We don't have synonyms for knarly yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are gnarly, snarly and knarry.
"When the swell struck, the North Shore got gnarly, and the wise ones hit the outer islands where the energy was just as juicy but a bit more organized. — Surfer Magazine, March 1977"
"When the swell struck, the North Shore got gnarly, and the wise ones hit the outer islands where the energy was just as juicy but a bit more organized. — Surfer Magazine, March 1977"