Rare 1-in-1,000-Year Storm Threatens Four U.S. States

Nearly 10 million Americans have been urged to prepare for potentially deadly flooding, with as much as a foot of rain capable of falling in just a few hours.

The National Weather Service (NWS) has issued a broad flood watch covering Arizona, New Mexico, Utah and parts of Colorado. Heavy monsoon storms are expected to sweep across the Southwest from Wednesday through Thursday evening.

Southwest monsoons typically bring late-summer thunderstorms, as warm, moisture-rich air moves north from the south and releases intense bursts of rain rather than prolonged, steady showers.

AccuWeather meteorologists say this week’s setup could produce a historic rain event, with isolated locations potentially receiving up to 12 inches.

Major cities such as Phoenix and Tucson could receive between two and four inches of rain before Friday.

Rainfall rates could periodically reach two to three inches per hour, forecasters said. Such intense downpours may trigger life-threatening flash floods in canyons, areas affected by wildfires and along 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.’

Flash flood warnings have been issued through Wednesday afternoon for several areas, including Phoenix and the Grand Canyon in Arizona, as well as parts of Utah and Colorado.

Authorities are urging drivers in the affected states to avoid unnecessary travel until the storm moves through. Entering flooded roads is among the leading causes of death during flash floods.

The NWS said: ‘Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles.’

At least one motorist has already required a rescue after becoming trapped by floodwater on an Arizona roadway, according to the Maricopa County Sheriff’s Office.

AccuWeather said the weather pattern is typical for this time of year, when shifting winds across the Southwest draw warm, humid air northward 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.

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 to fall well into next year.