The change in the 1888 LDS edition to the plural tempests is probably a typo. This change was probably caused by the immediately following plural earthquakes. The singular tempest works perfectly well since it is paired with the preceding singular fire. Elsewhere in the text, the word tempest(s) always agrees in number with the noun(s) conjoined with it, as in 1 Nephi 19:11 (“by tempest by fire and by smoke and vapor of darkness”). For a list of examples, see the discussion under 1 Nephi 19:11.
Summary: Retain the singular tempest in 2 Nephi 6:15, the reading of the earliest text.