The Cavendish Encyclopedia of Ornamental Trees says to prune in the Spring as growth begins.( M. grandiflora. )
If you have to prune now, I suggest not removing more than 20 to 25 percent.
Do your pruning on a cool day, when it isn`t raining or windy.
If some-one reading this has no pruning experience, please search on this site for pruning recommendations and techniques. Or go to the ISA website, or Dr. Alex Shigo.
Ansi also has guidelines available