My answer is similar to Yankee's.
The present progressive ("is beginning") can refer to a future event
when used in conjunction with an adverb or adverbial phrase.Examples:
It is beginning to rain. (no adverb/adverbial phrase)
The farmer is beginning his work on Tuesday. (adverbial phrase)
For the past progressive ("was beginning"), it is discussing something that was going for a while but not now. So, I don't know how the past progressive could refer to a future event if it can't involve the present.
It doesn't seem that "is beginning to rain" can refer to a future event very well; the best I can think of is this:
It is beginning to rain a deluge next week.
This sentence is really awkward with the "next week" (necessary for the event to be in the future), so the future tense (It will begin to rain...) would be a better option.