Science
Fast, wet and furious: How the North American monsoon floods the California desert
In the middle of summer, most Southern Californians would be surprised to see more than a handful of clouds in the sky (unless you count clouds of wildfire smoke). But on July 14 in Twentynine Palms, a rapidly developed thunderstorm dropped about 1.88 inches of rain in a couple hours — an overwhelming amount by historical standards for a town that typically gets less than 4 inches in an entire year.
The water rushed along streets and highways, picking up cars and driving debris to damage homes and businesses. Then, while residents were still taking stock of the destruction, nine days later the area was under a flash flood warning again as another thunderstorm moved through.
So why does the Mojave Desert — obviously an extremely arid place — receive all this summer rain while Los Angeles, less than 150 miles away, gets none?
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Late summer is the driest time of year for much of California; on average, Los Angeles International Airport receives no rain at all during the month of August. But if you go east over the mountains into San Bernardino County, the pattern suddenly switches — in Twentynine Palms, August is the wettest month. Keep going into Arizona and New Mexico and the late summer precipitation gets even more intense: In Santa Fe, almost half of the year’s rain falls between July and September.
This changeover is the result of a phenomenon called the North American monsoon, which was responsible for the deluge in Twentynine Palms. While it is less well-known than its South Asian counterpart, the North American monsoon plays an important role in the climate of the Four Corners states, bringing crucial moisture to areas that would otherwise be bone dry, but also at times leading to damaging flooding.
All monsoons are driven by the same source: a difference in temperature between land and ocean. During the hot months of late summer, the sun can deliver as much as 1,000 watts per square meter in the subtropics — in terms of power, that’s similar to running a space heater every four feet. Some of this power is reflected into space, but on average more than two-thirds of it is absorbed, either by the land surface or by the ocean.
The essential difference between the two is that the ocean is constantly mixing, which distributes the heat throughout approximately the top 60 feet of ocean water — something that is impossible on land. As a result, the top few inches of soil or rock heat up rapidly over the course of a day, and in turn warm the overlying air.
Monsoon flood waters buckled and damaged the roadway at Kelbaker Road and Mojave Road inside the Mojave National Preserve in August 2022.
(NPS)
Since hot air is less dense than cold air, the air over the land tends to rise, typically in the late afternoon after a full day of baking in the sun. As the air rises from the land, it pulls in moist air from the water — for the North American monsoon, this is the Gulf of California — to replace it. When this moist air reaches mountainous terrain like that of northern Mexico and the American Southwest, it is pushed up and drops its moisture, often in sudden, intense thunderstorms.
There are many places near the coast that don’t have monsoons — Los Angeles, for instance. One important factor is topography: Research suggests that a major reason why the South Asian monsoon is so powerful and consistent is the presence of the Himalayas, which act as a wall that prevents air from the dry Tibetan plateau from making it to the Indian subcontinent.
Another major consideration is something called the “subtropical ridge,” which is a series of persistent high-pressure systems that all occur around 30 degrees north (and south) of the equator.
The reason that the North American monsoon does not reach coastal California is the presence of the North Pacific High, which is a part of the subtropical ridge that typically sits northeast of Hawaii. The North Pacific High strengthens and expands during the summer, creating the hot, dry conditions that are typical for Los Angeles and crowding out the monsoon. In winter, however, the North Pacific High tends to weaken and shift south, allowing atmospheric rivers to reach the state.
For the Southwest, the North American monsoon can be both a blessing and a curse.
It brings much-needed rain to the region, but that precipitation typically falls in torrential downpours that the dry, hard-packed soil is unable to absorb. This leads to dangerous flash floods that can destroy roads and buildings and potentially claim the lives of those caught in their path. The rain and cool conditions delivered by the monsoon can be useful in extinguishing wildfires, but the lightning from the storms is also a major trigger for wildfires in the region.
As with many weather phenomena, climate change is expected to have some effect on monsoon rainfall, but the magnitude and direction of that effect depends on specific local factors.
For some parts of the globe, like South and East Asia, monsoons are predicted to become more intense because of climate change. It is thought that changes in aerosol pollution as China and India (hopefully) shift away from coal power will play a very important role.
In the southern hemisphere, models suggest a possible small increase in summer monsoon rainfall. Of the major monsoon systems, only the North American monsoon is expected to have substantial decreases in total precipitation, with the most likely outcome being a 1%-6% reduction in summer rainfall. The reasons for this predicted decrease are not entirely understood but warming sea surface temperatures off the coast of Baja California have been suggested as a possible explanation.
If the North American monsoon does weaken over the coming decades, it will put further stress on the dwindling Colorado River, which has a watershed that includes almost all of Arizona and large swathes of Colorado and Utah. Perhaps more significantly, it will represent a serious threat to ecosystems that are already fragile because of rising temperatures and outbreaks of the mountain pine beetle.
The future of monsoon systems across the globe is not certain or easily predictable but given the potential perils in either direction — more intense flooding or deepening drought — it is important that we prepare for both scenarios and act quickly to limit these changes including by rapidly cutting emissions.
Ned Kleiner is a scientist and catastrophe modeler at Verisk. He has a doctorate in atmospheric science from Harvard.
Science
Video: NASA Announces Artemis III Crew
new video loaded: NASA Announces Artemis III Crew
transcript
transcript
NASA Announces Artemis III Crew
NASA announced the crew of Artemis III mission, which will fly to low-Earth orbit to test rendezvous and docking maneuvers with one or two lunar landers.
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“I am excited to welcome you as the next crew in the Artemis journey to successfully return to the moon — this time to stay.” “I’m honored by the role that I’ve been given. I’m also very humbled by the task in front of us. But first and foremost, I’m grateful.” “So with that, the Artemis II crew, comrade, hands you the baton. You got the controls.” “As you know, we had a significant anomaly at our Launch Complex 36A on May 28. We’ve redoubled our efforts and are moving forward.”
By Alisa Shodiyev Kaff
June 9, 2026
Science
Santa Monica Mountains’ last steelhead trout survived the Palisades fire — and even had babies
Scientists feared the Santa Monica Mountains’ last remaining steelhead trout were dead, smothered by debris flows unleashed by the Palisades fire.
But the endangered fish surprised them: A team of biologists recently spotted 30 of the rare trout — and 21 babies — in Topanga Creek.
“There was a lot of happy dancing in the creek,” said Rosi Dagit, principal conservation biologist for the Resource Conservation District of the Santa Monica Mountains, which works with public and private landowners to conserve natural resources.
That’s because the steelhead here are endangered, at both the state and federal levels. Once, they swam in most streams of the Santa Monicas, but their numbers plummeted amid overfishing and coastal development. Increasingly frequent wildfire has further stressed their habitat. Topanga Creek, a biodiversity hot spot, is home to their last known population in the mountains that stretch from the Hollywood Hills to Point Mugu in Ventura County.
The trout that were spotted, including this one, are part of a distinct Southern California population that’s listed as endangered at the state and federal levels.
(RCDSMM Stream Team)
The California Department of Fish and Wildlife spearheaded a complex mission to rescue trout threatened by the Palisades fire that sparked in January 2025.
Time was of the essence. The fire hadn’t yet been fully contained. But rain was on the way, which would sweep massive amounts of sediment from the denuded hillsides into the water. Fish are often killed this way.
Crews stunned the fish with electricity, scooped them up in buckets, trucked them to a hatchery and ultimately moved them to Arroyo Hondo Creek in Santa Barbara County.
Within days, Topanga Creek was choked with mud. Some assumed the fish left behind were goners.
But in March, the conservation district’s team found four. The following month, when water conditions were clearer, they saw more.
“These fish continue to amaze me,” said Kyle Evans, environmental program manager for the state Department of Fish and Wildlife, who had seen the damage to the creek. “I had seen populations get wiped out in similar situations. So when I heard, I was thrilled.”
Evans surmises the fish that survived were in an area of the creek where less charred material and sediment were swept in.
“These fish likely hunkered down, were hiding under some rocks or places to try to get away from the main concentration of flow,” he said. “And luckily they weren’t buried.”
The ones that were spotted were fairly small, around 6 to 14 inches. Rainbow trout and steelhead trout are the same species, but with different lifestyles. If the fish remain in freshwater, they’ll be considered rainbows. However, they can migrate to the ocean and become steelhead, where they typically grow larger before returning to their natal waters to spawn.
Topanga Creek hasn’t fully recovered from the damage it sustained, but scientists say it’s looking better. Surveys last year were “so depressing,” Dagit said, with very few animals, and stretches that were essentially transformed into flat roads from all the sediment buildup. Some of the riparian canopy burned right down to the creek.
Then came 32 inches of rain over the last nine months, scouring out and moving sediment, creating deeper pools. Dagit said they recently found newt egg masses for the first time in years, as well as a few adult newts and many frogs. Plants that provide cover are starting to recover.
She provided photos comparing certain pools last year and this year, some dramatically transformed. In September 2025, the Shrine Pool could have been an overgrown hiking trail. This April, it was filled with shallow water.
The Shrine Pool in September 2025, left, and the same location in April 2026, right, with RCDSMM’s Isaac Yelchin donning a wetsuit.
(RCDSMM Stream Team)
Topanga Creek is home to another endangered fish, the small but hardy northern tidewater goby, often described as cute. Not long before the trout operation, Dagit led a rescue of hundreds of these fish too. Many were repatriated to the lagoon at the mouth of the creek in a moving ceremony last June.
There’s still the matter of what to do with the trout that were moved to Santa Barbara County last year. Evans would like to bring them home to the Santa Monicas at some point, but isn’t sure if it will happen. On one hand, they could bolster the small, genetically isolated surviving population. On the other, they might inadvertently bring in a disease or bacteria. There is some time to decide. Evans estimates the creek still needs to recover for two to three more years.
For now, the fish are functioning fine in their adopted creek. Experts worried the trauma wrought by the move would disrupt their spawning process, but they had babies that spring. This year, they spawned again.
Science
Pacifica pier cracks, another coastal casualty as seas continue to rise
The Pacifica Municipal Pier was shut down and taped off Thursday after city workers noticed cracks running through the landmark structure and concrete chunks falling into the ocean.
It’s just one of many coastal California structures that have recently crumbled under pressure from a rising and relentless ocean.
Officials from the small, beach city south of San Francisco said the pier was closed due to “cracking, separation, and displacement of the concrete walkway and structural elements.”
It will stay closed while structural engineers asses its safety.
Photos taken by city employees show a wide crack that runs from top to bottom and across the structure as well. Other photos show a large horizontal crack under the foundation of a small restaurant on the pier, the Chit Chat Cafe.
The cafe was also shut down.
This is not the first time the 53-year-old pier has shown signs of stress. In 2021, part of it was shut down after handrails along the edge collapsed. And in 2023, after a series of storms pummeled the Central California coast, damaging parts of the pier, the structure was partially closed for more than year.
Those same storms caused extensive damage in Aptos and Capitola, 70 miles south, where piers and waterfront infrastructure were swept away or damaged.
In 2024, a 150- to 180- foot section of the Santa Cruz wharf was ripped off by powerful waves.
At least 10 of the state’s dozens of coastal public piers were closed for part or all of 2024 due to structural damage sustained in winter storms since 2022. At least five others have longer-term upgrades planned to address structural issues.
“These things are costly to maintain,” said Zach Plopper, senior environmental director at Surfrider. “They are a part of our California coastal culture in many ways, but we’re going to need to reckon with, one, the state that they’re in, and two, the continuous and worsening threats they’re going to experience,”
He said most of the piers were constructed in the early 1900s, and they weren’t built to withstand decades of rough seas, storms and rising sea level.
“With this incoming El Niño, which is forecasted to be significant, and this marine heat wave we’re in the midst of, we’re kind of in uncharted waters as far as what this winter could bring in terms of storms and swells to the California coast, and we’re likely going to see a lot more damage,” he said. “Not just piers, but roads and other coastal infrastructure up and down the state.”
There was no storm in Pacifica earlier this week, so no single event could be blamed for the destruction.
However, a 2025 report from an outside engineering firm, GHD, found that several sections of the pier were in “poor” or “serious” condition, and they recommended closure before anticipated storms or events that could “subject the piles to high winds, swells and large waves.”
The firm found several areas of the pier where concrete was missing and rebar was exposed and corroding.
“The pier has continued to experience high winds and large waves in a harsh marine environment,” the engineers wrote in the report, noting that continuous exposure to seawater or marine spray was “detrimental” to the structure.
A 2023 city report estimated it would cost $19 million to repair.
That same year, a state law was enacted to require local governments along the California coast to plan for sea level rise in the coming decades.
Sea level has risen some 8 inches, on average, along the coast in the past 150 years, Plopper said, and researchers anticipate another foot in the next 25 years.
“We’re going to see profound shifts on our coastline, none that we have ever experienced before, and building static structures on the coast just doesn’t work all that well,” he said. “We’re going to have to make some really hard decisions.”
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