Summer is monsoon season in the deserts of California, Nevada, Arizona, Texas and maybe Utah

 And other states might be affected too. 

However, flash floods are really really serious in all these states. If you see rain clouds within about 50 to 100 miles of where you are in the deserts, especially regarding mountains and canyons nearby you need to just expect that that water might fill dry riverbeds and gulches and anywhere it looks like water has run at any time in the deserts. If you are not used to living or being in the deserts during rain storms within 50 to 100 miles of you that you see coming down or looking ominous you could easily be swept away not realizing what is going on.

Why?

Let me tell you a true story of myself where I could have easily died. This was likely around 1968 or 1969 when my 1968 Camaro I hadn't had more than a year or two at that point. I was driving from Yucca Valley up a road that has a filming Cowboy town built towards the base of the Big Bear mountains where a dirt road suddenly goes up through the mountains to Big Bear. However, I think land owners might have blocked the road legally or illegally now because I tried to drive this dirt road in my wife's 1989 4 runner before the engine seized during the early 2000s and couldn't get all the way to old woman springs road which heads down to Yucca Valley.

Well, suddenly as I drove near the mountain I noticed two things: First a wall of water was coming towards the hood of my car and two the rain clouds were ominous over Big Bear Mountains. So, I burned rubber and spun my car on a dime so I wouldn't be killed by the wall of water coming towards me across the desert. Were any other people there at the time? No. But, likely many animals like Rabbits and things like this didn't survive this or drowned in their holes in the sand unless they had made other arrangements. I did notice birds taking off because where they had landed now was underwater too. I'm okay ONLY because the paved road ran away from the wall of water. Otherwise, I would have been going sideways across the underbrush while the car turned over and over from the wall of water. This is something I survived and I have never forgotten it.

If you aren't used to what rain does in the desert either you need to go to high ground or else don't be there at all if you want to survive in the deserts especially near mountains in the south western states.