If I were you, I would talk to local beekeepers and find out the best time to do that split and still get a crop. I really find Mel Disselkoen's discussions on splitting helpful. His material is at mdasplitter.com.
+1 on what Vance said. I'm going to put Mel's methods into practice this year. I understand that before you can split a hive overnight temps have to consistently be above 50degF or the queen cells might not make it. Initially I plan to do simple walkaway splits and then using Mel's OTS queen rearing method raise as many queens as I can for fall splits and queen replacements. I want to make as many colonies as I can in August. That should give them time to brood up for winter and take advantage of the Goldenrod flow.
Good luck to you