The first major atmospheric river event of the season is parked over Northern California, and it’s being held in place by two bomb cyclones — one of them among the strongest on record — in an unusual meteorological event that is expected to feed a continuous surge of moisture into the West Coast for the next several days

As more rain falls, the risk for dangerous flash flooding, debris flows and landslides will increase as the ground becomes too saturated to hold it and streams and rivers continue to rise and overflow.

What can make atmospheric rivers difficult to forecast is that they are narrow bands of moisture, and they hold dramatically less moisture around their edges than they do in the center. So a specific location could see a huge variety in their potential forecast in the days and even hours leading up to an event. For this storm, that means a place south of the Golden Gate Bridge, like San Francisco, could end up seeing only an inch of rainfall, or by Friday, they could have seen more than six inches.

Here’s a look at what’s happened so far, what the region may face next, and why it’s so hard to be sure.

This week so far

The storm began to move ashore Tuesday evening, bringing pouring rain and howling winds first to the northern regions of Washington and then working its way south. More than 600,000 customers lost power at some point overnight into Wednesday, and on Thursday morning more than 300,000 in Washington remained without power, according to poweroutage.us, which tracks utilities.