If you could see this, it's already too late.
At a distance of 5 miles from a detonation similar to Hiroshima, you wouldn't die instantly. First, you might feel a tremor or vibration. This comes from the shockwave. As it moves toward you, so does the high-energy particles. They will reach you first. You might see your skin begin to melt like hot plastic. Surprisingly, you can't hear anything. Time might appear to be somewhat sluggish. In 20 seconds, you die.
Trinity Atomic Web Site and HEW Archive
Todd's Atomic Homepage
Your Exposure to Radiation
The Physics of an Atomic Weapon
(c) William Jessee 1998