December was quite a wet month in Northern California. Rainfall from about Monterey north was way, way above normal. To get an idea of how wet, here’s a map showing the percent of average precipitation for California. Areas of dark purple are between 400 and 800 percent above normal.

The north-south precipitation gradient ran for the entire month with areas in Southern California not receiving any rain at all (the dark red areas).

With all the rain Northern California received in December, the ground was fairly saturated when the last storm of the month hit. And man did it hit the Napa Valley hard. Napa’s average rainfall in December is 4.87”, but in 24 hours over the weekend Napa received 5.42”!

All of this precipitation was generated by a storm that tapped into tropical moisture. The north-Pacific water-vapor satellite image from 12/30 shows the weather system approaching the West Coast with a plume of moisture stretching all the way to the Mariana Islands.

Of course, all of this moisture had to go somewhere — straight into the Napa River. The Napa River crested just shy of the 30.5 feet it hit back on March 09, 1995.

Of course, check out Flickr for some shots of the flood. Here’s some and here. Here’s some shots of a mudslide that hit near by Mill Valley. And finally, some shots of the Truckee River which also flooded.