* WHAT...Dangerously hot conditions below 4000 feet, with daytime temperatures ranging from 103 degrees at Havasupai Gardens, to 112 degrees at Phantom Ranch.
* WHERE...Lower elevations of the Grand Canyon.
* WHEN...Until 8 PM MST Thursday.
* IMPACTS...Most individuals will be at risk for heat-related illnesses without effective cooling or adequate hydration, especially with prolonged outdoor exposure.