There is much to talk about here, and not least in MM's thoughts on how to approach these learning threads, and the fact that we don't have any guidelines to go on (that discussion perhaps suggests a thread of its own!)
But, to the issue immediately at hand:
It is a literary way to convert that proper noun to an indefinite one.
Example:
"Hey, don't ask me for more money. Do you think I'm some sort of a Bill Gates or something?!?
"Only in science fiction stories do you hear of a 'United States of Africa', and I don't think it would ever really happen."
That is
another acceptable use of the indefinite article before proper nouns, but it's quite different from what we're discussing here: i.e. the use of the article to emphasise the condition of a person or place at a a particular time.
I agree with MM san that the OP's example doesn't need the 'a' and it would add little, as the fact that
Hokkaido was sunny for once isn't one of the main thrusts of the sentence.
However I disagree that phrases like ' a sunny Hokkaido' are not acceptable in journalism - in fact, that is where i think we see them most often here in Britain!
Having thought about this more, I would think the careful nuance involved lends itself particularly well in journalism, where making maximum impact whilst being concise is such a consideration.
Reading The Times yesterday I quite came across three or four examples within ten minutes. (Interestingly, most of them seemed to be on the sports pages, or is that just where I spent most of my time?
)
I do wonder if this is far less common in US English, or perhaps only exists in the English of certain areas (the ones that have most in common with British English, perhaps?) I did a few minutes of Googling of similar phrases I guessed were likely, and interestingly couldn't find that many examples, but here are a few:
1) Outlines Emerge for a Shaken New Orleans. (A headline from
The New York Times)
2) Although either a victorious Bush or Gore would put up a brave show, insisting that a win is a win, reality is far different. (From
Business Week)
3) The Queen, a sprightly 81, gave an embarrassed Bush a gracious nod after he suggested she had celebrated the United States' founding in 1776. (From that bastion of journalistic integrity,
Fox News)
From a still-fascinated made of stone