* WHAT...Dangerously hot conditions expected below 4000 feet, with daytime temperatures ranging around 100 degrees at Havasupai Gardens, and 112 to 116 degrees at Phantom Ranch. * WHERE...Lower elevations of the Grand Canyon. * WHEN...From 10 AM Thursday to 8 PM MST Sunday. * IMPACTS...Most individuals will be at risk for heat-related illnesses without effective cooling or adequate hydration, especially with prolonged outdoor exposure.