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Monsoon rains could bust exceptional drought across the Southwest

(CNN)A surge of monsoonal moisture is bringing rounds of heavy rain and strong thunderstorms to areas of the Southwest that are currently suffering from extreme to exceptional drought conditions.

Nearly 10 million people are under flash flood watches across Arizona, Colorado, New Mexico and Utah.

While expansive rain and thunderstorms will affect much of the Southwest this weekend, the highest rainfall totals are expected across the state of Arizona where some locations could tally upwards of 6 inches. This amount of rain in such a short period of time have prompted significant flash flooding concerns.

Tucson, Arizona, was under a flood warning for much of the day on Friday where 1 to 4 inches of rain fell across the region, most of it accumulating in the overnight and early morning hours. The city has seen nearly 3 inches of rain so far this month which is more than an inch above the city's normal rainfall for the month.

The heavy rain Thursday night into Friday morning even submerged highway ramps north of Phoenix, and more rain is expected through the weekend.
Just outside of Tucson, the Vail School District alerted families on its Facebook page that due to the flooding, road closures could impact bus routes Friday. "If you live in an area that is prone to road closures due to flooding, you can assume that bus routes will be delayed or unavailable this morning".

"What we're expecting to see here is a widespread rain event across the majority of the state of Arizona," said National Weather Service Tucson meteorologist Rob Howlett. "Looking at just southeast Arizona a lot of the valleys are going to see rainfall amounts most likely between 1 to 2 inches, maybe higher in other spots."

The mountains and foothills across the Southwest could see the most rain with five or more inches possible, but even the metropolitan areas, including Flagstaff and Tucson, could see 3 or more inches of rainfall through the weekend.

When heavy rain falls in a brief period during the stronger thunderstorms, flash flooding can occur extremely quickly.

"Normally dry stream beds can instantaneously turn into torrents of fast-moving water, especially below burn scars," CNN meteorologist Chad Myers said.

The weather service office in Tucson is likening the weather pattern setup this weekend to that of the 2006 rain event that caused widespread flooding and damage to areas of Tucson. Similarities include large burn scars in the area and already saturate soils from previous flood events earlier this month.
"Numerous burn scars scattered throughout the area will be a significant cause of concern for widespread flash flooding events," according to the Weather Prediction Center. "As such, the Weather Prediction Center has issued a Moderate Risk for Excessive Rainfall for portions of Arizona and New Mexico that will be carried through Sunday morning."
Numerous fires have ravaged the state in 2021, with two of the largest fires in Arizona history scorching thousands of acres outside of Phoenix. The record 2020 wildfire season left its mark with a 119,978-acre scar on the Tucson foothills.

"The Bighorn Fire near Tucson, we are going to be watching that very closely for any heavy rainfall there as we have concerns downstream from that. There is the potential for some flash flooding along those washes and rivers," Howlett said.

Heavy rainfall runs off burn scars easily because the soil, once burned, becomes hydrophobic -- unable to absorb water -- and the vegetation that typically holds soil in place is destroyed.

Rain to put a dent in the drought

In the short term, this torrential rainfall will lead to dangerous flash flooding. But in the long run these types of rain events bring very beneficial rain that could break the ongoing drought across the region.

Nearly the entire state of Arizona -- 99% -- is under some level of drought, with more 80% of the state in either extreme or exceptional drought.

The extent of the drought improved across the Southwest over the last week due to monsoon rains that impacted the region last week. The highest level of drought fell from 58% to 36% and marked improvements are expected again next week, with this current burst of monsoon moisture.

The heaviest rain is concentrated in the worst drought regions in Arizona along the border of New Mexico up the eastern side to the Utah border.

"Usually, to make a really good dent in drought conditions you want to have that steadier rain that can infiltrate the soils more deeply, but this time of year that's just not the kind of scenarios that we see," said Howlett. "We get thunderstorms and a lot of that is runoff, but every drop counts."

The sporadic, intense nature of monsoon thunderstorms frequently leads to runoff instead of moisture soaking into the soils efficiently and reliving the drought-stricken earth. Nonetheless, any rainfall is welcomed during the hottest part of the year.

"It's going to make a difference, and we really count of rain this time of year to help us out with our drought conditions because for the entire year, half of our rainfall occurs during the monsoon season."

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