Actually this happened a few years ago. The storm came up the Gulf of Calfornia and into the Colorado River area. By this time it was a tropical depression but the desert areas got quite abit of rain. There was a lot of talk that it could strike more towards the coastal areas but in did not. I was preparing for the heavy rain.
We can certainly use the rain, the lake is down because more water is being drawn down this summer than usual for watering crops in the desert and the upper Colorado would benefit greatly, though they would probably prefer heavy snow to heavy rain.