PHOENIX — Nearly 10 million people across Arizona, New Mexico and western Texas are under flood watches Saturday, including Phoenix, Albuquerque and El Paso.

"The stage is set for southern Arizona and New Mexico to potentially receive prolific rainfall and widespread flash flooding today," as a low-pressure system brings moist, tropical air to the Southwest in the form of heavy rain and thunderstorms to add to the already active monsoon season across the region, the Weather Prediction Center said early Saturday morning.

Widespread rainfall totals of 2 to 3 inches, with locally higher totals of 5 to 7 inches, are forecast across the region — leading the weather center to issue a level three out of four "moderate" risk for excessive rainfall ahead of the wet forecast. That could mean widespread flash flooding across the Southwest.

On Saturday, a search and rescue operation for a missing person continued at Zion National Park in Utah following a flash flood on the Virgin River, according to a tweet from the national park.

Rangers were alerted about hikers being "swept off their feet" near the Temple of Sinawava on Friday afternoon, according to Zion National Park spokesman Jonathan Shafer. Most hikers have been located, but one remains missing as of Saturday afternoon.

"Park rangers found an injured hiker who had been pulled downstream several hundred yards," Shafer said. The injured hiker was taken to a hospital, according to Shafer. The hiker's condition is not known.

The flash flood at Zion National Park is associated with rain from the same system affecting the Southwest this weekend.

Related:

1 missing in flash flooding in Zion slot canyon; more floods expected this weekend

One person is missing after a flash flood at Zion National Park and a rescue team has been mobilized Saturday morning to search through The Narrows and down the Virgin River.

"Urban locations in addition to areas of complex terrain, slot canyons, arroyos, and burn scars are especially vulnerable for flash flooding and can quickly turn into very dangerous situations," the weather center said on Saturday.

The plume of moisture and heavy rainfall is expected to move into northern Texas beginning Sunday into Monday — where a level two out of four "slight risk" for excessive rainfall has been issued. Rainfall nearing 2 to 3 inches per hour is possible, according to the WPC.

"Urban areas will be the most vulnerable to flooding for the period even with the extremely dry, drought conditions."

Over 90% of the state of Texas is currently experiencing drought conditions, with nearly 62% experiencing extreme or exceptional drought conditions — the highest categories.

Potential Tropical Cyclone Four to bring heavy rains

Potential Tropical Cyclone Four has formed over the western Gulf of Mexico, with sustained winds of 35 mph, according to an 11 a.m. update from the National Hurricane Center. It was located about 165 miles south-southeast of the mouth of the Rio Grande.

The hurricane center uses the potential tropical cyclone designation to issue warnings for a system prior to it actually being named.

A tropical storm warning is in effect for the Gulf Coast of Mexico from Boca de Catan northward to the mouth of the Rio Grande — and across the lower Texas coast, from Port Mansfield southward to the mouth of the Rio Grande. Tropical storm conditions are expected across these regions in the next 12 to 24 hours as the system approaches the coast.

The system is expected to reach the coast of northeastern Mexico late Saturday afternoon, pushing inland through Sunday.

There is still uncertainty about whether the system will strengthen enough to become a named storm before landfall. If it does, it will be named Danielle.

An 11 a.m. National Hurricane Center advisory said "the chances of the disturbance (Potential Tropical Cyclone Four) becoming a tropical cyclone appear to be decreasing." The storm is located close to the coast and may not have enough time to strengthen before moving inland.

Potential Tropical Cyclone Four still remains disorganized over the Gulf of Mexico and does not have a defined center at or near the surface, according to recent data from the Air Force Hurricane Hunters.

Still, heavy rain of 1 to 3 inches, with isolated totals up to 5 inches, is forecast across portions of Texas and Mexico over the next 48 hours — which could lead to localized areas of flash flooding.

"Regardless of the system's status, the overall impacts are expected to be the same," the NHC said. "Winds to tropical storm force and heavy rains are expected to spread across northeastern Mexico and southern Texas later today and continue into Sunday."

The system is forecast to quickly weaken after it moves inland and eventually dissipates by Sunday night over southern Texas.

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