portentous (adj.)
- alarming
- amazing
- apocalyptic
- astonishing
- astounding
- awe-inspiring
- awesome
- bad
- baleful
- baneful
- black
- boding
- dark
- dire
- dreary
- earnest
- evil
- exceptional
- extraordinary
- fabulous
- fantastic
- fatal
- fateful
- foreboding
- formidable
- gloomy
- grave
- heavy
- ill
- ill-fated
- ill-omened
- ill-starred
- imposing
- inauspicious
- indescribable
- ineffable
- legendary
- lowering
- marvelous
- menacing
- mind-boggling
- miraculous
- momentous
- mythical
- noteworthy
- ominous
- phenomenal
- precautional
- precautionary
- precursory
- premonitory
- prodigious
- prodromal
- prodromic
- remarkable
- serious
- sinister
- sober
- solemn
- somber
- stupefying
- stupendous
- threatening
- unexampled
- unfavorable
- unfortunate
- unlucky
- unparalleled
- unprecedented
- unpromising
- unpropitious
- unspeakable
- untoward
- weighty
- wonderful
- wondrous
