A "bomb cyclone" is forecast to intensify an atmospheric river and bring snow to the northern Sierra Nevada starting on Tuesday, with heavy rain expected along the West Coast.
Hector Crespo, meteorologist at the National Weather Service in Reno, said Monday the western slopes of the Sierra around Donner Pass on Interstate 80 could see 4 to 8 inches of snow on Wednesday. Areas north of there, including parts of Shasta and Plumas counties, could see even higher accumulations.
Northern Nevada and the eastern side of the Sierra are expected to see rain and snow midweek and into the weekend. The impacts are being driven by moisture from the atmospheric river setting up off California's coast.
The National Weather Service also forecasts heavy precipitation for Northern California's coast to southern Oregon.
How a 'bomb cyclone' could affect the atmospheric river
A developing low-pressure system, which could intensify into a bomb cyclone, is forecast to enhance the impact of an atmospheric river set to hit Northern California this week.
Bomb cyclones occur when atmospheric pressure drops rapidly to a certain point over 24 hours.
For this system, the San Francisco NWS forecasts a pressure drop of 60 millibars in 24 hours -- far surpassing the 18-millibar threshold needed. Its strong circulation is forecast to pull subtropical moisture to Northern California's coast and intensify the atmospheric river.
While the bomb cyclone would fuel the atmospheric river, the NWS said the atmospheric river itself would bring the most significant impacts, including heavy rain, snow, and potential flooding.
What is an atmospheric river?
An atmospheric river is a narrow band of concentrated water vapor in the atmosphere. These weather systems can transport vast amounts of moisture from the tropics to other regions, often resulting in heavy rain or snow.
According to Zack Taylor, meteorologist at NWS' Weather Prediction Center, this atmospheric river has a lot of energy and will last longer than usual.
Atmospheric rivers typically move from north to south down the coastline, he said.
"So this one is just kind of stalling out, and that's ... why you're seeing some of these really impressive rainfall totals," Taylor said.
Taylor said he wouldn't be surprised if some areas got up to 15 inches of rain by the time the system moves through into early next week.