(CNN)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 WPC to issue a level 3 out of 4 “moderate” risk for excessive rainfall ahead of the wet forecast. That could mean widespread flash flooding across the Southwest. Read MoreSee if rain is expected where you live right here >>>“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 WPC added. The plume of moisture and heavy rainfall is expected to move into northern Texas beginning Sunday into Monday — where a level 2 out of 4 “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.Uncertain whether Potential Tropical Cyclone Four will strengthen Potential Tropical Cyclone Four has formed over the western Gulf of Mexico, with sustained winds of 35 mph, according to a 5 a.m. update from the National Hurricane Center. It was located about 230 miles south-southeast of the mouth of the Rio Grande. Tropical storm warning issued for South Texas and the East Coast of MexicoTropical storm warning issued for South Texas and the East Coast of MexicoTropical storm warning issued for South Texas and the East Coast of Mexico 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. “There remains uncertainty if the disturbance will be able to develop a closed surface circulation — and become a tropical cyclone — before it reaches the coast later today,” the NHC said. An Air Force Reserve reconnaissance flight scheduled later Saturday morning should provide more information. 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.

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