A plume of tropical moisture known as an "atmospheric river" streaming across the Pacific Ocean will bring some much-needed rain to parts of the West Coast tonight and tomorrow, but will do little to satiate the exceptional drought that California has experienced for the last few years.

This afternoon's water vapor imagery of the Eastern Pacific shows the atmospheric river stretching from the tropics to the western United States.

[Image via U.S. Naval Research Laboratory]

A strong low pressure system sitting off the coast of the Pacific Northwest is to blame (or to thank) for the incoming precipitation as it interacts with this atmospheric river of moisture. The storm will be a prime example of the saying "too much of a good thing," as areas from British Columbia in Canada southward through Oregon are at risk for flash flooding and mudslides as up to 10 inches of rain could fall in some spots over the next week. Below is the 7-day rainfall forecast from the NWS Weather Prediction Center, showing expected precipitation in inches.

[Image via NWS Weather Prediction Center]

While the rain and snow will help relieve some of the "abnormally dry" to "moderate drought" conditions in Washington and Oregon as indicated by the U.S. Drought Monitor, the scant rainfall expected over most of California won't make a dent in the exceptional drought the state is experiencing.

According to the National Weather Service, an overwhelming majority of the state of California has received less than 50% of its normal annual rainfall between March 2013 and now.

[Image at top of post via AP; image to the left via U.S. Regional Drought Monitor]