernatural speed," but it is apparent that the creature moves just as quickly, because with each flash of lightning Victor finds it at another location on the mountain. Then, in Chapter 10, Victor does use the words "superhuman speed" when the creature approaches him while he is on top of a mountain during a rainstorm. The passage is as follows:
"I suddenly beheld the figure of a man at some distance, advancing towards me with superhuman speed. He bounded over the crevices in the ice, among which I had walked with caution; his stature, also, as he approached, seemed to exceed that of a man. I was troubled: a mist came over my eyes, and I felt a faintness seize me; but I was quickly restored by the cold gale of the mountains" (67-68).
Based on the passages cited, it seems as though these all encompass what is asked for in the above question: someone trapped, a storm, and the creature moving with "superhuman speed." The first passage shows Walton trapped in ice when he sees the creature; the second one shows Victor in a storm when he sees the monster for the first time since he created it; and the third one has Victor in a rainstorm, witnessing the speed of the monster, but it doesn't disappear; rather, Victor mentions its speed with the closest phrase in the book to "supernatural speed," which is actually "superhuman speed."