Additional heavy rainfall is expected over parts of southern Colorado and Utah on Thursday and is forecast to spread farther north over the weekend.
The rain is welcome, but experts say it falls well short of alleviating the long-term drought conditions that have resulted from years of inadequate precipitation.
The downpours come with strings attached. At times, too much rain has come at once, flooding communities. Moreover, while instigating thunderstorms can help put out fires, they can be problematic when they move into areas with low humidity. There, their rain evaporates and they can unleash dry lightning that can ignite new fires. That is a particular concern in drought-plagued Northern California into the interior Pacific Northwest over the coming days.
A supercharged monsoon delivers beneficial rain but also flooding
The annual North American monsoon is driven by a reversal of winds. The wind shift brings moisture from the Pacific and Gulf of Mexico to the traditionally arid desert. It is caused by intense summer heating over land in the southwestern United States, which helps surface air to rise, creating an area of weak low pressure that draws in moist ocean air from the south.
Monsoon season officially runs from June 15 to Sept. 30 and can provide more than half of the annual precipitation for parts of Arizona and New Mexico.
This year, the monsoon arrived about a week ahead of schedule and ramped up quickly, transporting ample subtropical moisture north from the Gulfs of California and Mexico.
Plentiful rains have marked a sharp reversal from a hot, dry and fiery start to the summer — and from 2019 and 2020, when the monsoon’s showing was dismal to nonexistent. Last year’s monsoon was the driest and hottest on record. Many referred to it as the “nonsoon.”
In Tucson, a whopping 7.08 inches of rain has fallen this month, making it the city’s wettest July on record. Phoenix has already recorded more rain than it did during the previous two monsoon seasons combined. Parts of northern Arizona have received several times more rain than they did during the 2020 monsoon season.
On Monday, Death Valley, Calif., picked up 0.74 inches of rain, the rare summertime drenching marking the site’s second-wettest July day on record.
“We had these surge events start right at the beginning of the month, and they’ve continued for weeks,” said Mike Crimmins, a professor and climate scientist at the University of Arizona at Tucson.
Normally, monsoon storms are patchy and sporadic, he said, and are driven by topography as moist air rises over mountain ranges and enhances storm formation.
But this year, low-pressure disturbances in the upper atmosphere have helped trigger and organize that convection, or shower and thunderstorm activity, resulting in large storm complexes that can last for 12 to 36 hours.
“We’ve just had rounds and rounds of heavy thunderstorms that are really outside of the normal, garden-variety monsoon storms,” he said.
Flash flooding has replaced wildfires as the main summertime hazard in many western states, and that also means increased flood risk in recently burned areas. This week, weather offices across the Southwest desert issued a number of flash-flood and severe-thunderstorm warnings, followed by significant flooding in many areas, including Las Vegas, southern Utah and Death Valley.
Storms bring wildfire relief, boost lightning danger
The wet weather has largely shut down the Southwest’s fire season, which has burned 709,584 acres in Arizona and New Mexico this summer, according to the National Interagency Fire Center’s Southwest Coordination Center.
Moisture has also poured into Nevada, Utah and Colorado and will surge even farther north this weekend, with slow-moving thunderstorms expected to ease wildfire risk well into Idaho.
“A heavy rain event is possible this weekend as southerly flow will dramatically increase available moisture to near record values for this time of year,” the National Weather Service in Boise, Idaho, wrote in a forecast discussion.
This summer, several large wildfires have burned through timber areas of drought-plagued western states, including Utah, Idaho and Montana.
“There is one thing that can save us from a record fire season this year, and that’s a really healthy monsoon,” Shelby Law, a Predictive Services fire meteorologist with the Great Basin Coordination Center in Salt Lake City, said in an interview.
Law called the rain “a good break” and a “midseason slowdown” that will allow firefighting resources to rest and reserves to be sent to help with fires in other areas such as California.
“The monsoon moisture has a big impact on vegetation where fire potential is concerned,” she said.
Heavy rain also fell on parts of the 68,000-acre Tamarack Fire burning on the California-Nevada border, helping firefighters gain the upper hand on the blaze. Containment had climbed to 59 percent as of Wednesday.
However, fire risk remains high in California, the Pacific Northwest and parts of the Northern Rockies. Although monsoonal moisture travels far enough to trigger thunderstorms in these areas, they typically have less rain.
For example, a mix of wet and dry thunderstorms could develop in Northern California and Oregon this week. Dry thunderstorms bring the potential for lightning-ignited fires given historic levels of flammability there — but without the thunderstorm rainfall to counteract fledgling flames. Even in storms that do produce rainfall, lightning can strike outside of precipitation cores and ignite vegetation.
Is the monsoon helping with drought?
The U.S. Drought Monitor map reflected improvements as of Tuesday in Arizona, New Mexico, Utah, Nevada and Colorado because of recent rain, although drought has deepened in California and the Pacific Northwest.
Nearly two-thirds of the West is experiencing a “severe” or “exceptional” drought, the two most significant categories. In Arizona, however, some places improved a category thanks to recent heavy rains.
“Tucson received more rain in 6 days (4.20 inches fell from July 20-25) than during all of 2020, when annual precipitation of 4.17 inches was the lowest on record,” the U.S. Drought Monitor wrote.
Crimmins said that this summer’s monsoon will provide short-term drought relief but that the chronic water crisis on the Colorado River will take years to climb out of and largely depends on winter precipitation. Mountain snow followed by spring snowmelt is key to replenishing river flows and reservoirs in the region — runoff flows easily into rivers when soils in the drainage basin are saturated.
“The monsoon really only helps by laying down a good base of soil moisture for the following spring,” he said.
That’s not guaranteed, though, if the rains end early or if autumn is exceptionally warm and dry.
“We’re so excited and so glad to see [the rain], but even its relationship to runoff next spring is tenuous,” he said.
Matthew Cappucci contributed to this report.