Flash Flood Warning Update: More Than 13 Million Under Flood Advisory Across South California, Arizona, Nevada and Utah

A Flash Flood Watch has been issued by the National Weather Service (NWS) for parts of Southern California, most of Arizona as well as pockets of Nevada and Utah over the next few days, including in San Diego, Phoenix and Tuscon.

"A strong weather system will bring snow to the mountains and rain in lower elevations across the western U.S. over the next couple days. Flooding is possible today [Wednesday], particularly in Arizona and Southern California," the NWS said in a note posted on its official Twitter account.

"Heavy rainfall will continue over the Desert Southwest, particularly Arizona into Thursday. There continues to be the potential for locally significant flash flooding," the NWS notes.

"Flash flooding remains possible across the Desert Southwest for the next couple of days as rain associated with a developing low-pressure system moves slowly across the area. Up to a moderate risk is forecast for central Arizona through Wednesday night," the National Oceanic and Atmospheric Administration (NOAA) NWS Weather Prediction Center said in a post on its official Facebook page.

The weather systems heading into the Desert Southwest are remnants of Tropical Storm Raymond and are "poised to bring heavy rain and potential flash flooding to the area starting tonight and lasting into early Thursday morning for some," the NWS notes in a post on its Twitter feed.

More than 13 million people are estimated to be under the flash flood advisory, including in parts of Utah, CNN reports.

The wet weather that has been developing in Southern California from Tuesday is reportedly the first significant rainfall the region has seen since May, according to CNN. Last week, the NWS Bay Area noted much of California has been experiencing "abnormally dry" conditions, according to a post on its official Twitter account.

The areas under a Flash Flood Watch include the San Bernardino County-Upper Colorado River Valley of southeast California, Lake Mead National Recreation Area and Northeast Clark County in southern Nevada, as well as parts of northwest Arizona including Lake Havasu and Fort Mohave, Lake Mead National Recreation Area, Northwest Deserts, and Northwest Plateau, the NWS notes.

"[The Flash Flood Watch continues for] a portion of southern Utah, including the following areas, South Central Utah, Southwest Utah, and Utah's Dixie and Zion National Park," the NWS said.

One to two inches of widespread rain are expected through Wednesday afternoon, with thunderstorms could potentially bring locally heavier amounts of 2 to 4 inches of rain, especially in southern and eastern Mohave County of Arizona, the NWS said.

"Secondary roads which cross the Big Sandy River and Burro Creek will be at risk of flooding, as will any areas where thunderstorms occur," while "steady moderate to heavy rainfall overnight will turn more showery today [Wednesday]," it notes.

"Some of these showers could generate heavy rainfall rates which could be sufficient to generate flash flooding. Flash Flooding of slot canyons, normally dry washes, small streams and urban areas with poor drainage will be possible," it adds.

Locals are advised to monitor the latest updates at the website of the NOAA NWS Weather Prediction Center for excessive rainfall forecasts as well as surface fronts and sea-level pressures.

"A Flash Flood Watch means that conditions may become favorable for flooding of washes...streams and other drainage areas in the watch area. If you are in the watch area...you should closely monitor weather forecasts and be prepared to take immediate action should heavy rain and flooding occur or a flash flood warning is issued," the NWS advises.