Early Quenya
turwa
adjective. powerful
valma
adjective. powerful
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
An adjective for “powerful” in the Qenya Lexicon of the 1910s under the early root ᴱ√TURU “am strong” (QL/96).
Neo-Quenya: Since √TUR remains connected to power in Tolkien’s later writings, I’d retain this word for purposes of Neo-Quenya, but I’d use it for general or abstract forms of potency, as opposed to being physically powerful (turca or [ᴹQ.] poldórea) or politically powerful (túrëa). For example: turwa nus “a powerful smell” or turwa vangwe “a powerful storm”.