翻译成英语：天空乌云密布，电闪雷鸣，不一会儿，下起了瓢泼大雨。

the clouds in the sky heavily ,the lightning is accompanied with thunder ， after a while， it rains

There are dark clouds with lighting flashes and thunder in the sky, it rains heavily in a moment.

天空乌云密布，电闪雷鸣，不一会儿，下起了瓢泼大雨。

The sky was covered with thick dark clouds, and soon followed by lightnings and thunders. In no time at all, it was down pouring.

以上是最标准的英语翻译，希望会对你有帮助。