warningly

[…] Moby Dick moved on, still withholding from sight the full terrors of his submerged trunk, entirely hiding the wrenched hideousness of his jaw. But soon the fore part of him slowly rose from the water; for an instant his whole marbleized body formed a high arch […] and warningly waving his bannered flukes in the air, the grand god revealed himself, sounded, and went out of sight.