Portentous, a.
Etymology: [L. portentosus.]
1. Of the nature of a portent; containing portents; foreschadowing, esp. foreschadowing ill; ominous. For, I believe, they are portentous things. Shak. Victories of strange and almost portentous splendor. Macaulay.
2. Hence: Monstrous; prodigious; wonderful; dreadful; as, a beast of portentous size. Roscommon. -- Por*tent"ous*ly, adv. -- Por*tent"ous*ness, n.
Shades of meaning
Fernald has no entry of its own for portentous; he lists it under awful.
He lists it without comment; the entries above explain its neighbors.