I was wondering how different the weather might be in Zion and Escalante late May vs Early/mid September.

Temperature, precip, etc.

Thanks.

I think the temperature and precipitation is pretty close for those two times. The afternoon storms are possible for both times. May will have more hours of daylight if that is important to you, however. If you go to www.weather.com you can find averages for both places.

Myer,

Based on my experience you have a higher chance of thunderstorms in Sept than May. Temps probably will be about the same - high 80's , low 90's. Late May in Zion is when the gorgeous wildflowers start blooming and create the hanging gardens on the canyon walls along the Riverside Walk and the Weeping Rock especially. Beautiful yellow columine and sometime purple monkeyflower too, plus others. Sept wildflowers are over, except an occasional Indian Paintbrush.

For Zion, I prefer Oct over Sept. May is very good too.
For other areas, higher altitude means a noticable temp change, but you already know this about southern UT.
Zion, of course, is usually the hottest, followed by Moab.