LAKE COUNTY, Calif. – Much of Northern California – including Lake County – is being urged to prepare for a fierce storm that forecasters say will arrive later this week.

The National Weather Service issued a special weather statement on Sunday warning of a major storm that's set to arrive on Wednesday night – bringing with it heavy rain – and expected to continue through Friday.

The threat level is severe, according to Sunday reports from the National Weather Service, which said the storm is expected to be of the magnitude that hits Northern California once every several years.

The agency said the rain in the forecast ahead will be the result of an “atmospheric river” – a narrow region in the atmosphere, between 250 and 400 miles wide – responsible for most of the horizontal transport of water vapor outside of the tropics.

Atmospheric rivers, the National Weather Service reported, move with the weather, are present somewhere on the earth at any given time and “provide beneficial rain or snow that is crucial to water supply.”

They're responsible for between 30 and 50 percent of the West Coast's annual precipitation, the agency said.

The heaviest rain is forecast to occur on Thursday, with thunderstorms possible that day and on Friday, the National Weather Service said.

The early forecast expects as much as 10 inches of rain in Northern California's mountains and wind gusts up to 60 miles per hour.

Early precipitation forecasts for the Lake County area predict a seven-day rain total for the period starting Monday and ending at 4 a.m. Sunday of between 5 and 7.5 inches.

While forecasters said the details of what areas can expect in the storm are expected to change over the next several days, they believe that there will be a combination of heavy rain and high winds that could lead to downed trees, power outages, stream flooding and – in high mountain areas – between 1 and 2 feet of snow.

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