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Fox News Weather Center

Rain Poised to Wallop Southwest US, Raise Flooding Risk Monday to Wednesday

Heavy rain, thunderstorms and the risk of flash flooding will expand from northwestern Mexico and into the southwestern United States into Wednesday.

A surge of tropical moisture from the eastern Pacific will unleash an expanding area of torrential downpours and locally gusty thunderstorms. The downpours could be more widespread and heavier than the typical storms.

While the atmospheric event will bring beneficial rain to some drought-stricken areas and greatly aid firefighting efforts, the same system will threaten lives and property.

The combination of heavy rain and rocky, rugged terrain in the region will lead to the risk of flash flooding and mudslides.

Rainfall in this swath is likely to average 1-3 inches (25-75 mm) with local amounts of 5 inches (125 mm) possible, especially along the southern- and southwestward-facing slopes of the mountains.

Even where rain may seem insignificant at a particular location, a torrential downpour over a nearby mountain or canyon could lead to flash flooding within minutes.

RAW VIDEO: Two Vehicles Wash Away in Utah's Catastrophic Flooding

Remember to never attempt to drive through a flooded roadway. To do so could not only put yourself and other occupants in your vehicle at risk, but also your would-be rescuers.

The risk of debris flows (mudslides and landslides) will be greatest in, but not limited to recent burn areas.

According to AccuWeather Meteorologist Courtney Spammer, "Baja California and the west coast of the Mexico mainland are expected to be the first in line to experience the disruptive and dangerous downpours this weekend."

Mexico cities that can be hit with dangerous and disruptive downpours include La Paz, Hermosillo and Mexicali.

As the system rolls northeastward, the downpours will expand to affect much of Southern California, Arizona, New Mexico and southern Nevada during Monday and Tuesday. Residents and those traveling to San Diego, Los Angeles and Palm Springs, California; Las Vegas, Nevada; and Phoenix and Flagstaff, Arizona, should expect disruptions and localized flooding.

Toward the middle of the week, the downpours may become more sporadic and are likely to lift northeast of Southern California and southern Arizona. However, there will still be the potential for highly localized flooding in portions of Utah, Colorado and New Mexico.

According to AccuWeather Meteorologist Brett Rathbun, "Los Angeles has already recorded 2.39 inches of rainfall this month, or 10 times that of average.

"All of the rain fell during last Tuesday and resulted in flash flooding and numerous water rescues," Rathbun added.

WATCH: Los Angeles Fire Department Rescues People Trapped in Floodwaters

Depending upon how much rain falls near the coast of Southern California, the record for the wettest September on record set in 1939 could be challenged. On Sept. 25, 1939, an unnamed tropical storm moved ashore near Long Beach, California. That storm contributed to 5.67 inches of rain that month.