"The black clouds foreboding rain cleared" sounds rather formal, although it is correct. The verb
forebode is not a very common verb;
foreboding is far more often used as either a noun or an adjective. The reader has to pause for a microsecond or two to realize you are using the word in its less frequently used verbal form. For that reason alone, I give it low marks on the "natural" scale, even though, again, it is a grammatically correct sentence.
"We were thus spared the onslaught the rain" is wrong because you need to say this:
"We were thus spared the onslaught of the rain."
With
of in there, it's a good sentence.
Onslaught works well here, if that's what you're worried about. I give it higher marks on the "natural" scale.
Hope this helps.
Greg
Hi Ann. Beat me again.
