Parts of California have already received 6-8 inches of rain.
That number is expected to double (or more) today and Wednesday.
January is typically a rainy month for California, but the current weather pattern is especially wet. That's because the jet stream has shifted into a pattern known as the “Pineapple Express.”
This occurs when high pressure bubbles up off the west coast of Canada and forces the jet stream farther south than usual. The jet stream then moves just north of Hawaii and travels east-northeast and brings plentiful tropical moisture to the entire West Coast of the United States.
Pineapple Express patterns have been responsible for many major floods in the west, and the current situation is no exception.
Flood watches and warnings have been in effect across the northern half of California, southwest Oregon and even parts of northern Nevada for the last few days now. In higher elevations, Winter Storm Warnings have been issued, calling for feet of snow.
Several rivers have already reached flood stage, and they're expected to keep rising throughout the next couple of days.