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).