The storm predicted to strike Northern California starting Wednesday night originated in the complex high-altitude wind currents constantly whipping around the globe. In this case, those winds are focusing into a narrow jet that is poised to sling heavy rain and wind unlike anything the state has seen in years.
The storm has taken the shape of an “atmospheric river,” a condition some experts have described as a kind of horizontal hurricane. Miles above the Earth, swirling winds of up to 200 mph are funneling into a narrow band, pulling subtropical moisture all the way across the Pacific Ocean. That funnel will be emptying on California.
“If this continues to intensify, this is going to be a lot more rain than anybody has seen in a long time, because everything is so dry,” said William Patzert, a climatologist at NASA’s Jet Propulsion Laboratory in Pasadena. “Any way you look at it, you could say the Pacific Ocean decided to declare war on the California drought this week.”
Because of California’s long drought, and the particular intensity of this atmospheric river, the approaching storm is attracting attention as some kind of monster event.
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In reality, the drought is so severe because California has not received enough of these storms. Atmospheric river events normally occur four to six times every year, weather experts say, and collectively they account for 30 percent to 50 percent of all the precipitation California receives each winter. They often occur without any particular fanfare.
This storm can also be called a “Pineapple Express,” a less-technical term that has been applied to strong atmospheric river events that cross the Pacific Ocean near Hawaii, said Marty Ralph, director of the Center for Western Weather and Water Extremes at the UC San Diego Scripps Institution of Oceanography.
“On average, each one lasts a day,” said Ralph, an expert on atmospheric rivers. “The longest-lived ones, sometimes they’ll stall and will last up to two days, and those are the kinds that can produce floods. This I would qualify as a strong one.”
He said this atmospheric river is also being fed by an extratropical cyclone – a mass of wind swirling counter-clockwise that has formed just off the California coast. This cyclone will act to fling the wind and moisture in the atmospheric river at the state with greater force.
“I think we’ve got the makings of a significant storm that is going to help provide beneficial drought relief,” Ralph said.
Strong atmospheric river storms are responsible for the worst flooding events throughout California history. No catastrophic floods are expected from this storm, partly because the state’s major reservoirs are so depleted by drought.
But there may be enough rain in this storm to cause modest flooding, said Alan Haynes, coordination hydrologist at the California-Nevada River Forecast Center, a branch of the National Weather Service in Sacramento.
This could include large rivers like the Eel, Russian and Sacramento. The latter is expected to exceed flood stage in parts of the northern Sacramento Valley near Tehama on Thursday night. No significant damage is expected, as these are rural areas that routinely flood in significant storms.
Haynes said flooding could worsen if the storm slows down and parks itself over a particular region, which is known to happen with atmospheric rivers.
“Even with the reservoirs pretty empty and holding back all the water, there will be enough from the side creeks to cause the Upper Sacramento to go above flood stage, potentially,” Haynes said. “It’s surprising, because here we have this drought going on, and still we’re going to probably see some flooding.”
The northern Sacramento Valley could see as much as 6 inches of rain between Wednesday and Friday. The southern Valley, including the city of Sacramento and the surrounding metro area, could get 2 to 4 inches.
Eric Kurth, a meteorologist at the National Weather Service in Sacramento, said the storm could end up ranking among the Top 10 wettest events in Sacramento’s weather history, which dates to 1877.
Along with the rain will come strong winds – exceeding 60 mph at times in the Sacramento Valley. The region hasn’t seen a rain and wind combo like this since about 2008, said Kurth.
Flooding is also expected on small streams and urban drainages and is likely to be worse than last week’s storm. That’s because the earlier storm soaked the soil, so it will not absorb rainfall as readily. Many storm drains will be clogged by fall leaf litter, and will likely become more clogged by wind-thrown debris.
Thursday is predicted to be very rainy all day, with the heaviest rain expected to coincide with the evening commute. The worst wind will be earlier, likely timed with the morning commute.
“It’s going to be a challenging day all around, with that combination of strong winds and heavy rain,” Kurth said. “People should leave a lot more time, or if they don’t need to travel it might be good to try to do it another day.”
Snow levels in the Sierra Nevada will be relatively high – around 5,500 feet – at least initially. Yet it will be cold enough to shroud major highway passes in as much as 2 feet of snow. Higher elevations could get 4 feet of snow. Strong winds are likely to create whiteout conditions, which will make driving especially treacherous.
“This is not the storm of the century,” said Patzert. “But this is definitely a break in the pattern. When you haven’t had a winter wet season for three years, when it finally comes back it’s a shocker.”
Call The Bee’s Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.