I realized that I keep reaching for long impressive words here, when there's some perfectly lovely short words to be used. Not that the meaning of this is lovely, but the word itself is. So -- this is an old one, attested in the 13th century in the spelling louren, meaning to scowl and to lurk, of origin obscure but there are several Germanic cognates.
The sky this morning loured heavily enough the dawn was almost colorless.
(True statement, except that it as of writing it should be present tense.)