Flooding downpours, locally severe storms to threaten parched southwestern US this week

Renee Duff

An increase in downpours across the southwestern United States early this week will be beneficial for the ongoing drought but could pose the risk for flash flooding. 

Drought conditions have grown considerably across the Southwest over the past few months due to a lackluster monsoon season. While Arizona was free of drought during the middle of June, over 85 percent of the state has succumbed to moderate to severe drought, according to the latest outlook by the U.S. Drought Monitor.

While the upcoming rainfall will go a long way in helping to ease the dry conditions, AccuWeather meteorologists are concerned the rain may trigger flash flooding and debris flows in the arid terrain.

Arizona looks to be the main target for widespread showers and thunderstorms, as well as flash flooding concerns from Monday into Tuesday, with lesser impacts on surrounding areas, according to AccuWeather Senior Meteorologist and western U.S. blogger Brian Thompson.

A potent storm system in the upper levels of the atmosphere will dive southward across the West into Tuesday. At the same time, tropical moisture will surge northward from the eastern Pacific Ocean.

These two factors will combine to generate the widespread downpours.

An AccuWeather Local StormMax™ of 6 inches is forecast in Arizona during this event.

"While the rainfall is needed, if the rain comes in bursts from heavier thunderstorms, flash flooding will be a big concern, especially in mountainous and urban areas like Phoenix and Tucson, Arizona," Thompson said.

People should avoid dry stream beds, known as arroyos, which may suddenly fill with a torrent of water.

The Arizona Department of Transportation was alerting motorists of the heavy rain threat on Twitter, reminding them to inspect windshield wipers and slow down when the rain starts.

Motorists will also need to be on the lookout for flooded roadways. Remember to turn around and find a safer, alternate route when high water is encountered.

"Aside from the heavy rain, flooding rain and mudslide threat, strong thunderstorms will be a possibility as well," Thompson said.

The strongest thunderstorms can contain large hail and damaging winds, with Monday likely posing the highest threat for these hazards.

"The threat for at least spotty showers and thunderstorms will probably linger into Wednesday and Thursday," Thompson said.

The rainfall should douse active blazes across the region and substantially lower the risk of new wildfire ignition.

However, it will be a different story on the northwestern side of the storm system, where warm, dry winds will heighten the fire danger in Northern California from Monday to Wednesday.

"We're now heading into prime wildfire season across California, so staying on guard is important," Thompson said.

By the end of the week, the fire danger will likely decrease in Northern California as a significant, winterlike storm is expected to target the West.

This new storm has the potential to bring a significant reduction in temperatures and unleash a large amount of early season snow in the northern Rockies.