Monsoon rains to bring flash flooding to West; California to experience scorching temperatures

The Western U.S. is bracing for more extreme weather as some regions prepare for torrential rain and others for continuing extreme heat.

Monsoon rains are forecast to continue over the deserts in the Southwest and up into the Rocky Mountains, with potential for flash flooding in Arizona, Utah, Colorado and Wyoming on Sunday.

A flood watch is in effect for more than 8 million Americans from Phoenix and Las Vegas all the way into Wyoming as that steady stream of moisture leads to flooding rain.

Among the first places to watch for flash flooding is along the burn scars of recent wildfires.

But given the extreme drought over much of the region and how dry the soil is, it will not take much precipitation to create a flash flooding scenario, officials said.

The flash flooding that has occurred in the region so far has been costly and even deadly.

Another low-pressure system with tropical characteristics is heading toward Texas from the Gulf of Mexico, which will bring between 6 to 10 inches of rain to some places near Corpus Christi, Texas. But, drought-stricken areas may also be flooded quickly due to the dry soil.

Other parts of the West and the central U.S. are expecting sweltering heat on Sunday.

Triple-digit temperatures are expected in states like California, Arizona, Texas, Oklahoma, Kansas and Colorado.

California's central valley region is gearing up for a week of scorching conditions -- with highs expected to hit 105 or more from Tuesday through Friday, prompting an excessive heat watch for more than 7 million residents.

The heat will have a compounding effect over the megadrought that has been plaguing the region for decades, creating further breeding grounds for wildfire.

ABC News' Daniel Amarante contributed to this report.