SAN FRANCISCO — In the midst of an unrelenting winter that has already dumped near-record amounts of snow and rain on California, yet another "atmospheric river" is taking aim at the Golden State.
Unlike the recent series of cold storms that have blanketed lower elevations across the state with snow, the coming "Pineapple Express," which the National Oceanic and Atmospheric Administration defines as "a narrow region of atmospheric moisture that builds up in the tropical Pacific," will arrive late Thursday with warmer air and a heightened risk of flooding.
WPC has initiated Key Messages for a winter storm set to deliver heavy rain & mountain snow to California late week. Expected impacts include (but are not limited to) treacherous travel in higher terrain & flooding from a combination of heavy rain & snowmelt. #CAwx pic.twitter.com/BEv7t1OyDV
— NWS Weather Prediction Center (@NWSWPC) March 7, 2023
While mountain elevations above 6,000 feet are expected to receive multiple feet of additional snow from the storm, which will last into next week, elevations below 5,000 feet, such as the Sierra Nevada foothills, will see anywhere between 2 and 7 inches of rain. Warmer precipitation at those elevations will hasten snow melt, increasing the risk of flooding.
Temperatures in foothill towns like Grass Valley are expected to rise into the low 50s over the weekend, but not to levels that, even with the heavy rain, are likely to result in a catastrophic melting event.
“Will there be a moderate to strong & warm #AtmosphericRiver storm this weekend in CA? Yes. Will there be flooding from hvy rain & snowmelt? Also yes,” UCLA climate scientist Daniel Swain tweeted on Tuesday. “Do meteorologists and hydrologists, using the best available data at this time, expect a historic flood event? No.”
This latest deluge comes in a winter in which the state saw extreme drought conditions wiped from the map due to a parade of atmospheric river storms.
“From late Dec. 2022 into Jan. 2023, a series of nine ‘atmospheric rivers’ dumped a record amount of rain and mountain snow across the western U.S. and Canada, hitting California particularly hard,” the National Environmental Satellite Data and Information Service said on its website. “More than 32 trillion gallons of water rained down across the state alone, and the moisture also pushed into much of the Intermountain West.”
While California's winter has so far been “above average” in terms of precipitation, the more atmospheric river events that occur before the arrival of the dry season will further aid drought relief, but could also pose more extreme flood risks.
“If we get another couple of big storms, which looks like it’s possible, we could end up with a really wet year. The consequences of that depend on how we get the water and when we get it and where we get it,” Peter Gleick, climate scientist and the founder of the Pacific Institute in Oakland, told Yahoo News.
This round of heavy rain and snow will affect the central and northern parts of the state, the National Weather Service said. After heavy precipitation falls starting Thursday, a second blast is expected Monday and Tuesday.
Showers continue to move across the area this morning. A warm, wet storm will bring major impacts to interior NorCal Thu - Sun with rain and flooding, high elevation snow, and wind. Another system could bring additional widespread precip early next week. #cawx#cawx pic.twitter.com/XWxJYTvNNE
— NWS Sacramento (@NWSSacramento) March 8, 2023
In historical terms, 2023 has not yet exceeded the all-time snowpack record in the Sierra Nevada, according to the University of California Berkeley's Central Sierra Snow Lab. But with heavy snow on the way this weekend and more systems on the horizon, this year appears likely to be one of the top four in recorded history.
We have had 10.6" (27 cm) of #snow over the last 24 hours at the lab. That brings our total to 603" (15.32 m) since Oct 1st. This is now the 5th snowiest year since the CSSL was built in 1946.
More snowfall in the forecast and it's likely it will be moving up!#CAwx #CAwater pic.twitter.com/j5Olp7JpC6
— UC Berkeley Central Sierra Snow Lab (@UCB_CSSL) March 8, 2023
As for the coming days, the latest atmospheric river will have an impact on more than just the Sierra Nevada and its foothills. Heavy winds are forecast for a wide area that includes the San Francisco Bay Area and the Central Coast, with downed trees and limbs expected that could disrupt power.
Along with widespread rainfall from the upcoming atmospheric river, strong southerly winds will develop on Thursday with wind gusts up to around 50 mph. These winds may blow down tree limbs and/or trees resulting in isolated to scattered power outages & blocked roadways. #CAwx pic.twitter.com/1mZm2Qwb2S
— NWS Bay Area 🌉 (@NWSBayArea) March 8, 2023