Dark clouds massed on the horizon.

天边乌云密布。

《牛津词典》

There were dark clouds overhead promising rain.

天上乌云密布，预示就要下雨。

《牛津词典》

Lightning flashed among the distant dark clouds.

远处的乌云中电光闪闪。

《柯林斯英汉双解大词典》

The challenge is to pick out oil from other dark spots, such as shadows made by clouds or patches of seaweed.

VOA: standard.2010.07.05

The clouds are really dark and it's getting quiet around the time,

乌云密布，四周都是暴风雨前的宁静。

I'm going to 课堂 - SpeakingMax英语口语达人

Friday started out stormy, but as the 14 soldiers, Marines and their families boarded a yacht for a sightseeing cruise around lower Manhattan, the dark rain clouds parted giving them a glorious view of the Statue of Liberty.

VOA: standard.2009.08.01

Unfortunately, dark clouds are gathering as pursuit of that path seems less likely this afternoon.

FORBES: Congress Poised to Escalate U.S.-China Trade War

Dark clouds filled the sky, as strong winds could be heard whipping against the microphone.

CNN: STORY HIGHLIGHTS

Moreover, dark clouds are now swirling over the oil and gas price jewels as well.

CENTERFORSECURITYPOLICY: In Russia, a landslide to tyranny