California’s not done. Three more atmospheric rivers are on the way.
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San Francisco is facing one of its wettest stretches on record. A whopping 10.33 inches of rain — equivalent to more than two months of water — has fallen in 10 days, causing widespread flooding and scattered rockslides. The culprit? A series of atmospheric rivers, or tongues of deep tropical moisture, invade the Golden State, which are also accompanied by strong winds and high surf. And the watery, windy pattern isn’t going away anytime soon.
At least three more storm systems, one of which appears to be particularly important, are on the way in the next seven days. The most significant is expected between Sunday night and Monday night, possibly bringing “widespread flooding, damaging winds and hazardous beach and marine conditions.” according to the National Weather Service. Then, signs point to a persistently active pattern that will keep the fire hose of moisture toward the state through at least mid-to-late January.
“A series of powerful weather systems will affect our area this weekend into next week,” the National Weather Service serving the Bay Area wrote in a forecast discussion Friday. “Please stay current with forecast updates as there is an immediate threat to life and property from these impacts.”
California is getting flooded with rain. Will it ease the drought?
While the floods bring a welcome blow to the drought that has gripped the state for years, they are proving to be very good. Flood watches are in place again for the northern two-thirds of California, with winter storm warnings in the Sierra Nevada.
Just last week, approximately 81 percent of California was affected by a “severe” or worse drought, according to the US Drought Monitor. That number has dropped to 70 percent, but the biggest story has been the elimination of the “exceptional” high-level drought story. It is unlikely that the multi-year drought will be broken, but a deeper deficit impasse is at least possible.
As much as 22 trillion gallons of water could fall on California from the next series of storms, according to Michael Snyder, a meteorologist based in Seattle.
What is an atmospheric river?
Atmospheric rivers are narrow filaments of tropical moisture that can stretch thousands of miles or more. They are often only a few hundred miles wide, but can transport more than a billion pounds of moisture aloft every second. Most of that moisture remains as water vapor or condensate in the form of cloud cover, but some falls as rain or snow. Surprisingly, rainfall totals can add up quickly.
Most atmospheric rivers hold most of their moisture a mile or more above the ground. This is why the greatest amounts of rain or snow are usually found at higher elevations. In addition, the moist air forced up the mountains often cools to the dew point, becoming saturated and relieving itself of excess moisture in the form of heavy precipitation.
It is not uncommon for precipitation rates to approach or exceed half an inch per hour in the foothills, with snowfall rates of 5 inches per hour or more in the Sierra Nevada.
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At the UC Berkeley Central Sierra Snow Lab, 57.9 inches of snow have fallen in the last week, and there’s plenty more to come.
A new 11.6″ (29.5 cm) of #snow in the last 24 hours and 20.3″ (51.5 cm) in the last 48 hours. It’s been a deep week with almost 5 FEET of snow (57.9″, 147 cm) falling in the last 7 days!
More moisture over the next 7-10 days with many feet of snow possible. #CAwx #CAwater pic.twitter.com/SuZsFvLf3c
— UC Berkeley Central Sierra Snow Lab (@UCB_CSSL) January 6, 2023
Why can’t California catch a break?
California sees atmospheric rivers every winter; they are a principle of the Western cool season weather pattern. But it’s been an extremely wet stretch as of late. San Francisco received 11.6 inches of rain during December, including 5.46 inches on December 31, which was the city’s second wettest day since 1849.
A contributing factor has been the zonal, or west-to-east, pattern of the jet stream. This has kept a wet westerly flow in California. This means that atmospheric rivers, rotated eastward by low-pressure systems, have exited eastward in California instead of sliding northeastward into Oregon or Washington.
This pattern looks set to continue for weeks to come.
What’s next for California?
Another conga line of atmospheric rivers is on its way to California. The first may be modest, but the second, expected to arrive at the beginning of the work week, appears to be more frightening.
Event no. 1: Saturday to Sunday
The first event is actually a pair of low-level atmospheric rivers. They’re pulling into California as they wrap northward in a pair of dying low-pressure systems a few hundred miles west of the Oregon-Washington state line.
The initial atmospheric river will begin to flow against the extreme northern California coast Friday night before falling south overnight, targeting the Bay Area as it fades on Saturday. By evening, another pulse of moisture from the second inducing low pressure system will roll into the region.
Here we go again! There will be heavy rain in the following days. A flood watch begins for Saturday morning in North Bay, expanding across the area Saturday afternoon, lasting into Tuesday.
In addition to flooding, expect high winds, downed trees and power outages. #CAwx pic.twitter.com/33Kh7hJflR
— NWS Bay Area 🌉 (@NWSBayArea) January 6, 2023
Up to 5 inches of rain is expected in the Coast Range, with 2 to 3 inches in the lower elevations north of the Bay Area. San Francisco may be on the verge of the first episode, but it could still see more than an inch. Several feet of snow is expected in the higher elevations of the Sierra Nevada.
Event no. 2: from Monday to Tuesday
A more significant atmospheric river — likely a 4 out of 5 on the scale from the Center for Western Weather and Water Extremes, which rates the phenomenon — will arrive Sunday evening and continue into Tuesday. Very heavy rainfall is likely, with totals of 2 to 4 inches along the coast and in the Central Valley. At higher elevations, where it is too warm for snow, more than 4 inches of rain is possible and as much as 8 to 10 inches.
In the Sierra Nevada, 2 to 5 feet of snow is expected, mostly above 7,000 feet. Along the coastline, winds could gust over 55 mph, with 65 mph gusts expected on mountain peaks above 6,000 feet.
“[W]Widespread and potentially significant flooding is expected,” the National Weather Service office serving the Bay Area wrote. “Strong winds are also forecast on higher ground, which could bring down more trees, causing power outages, travel problems and adding debris to flooded waterways.”
Although the rain should gradually ease on Tuesday, many waterways may overflow their banks. “Tuesday is probably the day you will need to keep a close watch on the weather as the potential for widespread flooding of rivers, creeks, streams and roads and urban flooding will be at its maximum over the next week as all runoff and heavy rainfall combine to result in a mess,” wrote the Weather Service office serving the area around Sacramento.
Event no. 3: At the end of next week
A third storm surge will affect parts of the West Coast late next week. While details are hazy, it appears that significant rainfall is again possible.