Is there an idiom related to the idea of a "bad omen"?

Question

I am thinking there's an expression or an idiom that means something along the line of "This is or might be a bad omen to bad things about to come about." It doesn't have to exactly mean that, but it must be something we can pretty much use in the same context.

Answer 1

The word “portend”. From Merriam-Webster dictionary:

1: to give an omen or anticipatory sign of

2: INDICATE, SIGNIFY

Examples

The distant thunder portended a storm.

If you're superstitious, a black cat portends trouble.

Answer 2

• Red skies at night, a sailor's delight.
• Red sky in the morning, sailors take warning.

These can be called signs.

"He's late for work again. That's a bad sign."

Something is said to be a bad sign.