President-elect Donald J. Trump has picked Brandon Williams, a former Navy officer and one-term congressman, to become the keeper of the nation’s arsenal of thousands of nuclear bombs and warheads.

Mr. Trump’s selection is a shift from a tradition in which the people who served as administrator of the National Nuclear Security Administration typically had deep technical roots or experience in the nation’s atomic complex. What’s unknown publicly is the extent of Mr. Williams’ experience in the knotty intricacies of how the weapons work and how they are kept reliable for decades without ever being ignited.

Terry C. Wallace Jr., a former director of the Los Alamos weapons laboratory in New Mexico, expressed surprise at Mr. Trump’s pick.

Dr. Wallace said he had “never met him or had a meeting” with Mr. Williams and characterized him as having “very limited experience” with the N.N.S.A.’s missions, based on his own decades of work in and around the nation’s atomic complex.

Hans M. Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists, said Mr. Williams “will be facing an incredibly complex, technical job.”

Mr. Williams did not return calls for comment on his selection by Mr. Trump or his credentials.

The credentials and credibility of whoever becomes N.N.S.A.’s new leader may face close scrutiny because advisers to Mr. Trump have suggested that the incoming administration may propose a restart to the nation’s explosive testing of nuclear arms. That step, daunting both technically and politically, would end U.S. adherence to a global test ban that sought to end decades of costly and destabilizing arms races.

From 2023 to early this year, Mr. Williams, a Republican, represented New York’s 22nd Congressional District, an upstate area that includes the cities of Syracuse and Utica. He was defeated by a Democrat in the November election.

Mr. Williams joined the U.S. Navy in 1991 and served as an officer on the U.S.S. Georgia, a nuclear submarine, before leaving the service as a lieutenant in 1996.

In his congressional biography, Mr. Williams said he made a successful transition during his Navy career into nuclear engineer training, calling it “a very steep learning curve” that he met “against significant odds.” The program is widely considered one of the U.S. military’s most demanding.

Mr. Trump announced his choice of Mr. Williams as the nation’s nuclear weapons czar in social media posts on Thursday morning, calling him “a successful businessman and Veteran of the U.S. Navy, where he served as a Nuclear Submarine Officer, and Strategic Missile Officer.”

According to his congressional biography, Mr. Williams founded “a software company that now helps large industrial manufacturers modernize their production plants, secure their critical infrastructure from cyberattacks, and paves the way for reduced emissions through advances in artificial intelligence.”

Chris Wright, Mr. Trump’s nominee for secretary of energy, the cabinet-level post that oversees the N.N.S.A., called Mr. Williams “a smart, passionate guy” who wants to “defend our country and make things better,” according to an interview on Wednesday with the website Exchange Monitor.

A lengthy 2022 profile of Mr. Williams described him as a multimillionaire who starts each morning by reading a section of the Bible. After high school, it said, Mr. Williams went to Baylor University, a private Christian school in Waco, Texas, and then transferred to Pepperdine University in Malibu, Calif.

His congressional biography says he earned a bachelors from Pepperdine in liberal arts, and later an MBA from the Wharton School, a contrast with the advanced degrees in physics or engineering that typically dot the résumés of weaponeers who end up in senior positions of the nation’s atomic complex.

The outgoing administrator of the National Nuclear Security Administration, Jill Hruby, offers a striking contrast with Mr. Williams in terms of technical background and nuclear experience. Before her 2021 nomination to the post, she had a 34-year career at Sandia National Laboratories, retiring in 2017 as director. By training, she is a mechanical engineer.

Sandia is one of the nation’s three nuclear weapons labs, with its main branch located in Albuquerque. It is responsible for the nonnuclear parts of the nation’s arsenal of atomic bombs and warheads.

Other N.N.S.A. administrators have had backgrounds in national security, nuclear operations, the military or scientific fields related to nuclear technology. The first was an Air Force general and a former deputy director of the C.I.A.

The overall responsibilities of the N.N.S.A. include designing, making and maintaining the safety, security and reliability of the nation’s nuclear arms; providing nuclear plants to the Navy; and promoting global atomic safety and nonproliferation. In Nevada, the agency runs a sprawling base larger than the state of Rhode Island, where the United States in the latter years of the Cold War tested its weapons in underground explosions.

Dr. Wallace, the former Los Alamos director, said he had tracked Mr. Trump’s search for an agency leader and found that “any candidate will be making a pitch for resumption.” He added, “That more or less disqualifies any recent director of any nuclear weapons lab.”

Many experts see a restart as unnecessary given the depth and breadth of the nation’s nonexplosive testing program, which the N.N.S.A. runs at an annual cost of roughly $10 billion. Experts argue that the program’s decades of analyses have led to deeper understanding of nuclear arms and greater confidence in weapon reliability than during the explosive era.

Dr. Wallace said Mr. Trump was aided in his hunt for a nuclear czar by Robert C. O’Brien, his national security adviser from 2019 to 2021. Last year in Foreign Affairs magazine, Mr. O’Brien, a lawyer, argued that Washington “must test new nuclear weapons for reliability and safety in the real world.” He added that the freshly tested arsenal would be a deterrent to China and Russia.

Republicans have long criticized the test ban and urged a testing restart. President Bill Clinton, a Democrat, signed the accord in 1996. In 1999, however, he suffered a crushing defeat when the Senate refused to ratify the test ban treaty.

In spite of the treaty’s defeat, successive administrations have informally abided the terms of the test ban. That position began to come under fire during Mr. Trump’s first administration.

In 2018, the Defense Department declared that “the United States must remain ready to resume nuclear testing.” John R. Bolton, Mr. Trump’s national security adviser from 2018 to 2019, reportedly argued for a restart but made little headway.

In 2020, when Mr. O’Brien was the national security adviser, the Trump administration reportedly discussed whether to conduct nuclear test explosions in a meeting with national security agencies.

Opponents of a restart see the nonnuclear tests as more than sufficient to ensure arsenal reliability. “We have more confidence today than when we stopped explosive testing,” Victor H. Reis, the program’s architect, said in an interview.

Siegfried S. Hecker, a former Los Alamos director, argued that a restart would probably start a chain reaction of testing among the world’s atomic powers and perhaps among the so-called threshold states. Like Iran, they’re considered close to being able to build a bomb.

Dr. Hecker noted that during the Cold War, China conducted 45 test explosions, France 210, Russia 715 and the United States 1,030. He said that Beijing, which in recent years has rebuilt its base for nuclear tests, had a major incentive to design and explosively test a new generation of nuclear arms. He argued that the arms could make its expanding missile force more lethal.

“China,” Dr. Hecker added, “has much more to gain from resumed testing than we do.”

A couple in Canada were returning home from walking their dogs some months ago when they found a burst of dusty debris on their walkway. They turned to their security-camera footage for answers and found it showed a mysterious puff of smoke appearing on the tidy walkway where the mystery splotch was.

The source of the splotch was officially registered on Monday as the Charlottetown meteorite, named after the city on Prince Edward Island, in eastern Canada, where it landed.

Only 69 meteorites have been found and registered in the country, and this was the first recorded in Prince Edward Island, according to the Meteoritical Society, an organization that records all known meteorites. The Charlottetown meteorite was more significant because of the security-camera footage, said Chris Herd, a professor at the University of Alberta and the curator of its meteorite collection.

“To the best of my knowledge, it’s the first time that a meteorite hitting the surface of the Earth has been recorded on video with sound,” said Dr. Herd, who identified the space rock after the couple sent the video to the University of Alberta’s meteorite reporting system.

About 99.9 percent of the rocks that people have submitted to the reporting system have turned out not to be meteorites, Dr. Herd said. When he saw the video from July 25, 2024, which showed a small explosion with a sound resembling crackling ice or glass, he thought that what it documented was significant.

For the homeowner, Joe Velaidum, the video did not just capture something of value to science but also recorded a brush with luck. Just before the meteorite hit, Mr. Velaidum and his partner, Laura Kelly, had left the home to walk their dogs, The Canadian Press, a news agency, reported.

“I was standing literally over the exact spot where the meteorite hit just a couple minutes later,” Mr. Velaidum said. “I have been thinking about it a lot because, you know, when you have a near-death experience it kind of shocks you.”

In another fortuitous bit of timing, the meteorite crashed about 10 days before Dr. Herd was scheduled to go to Prince Edward Island for a family vacation. Dr. Herd took a side trip with his wife, older son and his son’s girlfriend to meet Mr. Velaidum and Ms. Kelly and to evaluate the debris they had collected.

Using a home kitchen scale and a teaspoon, Dr. Herd took measurements with the help of his son.

It is rare to find a meteorite, and to capture its fall to Earth, but, Dr. Herd said, “when it literally ends up on your front doorstep, it’s obviously a lot easier.”

While winds and flames continued to ravage Los Angeles, small teams began creeping onto the charred soils left in their wake.

Roughly a dozen members of the California Watershed Emergency Response Teams and the United States Forest Service are studying the edges of the Eaton and Palisades fires to determine what patches of land burned most severely. Soon, they’ll issue hazard maps to help people prepare for what comes next: the near-certain threat of floods and landslides that will loom for days, months and even years while the city recovers.

“After a wildfire, the hazard to the public is not over,” said Jeremy Lancaster, California’s state geologist. He and his team spent Wednesday hiking in the steep canyons that flank the San Gabriel and Santa Monica Mountains. When it rains hard enough, the sediment on slopes like these can swiftly tumble downhill onto houses that increasingly push up against the fire-prone foothills.

The two major hazards after a wildfire are flash flooding and post-fire debris flows. While spongy soils typically absorb water, burned soils can become hard packed like concrete, repelling water as a raincoat would. Water then funnels downslope without much, or any, vegetation left after a fire to keep it in check.

Hazard maps use a combination of satellite images and field testing of soils to show where patches of moderately to severely burned soils could make these post-fire risks more likely. Recommendations for emergency services to engineer barricades against the danger accompany the maps.

The Palisades burn map was released on Thursday, and the Eaton map is still under review. Experts said the Palisades fire had mostly low to moderate burn severity, while the Eaton fire was likely to have more moderate to high burn severity.

Debris flows require three ingredients — steep slopes, burned soils and rain — and they’re often more dangerous than floods because the sediment they draw in claws at the landscape, creating a snowball effect that pulls a tumult of trees, vegetation, soil, rocks and anything else in its way.

“A debris flow is like a flood on steroids,” said Jason Kean, a research hydrologist with the United States Geological Survey. “It’s all bulked up with rocks and mud and trees.” While floods often have a longer reach, water churns faster in debris flows, which are less common but more destructive.

After the Thomas fire in 2017, a debris flow in Montecito, Calif., killed 23 people and damaged or destroyed more than 400 homes.

Neither homeowners’ insurance nor federal flood insurance covers the impact to properties of debris flows, which are defined by the Geologic Survey as landslides.

The fire-flood cycle is a long-studied relationship, but scientists say a warming planet has made the post-fire threat more likely. Fires burn bigger and more severely. Rains hit harder and more often. Those changes expand the target area for post-fire hazards, which could increase the size and the frequency of floods and flows.

“The fire scientists are telling us that wildfires are increasing in size and severity,” Dr. Kean said. “From that fact alone, you’re exposing more terrain and making more terrain vulnerable to post-fire problems.”

With a dangerous combination of very steep terrain, lots of sediment, high-fire activity and a lot of people pushed up against the mountains, Los Angeles faces an extreme risk.

“The Los Angeles area and Southern California are the world capital for post-fire debris flows,” Dr. Kean said.

Debris flows are so common in Los Angeles that at the edge of the San Gabriel Mountains, where the Eaton fire burned, the state has carved debris basins to collect waste from major flow events. In Southern California, more than two million people live on alluvial fans, landforms that are conducive to flash floods and debris flows, according to Dr. Lancaster.

The National Weather Service in Los Angeles collaborated with the Geological Survey to start the nation’s first early warning systems for these post-fire hazards in 2005. The geological survey sets a rainfall threshold that could set off landslides, and the Weather Service issues a warning if the rainfall they expect nears or surpasses it.

Jayme Laber, a senior hydrologist with the Los Angeles forecast office, has spent the last two decades issuing such warnings. While there’s still no sign of rain in the seven to 10 day forecast he issued on Wednesday, a garden-variety rainstorm that Angelenos see at least once or twice a year could be enough to kick off the next wave of hazards, which can develop within minutes.

“In a burned area, the kind of rain that would not cause problems would be a really light drizzly rain that just goes on and on,” Mr. Laber said. But at some point this winter, he added, “we’re going to get rainfall that has the potential to cause flash flooding and debris flows in these newly burned areas.”

This video of a small debris flow from 2016, which is close to the site of the current Eaton fire, shows how quickly large objects, like a six-foot boulder, can be sent rushing downhill.

Mr. Laber advised residents to prepare in case of a future evacuation, monitor the forecast and pay attention to local emergency officials if a warning is issued.