The park closed much of the backcountry today as the fire blew up overnight. It's not looking good. Even if you do go, expect the air quality to be not-so-good. Here's a pretty informative article from the SL tribune:
http://www.sltrib.com/ci_6414304
My hubby works in fire central for the USFS and he said they had the most number of active fires ON RECORD yesterday. Here is their website (just click on a number for information about the fire; updated each morning. The Zion fire is on there): http://activefiremaps.fs.fed.us/
Humidity is very low and lots of lightning over the last couple of days. We have a new big-ish fire south of here that started this afternoon and another going to the west that appears to have blown up a bit this afternoon.