Nearly 10 million Americans have been urged to brace for potentially life-threatening flooding, with some areas facing the prospect of up to a foot of rain in only a matter of hours.
The National Weather Service has placed large parts of Arizona, New Mexico and Utah, along with sections of Colorado, under a flood watch as intense monsoon rainfall is forecast to batter the Southwest from Wednesday through Thursday evening.
Southwest monsoon storms typically develop in late summer when warm, moisture-laden air moves north from the south, unleashing bursts of torrential rain rather than prolonged, steady showers.
AccuWeather meteorologists said the conditions could produce a historic rainstorm this week, with isolated locations potentially receiving as much as 12 inches of rainfall.
Major cities including Phoenix and Tucson are expected to see between two and four inches of rain before Friday.
Forecasters cautioned that rainfall could intensify to rates of two to three inches an hour at times, creating a serious flash flood threat in canyons, around burn scars and near local waterways.
AccuWeather expert meteorologist Tyler Roys said: ‘A foot of rain across the region is a 1-in-1,000-year event and though we do not expect widespread rainfall of 12 inches, it is certainly possible locally.’
‘This storm has a lot of characteristics of a Tropical Rainstorm and at times will have heavy rainfall rates that could create significant flash flooding risks.’
Heavy monsoon rain is forecast across the Southwest this week, prompting flood watches affecting millions of residents
Forecast models indicate isolated areas could receive up to 12 inches of rain
Urgent flash flood warnings have been issued through Wednesday afternoon in multiple areas, including Phoenix and the Grand Canyon in Arizona and in parts of Utah and Colorado.
Drivers in these states have been urged to stay off the roads until the storm passes, as driving into flooded roadways is one of the leading causes of death during flash flooding.
NWS added in a statement: ‘Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles.’
First responders have already been called to rescue at least one motorist caught in roadway flooding in Arizona, according to the Maricopa County Sheriff’s Office.
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The AccuWeather team noted that it is common around this time of year for winds over the Southwest to shift and pull warm, humid air north from Mexico and the Gulf of California.
That moisture rises up and turns into thunderstorms, especially over more mountainous terrain throughout the region.
This week’s storm has seen extremely deep moisture and slow-moving weather systems are letting storms form again and again instead of quickly moving on.
Although rain has been desperately needed throughout the Southwest, especially in Colorado, New Mexico and parts of Nevada, a sudden burst of heavy rainfall can actually make things worse for areas suffering from severe drought.
Multiple drivers have been stranded on flooded roadways in Scottsdale, Arizona on September 16
A tractor trailer crash on I-17 is seen in Arizona on September 16 during heavy rains in the US Southwest
A Phoenix, Arizona firefighter carries a motorist from his car after being caught in flood waters in October 2018 (File Image)
Due to the extremely dry conditions, the soil in many areas so likely dried out this summer or become so parched that it will not be able to absorb all the rain forecasted to fall this week – causing dangerous flash floods to form within minutes or hours.
The weather agency did note in their latest forecast that the heavy rain would still benefit portions of the Colorado River Basin, which has been experiencing ‘exceptional’ drought this year, leading to record-low water levels at nearby reservoirs.
AccuWeather Senior Meteorologist Dave Houk suggested that this severe storm could be the last major surge of rain for the Southwest this monsoon season.
However, that prediction may be upended by the extreme flux caused by the emergence of a record-setting El Niño this summer.
The weather phenomenon is a natural event which simply means ocean temperatures in the Pacific have risen well beyond their normal averages.
The result flips global weather patterns and drastically shifts what people are used to seeing to most regions of the country. In the Southwest, El Niño is expected to cause much more rain and mixed precipitation to fall well into next year.