or: akkor jövök, ha napos lesz az idő (the conditionality is a bit more stressed)

sounds better than akkor, ha.

However, if you wish to emphasize ONLY if, then CSAK AKKOR, HA ... sounds perfect.

While the other answers are all acceptable they lack the importance of stressing the verb tense. This is a conditional sentence where the action will take place only upon meeting a condition. Therefore the action has to have a definite form expressed with perfect tense. In the above example in colloquial Hungarian you may hear jövök instead of eljövök and everyone will understand it, but it sounds exactly the same and as erroneous as the original English: I am coming to the party if it's sunny, which should be: I will come to the party if it's sunny. This should not discredit the other answers, but since this was a linguistic question from someone who is trying to learn a language I needed to elaborate on this minor, but important detail. For practical use other colloquial forms are also acceptable, but not for learning proper Hungarian. These may inlcude: Jövök a bulira, ha süt a nap. Akkor jövök a bulira, ha süt a nap. Ha süt a nap, jövök. Csak napos időben jövök a bulira. etc.