It is cooler along the coast because the air blowing off the Pacific Ocean is cooler. There is very little rain, but most of the rain falls in the mountains during thunderstorms. The hottest recorded temperature in the Southwestern region is 134 degrees Fahrenheit in Greenland Ranch, California on July 10, 1913. The coldest recorded temperature is -69 degrees Fahrenheit in Peter's Sink, Utah on February 1, 1985. For more climate data, look at the Weather Research Center site.
Climate data is listed for many cities, including temperature and precipitation information.