Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
altĭtŏnans, antis, adj. [alte-tonans], thundering from on high: Juppiter, Enn. Ann. 2, 6: pater altitonans, i. e. Jupiter, Cic. Carm. Div. 1, 12.
Hence, in gen., of wind, loud-roaring: Volturnus, * Lucr. 5, 745.