Since you are talking about things past, you should use past tense.
If you say "それが分かったら、大会に出る。" (Quick grammar fix を -> が) it sounds like you are talking about the future. Which means you haven't know it (it's going to be so hot or not) but you will know it later. And you will decide to join the competition depending on it.
** Description on the quick grammar fix **
I fixed grammar それを -> それが, but one of my answer is それを... Here is the reason why.
First off, the full sentences of my two answers are:
The two sentences have almost the same meaning, but the nuance is different. The subject of the first sentence is 私, so the sentence focus on 私 weather or not know about それ (it's going to be so hot or not). Which has nuance that speaker (it is equals to 私 in this case) know someone who know about それ. That means, 私 didn't know what the weather going to be before the competition, but now 私 knows.
The subject of the second sentence is それ. It has extra nuance that 私 might has other information (like where the competition held, how many people are joining it, etc) but それ (it's going to be hot or not) is the important to make decision.
Now, get down to your phrase. The full sentence of "それを分かったら、大会に出る。" is:
Since 私 is a subject, it sounds like speaker (the speaker is equal to 私 in this case) know someone who know about それ. If so, 私 usually don't speak like that. Maybe "それを聞いてから決めます。" (I'll make decision after I heard it.)
The full sentence of それが分かったら、大会に出る。 is:
Which has extra nuance you know some other information, but what you need make decision to join competition is それ.