What about *since* and *for*?

> There are a lot of clouds, hence it is raining.

> Since there are a lot of clouds, it is raining.

> It is raining, since there are a lot of clouds.

> It is raining. For there are a lot of clouds.

I've seen *hence* and *since* forming a pair quite often. Furthermore, it is not at all unusual or frowned upon to begin a sentence with "For, ..." (even though I personally do not like it).