Temperature: the hottest place you've listed is Death Valley: expect temperatures as high as 120°F (49°C) in the shade! Next highest would be Las Vegas between 90°F (32°C) and 100°F (43°C). California will mostly be between 80°F (30°C) and 100°F (43°C) except for parts of coast and the San Francisco bay, which can be down below 70°F (24°C) in fog! The joke is that you can spot tourists walking the Golden Gate Bridge in San Francisco sweatshirts (which they just bought) and shorts! Note that although the temperatures are high during the day, the humidity is quite low (except when foggy) and so can be more comfortable than it sounds. Never the less, places like Death Valley can easily kill you if you're not careful!
You can expect the nights to be 10-20°F (6-12°C) cooler.
Rainfall: for the most part, California and Nevada get no rainfall from sometime in May through October! However, it is possible to have thunderstorms create flash floods in the desert.
FYI, I'm a California native and have lived here my entire life (59 years).