The first in a series of back-to-back atmospheric rivers arrived in California on Wednesday, as forecasters warn the severe storms will batter the state with heavy downpours, strong gusty winds and risks of flash floods.
Both “pineapple express” atmospheric river systems – which pull streams of moisture from the Pacific near the Hawaiian islands before making landfall along the west coast – have the potential to wreak havoc, especially in vulnerable areas where past wildfires have left burn scars, or where the landscape has already been saturated by previous rains. The storms will also dump heavy snow over the Sierra and intermountain west region and churn up high surf along the coasts.
What is an atmospheric river? And other weather terms explained
Here's a short breakdown of the different kinds of storms that have lashed the west coast of North America this winter.
Atmospheric river
The National Oceanic and Atmospheric Administration refers to these as “rivers in the sky” for good reason. Characterized by long streams of moisture in the atmosphere, the average atmospheric river carries an amount of water vapor that rivals the flow at the mouth of the mighty Mississippi River – and strong ones can hold up to 15 times that amount. That moisture is released as rain or snow when ARs make landfall and typically are accompanied by strong, gusty winds adding to their destructive tendencies.
Pineapple express
These particularly strong atmospheric rivers are named for their origin. Pulling moisture from the Pacific around Hawaii, Pineapple Express storms have been known to unleash torrents of precipitation when they reach the west coast of the US and Canada – and have dumped roughly 5in of rain on California in a single day, according to the National Ocean Service.
Bomb cyclone
These low-pressure storm systems help create atmospheric rivers, pushing them from the Pacific to the coast. Unlike hurricanes or other storms where the center is the strongest, bomb cyclones can generate the worst weather at their edges.
El Niño
This is a climate pattern characterized by unusually warm surface ocean temperatures in the tropical Pacific. Along with its counterpart La Niña – which, in turn, refers to a period of colder-than-average sea surface temperatures – these patterns can impact weather around the world. While the weather doesn’t always align, El Niño is associated with warmer temperatures, and generally delivers drier conditions in the northern US and Canada, and wetter ones – bringing increased flood risks – through the south.
– Gabrielle Canon, US climate and extreme weather correspondent
“This system has the potential to bring a six- to 12-hour period of heavy to locally very heavy rainfall in NorCal on Wednesday,” said the climate scientist Daniel Swain in a discussion of the storm posted on Tuesday, warning that widespread roadway and small-stream flooding was likely.
Officials across the state readied resources on Tuesday ahead of the storms’ arrival, and urged residents to prepare for power outages, downed trees and hazardous road conditions through the week. The state’s governor, Gavin Newsom, activated the state operations center and the flood operations center to coordinate emergency responses across affected areas, and pre-positioned personnel and equipment in communities most at risk.
“The state is working around the clock with our local partners to deploy life-saving equipment and resources statewide,” Newsom said in a press statement.
The storm rolled into California’s far north first before moving down the coast. Dark clouds gathered over the San Francisco Bay area on Wednesday morning, with rain arriving by the afternoon.
Much of the first storm’s heaviest downpours and mountain snow was due to arrive late on Wednesday and overnight into Thursday. Forecasters expect an even more powerful storm to follow it on Sunday.
The storms have put residents on edge. Last winter, California was battered by a string of atmospheric rivers that unleashed extraordinary amounts of rain, causing flooding and waves that hammered shoreline communities, as well as extraordinary snowfall that crushed buildings. More than 20 people were killed by the historic weather.
The memory was in mind in Capitola, along Monterey Bay, as Joshua Whitby brought in sandbags and considered boarding up the restaurant Zelda’s on the Beach, where he is kitchen manager.
“There’s absolutely always a little bit of PTSD going on with this just because of how much damage we did take last year,” Whitby said.
Even though Wednesday’s storm could deliver some damaging impacts, the stronger system is expected later into the week. “All of the ingredients,” Swain said, “will be in place for a potentially very high-impact storm on Sunday-Monday.” Noting that the details were still uncertain, he added that heavy, widespread precipitation and damaging winds were probably in store for southern California.
While the storms would be intense on their own, the biggest problems could be posed by their cumulative impact, especially in areas already saturated from previous downpours. Communities in San Diego and Ventura counties are still grappling with the aftermath of serious flooding this winter and both areas are going to see significant rainfall this week.
Along with potentially damaging rains and winds, forecasters said the storms could dump a fresh layer of snow in the mountains, a promising sign for the meager snowpack that’s now just 52% of average. The second snow survey of the year, conducted on Tuesday, showed a big improvement from last month, which stood at just 28% of average. Snow is a critical part of California’s water supply, acting as a kind of water savings account for the year ahead.
“Even though the storms during January slightly helped out our snowpack, we’re only about halfway of where we should be for this time of year,” said Sean de Guzman, the flood operations manager for the department of water resources.
The Associated Press contributed reporting
