A Tropical Storm in August? California’s Year of Extremes Grinds On

For California, where punishing droughts over the past two decades have shriveled crops and caused wells to run dry, it has been another year of extremes. Only this time, they’re of the opposite kind.

It started with winter storms that drenched cities and towns, buried the Sierra Nevada in snow and caused an enormous long-vanished lake to reappear in the Central Valley. And it is poised to pass another milestone this weekend, as Hurricane Hilary lashes Southern California and its bone-dry inland deserts, which normally receive only a scant few inches of rain a year.

All of this is quite a turnaround from the past three years, the state’s driest on record, when officials were imposing strict controls to save water.

Hilary, which forecasters say could weaken to a tropical storm by the time it makes landfall in California, has no direct meteorological connection with the storms from early this year. But, taken together, they reinforce a key maxim about the weather in California: There’s no such thing as an average year — only very wet, or very dry.

“This year is going to be known as just a tale of extremes that worked all the way through the year,” said Michael Anderson, California’s state climatologist.

In a warming climate, we should expect to see more of such extremes, Dr. Anderson said. Still, “to have them all happen in the same year, in and of itself might be its own extreme,” he said.

Nowhere in the contiguous United States does precipitation vary year to year more than in California, and southeastern California in particular. The state’s Mediterranean climate — with hot, dry summers and cool, wet winters — means the atmospheric-river-fed storms that hit the state between November and March deliver most of the water it gets for the entire year. This variability is a major factor in the state’s perennial struggles to supply water to both its giant population and its farm sector.

California often receives more rain during periods of El Niño, the recurring climate pattern related to sea-surface temperatures in the Pacific. But this past winter’s storms swept through during its opposite phase, La Niña. El Niño conditions arrived in late spring and are expected to persist into next year, which could mean another wet winter is ahead for California.

Also in the background: climate change. As societies burn fossil fuels and heat the planet, the warmer atmosphere can hold more moisture. This means storms in many places, California included, are more likely to be very intense.

Especially in the context of some other extreme weather that North America has experienced this year — exceptional heat waves in the Southern United States; wildfires exacerbated by warmth and drought in Canada; torrential rain and flooding in Pennsylvania, Kentucky, Vermont and other regions — California’s storms fit a pattern, said Michael Dettinger, a hydrologist and climatologist at the Desert Research Institute in Reno, Nev.

Different atmospheric mechanisms are at play in each of these extreme events, he said. But “the unrelenting nature of these compounding events sure seems to reflect something deeper than the individual events, by which I imply climate change unchained,” Dr. Dettinger said.

An unusual confluence of factors is leading Hilary to menace Southern California, where a tropical storm hasn’t made landfall in more than 80 years.

The waters of the Pacific off the coast of Mexico have been warmer than normal, which allowed Hilary to acquire extra energy as it formed over the ocean. A heat dome over the central United States and a low-pressure system off the California coast have also been steering the storm toward California and the Southwest rather than out to sea.

“Occasionally Mother Nature lines everything up,” Dr. Anderson, the state climatologist, said.

The feast-or-famine nature of California precipitation means that even a very wet year like this one can only boost water supplies so much before scarcity becomes a problem again.

“The reservoirs might be full, but the ground is still dry,” said Jay Cordeira, an atmospheric scientist with the Scripps Institution of Oceanography, which is part of the University of California, San Diego. He pointed to this month’s uptick in wildfire activity, including in the state’s northern forests, where evacuations have been ordered near the Oregon border.

Fundamentally, California has “a baseline-drought climate punctuated by rainy periods,” Dr. Cordeira said.

One way growers and landowners are trying to cope with these swings is by taking water from downpours and channeling it into the earth, where it can effectively be held in reserve for later use. In principle, this can reduce flood threats to homes and communities while also helping build up a lifeline for farmers against future droughts.

But making it work on a large scale takes lots of planning and infrastructure, including pumps, canals and basins. There are also knotty legal complexities: California regulates who gets to reroute water from creeks and rivers, to protect the rights of people downstream.

State authorities have worked to help local water districts overcome these hurdles and replenish their aquifers. Earlier this year, 92,410 acre-feet of potential floodwaters were diverted underground in response to an executive order from Gov. Gavin Newsom, according to the state’s Department of Water Resources. (An acre-foot is the amount of water used by two to three households a year.)

The progress has been heartening to see, said Philip Bachand, an engineer who works on groundwater recharge projects in different parts of California. But, he said, the state still needs to be putting much more water into the ground each year if it is to have any hope of reversing the damage from decades of aquifer depletion and overuse. And the obstacles to making that happen — logistical, technical, legal — remain great.

“I just don’t know if it gets worked out in time,” he said. “I really worry about that.”