San Francisco Chronicle - (Sunday)
2023 was a year of extreme weather for state
Last December, California was heading into a fourth year of drought, with La Niña conditions portending another dry winter.
But, as Californians know all too well, the next months defied expectations.
“A heavy rainfall season was not on people’s radar,” said Jan Null, a meteorologist with Golden Gate Weather Services and adjunct professor at San Jose State University.
From atmospheric rivers to tropical storms, here are some of the wildest weather events that the state saw in the past year.
New Year’s Eve storm
On Dec. 31, 2022, an atmospheric river fueled historic downpours across the San Area. In San Francisco, approximately 5.5 inches of rain fell — the second-largest amount for a single day in the city’s 174 years of record.
Some neighborhoods saw even more rain, according to data from the San Francisco Public Utilities Commission. Lower Pacific Heights, Twin Peaks and the Mission District all logged more than 7 inches on New Year’s Eve.
“The numbers with that (storm) were really phenomenal,” Null said.
Extreme rainfall drove flooding that inundated streets and soaked homes and shops. The storm also caused millions of gallons of Bay Area wastewater to spill into the streets and San Francisco Bay.
January parade of atmospheric rivers
The New Year’s Eve deluge was part of a three-week stretch of storms that began in late December. During this period, the jet stream ferried atmospheric rivers and low-pressure systems across the Pacific right toward California.
“The storm track was unusually locked,” said John Monteverdi, emeritus professor of meteorology at San Francisco State University. Across the state, an average of 11.47 inches of precipitation fell from Dec. 26, 2022, and Jan. 17, 2023. That’s more than 32 trillion gallons of water overall.
One especially impactful event was an early January bomb cyclone, which produced heavy rain and strong winds, including gusts topping 100 mph.
The January storms caused devastating floods, especially in Santa Cruz and Monterey counties. The San Lorenzo River exceeded its flood threshold by 8 feet, reaching a level recorded only once before. Floodwaters sloshed into neighborhoods, spurring evacuations amid downed trees and roads blocked by landslides.
Bay Area snow
In February 2023, the Bay Area and Central Coast transformed into a winter wonderland as snow blanketed places like the East Bay Hills and Santa Cruz Mountains. While peaks like Mount Diablo and Mount Hamilton regularly see snow, accumulation is much rarer across lower elevations.
Santa Cruz beaches and Half Moon Bay even saw unusual precipitation known as graupel, which has a texture somewhere between snow and hail.
Relentless atmospheric rivers continued
March brought more storms fueled by atmospheric rivers, with catastrophic consequences. The West Coast saw an astounding 31 atmospheric rivers from October through the end of March, according to researchers from the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography.
A fierce storm on March 10 caused a levee break on the Pajaro River, flooding the town of Pajaro in Monterey County and prompting more than 50 water rescues overnight. Floodwaters damaged more than 900 homes and businesses.
Another storm hammered Northern California on March 21, developing one “eye” as it rapidly intensified, then another as the system split in two. One storm made landfall near Santa Cruz while the other spun toward the Peninsula and San Francisco.
“I was looking out at my deck (in Oakland) and I could see sun
and it was pouring rain here because the eye was over San Francisco Bay,” Monteverdi said.
At least five people died in the Bay Area as strong winds uprooted trees and tore down power lines.
Tropical Storm Hilary
In August 2023, the National Hurricane Center and National Weather Service issued Southern California’s first-ever tropical storm watch as Hilary churned northward through the eastern Pacific.
The tropical cyclone peaked as a Category 4 hurricane, but weakened
to tropical storm status by the time it crossed into California. Tropical Storm Hilary had significant impacts, dropping a whole year’s worth of rain in some desert communities and more than nine inches in the San Bernardino Mountains.
Future extreme weather
Scientists expect that California will swing even more wildly between dry and wet periods in the future — a pattern referred to as “hydroclimate whiplash.” One reason is because, in a warmer world, the atmosphere can hold more moisture. That means
stronger storms and wetter wet periods.
Some of the storms in the past 12 months were intense enough to qualify as “100-year events,” meaning they only have a 1% chance of popping up each year. But that probability is based on the climate of the 20th century.
“In the 21st century, we expect more of those events,” Monteverdi said. “What used to be a 100year event … some projections say will become a 25- to 50-year event.”