A "bomb cyclone" will unleash an atmospheric river Saturday night into Sunday across Northern California.
"By Saturday night, a rapidly intensifying Pacific cyclone directing a powerful atmospheric river squarely at the West Coast delivers a fire hose of rich subtropical moisture into California," the Weather Prediction Center (WPC) said Friday.
These two simultaneous weather phenomenons will result in the season's first snow event in the Sierras and torrential rains for the coastline and valleys across central and Northern California.
"You might hear this term referencing the Sunday-Monday storm coming our way. A bomb cyclone is simply a storm that gets very strong very quickly. It drops at least 24 mb (a unit of pressure) in 24 hours. The lower the pressure, the stronger the storm," said Sacramento-based KTXL's Adam Epstein.
In Northern California, rainfall estimates through the end of the weekend are around 2-4 inches. In San Francisco, estimates are upwards of 3 inches.
WPC warns that some areas could receive 8-10 inches.
⚠️A HIGH Risk of Excessive Rainfall is in effect for portions of Northern California tomorrow. A strong atmospheric river will produce rainfall of 8-10 inches in the region, leading to significant and life-threatening flash floods and mudslides, particularly over burn scar areas. pic.twitter.com/FvACBTpNID— NWS Weather Prediction Center (@NWSWPC) October 23, 2021
The rare level 5 atmospheric river event could be enough rain to alleviate drought-stricken areas ravaged by wildfires.
"An atmospheric river marked as a category 4 or a 5 is capable of producing remarkable rainfall totals over three or more days, likely to exceed 10% to 15% of a typical year's precipitation in some locations," said Marty Ralph, director of the Center for Western Weather and Water Extremes at the University of California San Diego.
In higher elevations, wet snow across the Sierras could amount to 1-3 feet.
SEVERE STORMS: Tens of millions of Americans across the west are bracing for heavy rain, mudslides and more than a foot of snow. Here’s what you need to know: pic.twitter.com/AZqN0qCUTZ— CBS Evening News (@CBSEveningNews) October 23, 2021
The news gets better for Northern California and the Pacific Northwest as WPC has declared La Niña conditions, which means wetter than average conditions will ease areas plagued by drought. As for Southern and Central California, La Niña means a drier than average winter.