Flash Flood in Marin County, California

Heavy rainfall produced significant rises on Novato Creek between highways 101 and 37 on the east side of the city of Novato. This led to an intentional breach of the levee on a 20 foot section to prevent floodwaters from reaching downtown businesses and nearby subdivisions. This same flooding also resulted in failure of a smaller portion of the levee and overtopped another section. Caltrans levees on highway 37 were breached and a culvert was washed out.

An Atmospheric River event brought heavy rain and gusty winds with a strong winter storm that impacted the Bay Area for several days in mid-December. Cyclogenesis occurred with a 979 mb low forming off the Northern California coast. On December 11, the intense surface low moved up the coast and drug a narrow cold frontal rainband (NCFR) through the Bay Area that morning and early afternoon. NCFRs are rapidly moving fronts that are notorious for intense rainfall and gusty winds despite having limited instability.

The NCFR wasn't a major wind producer, but the rainfall rates were impressive. Instantaneous rainfall rates of 1.5-2 inches/hr were reported. Observational data from the Big Sur Coast were also impressive with Big Sur receiving 4.43 in 2 hours and Flores Camp receiving 3 in 1 hour. It wasn't so much the total amount of rainfall for many locations, but the intensity and how much was received in a short amount of time.

Many locations around the entire Bay Area had flooding: urban flooding of streets and highways, flooding of creeks and even one large river in the North Bay. Eventually the NCFR slowed around the Big Sur Coast. The stalling was likely due to another 'wave' in the atmosphere, farther to the southwest, riding along the boundary. The end result was to have the weakened NCFR lift back northward, almost like a quasi-warm front, producing another round of moderate to locally heavy rainfall around the Bay Area, compounding flooding concerns.

In addition to the impressive rainfall, winds also became a problem for the water soaked ground, drought stressed trees and diseased trees. Peak winds occurred over the higher terrain with the Lick Observatory reporting 83 mph. Most low elevation areas saw much lower wind speeds.

Lastly, the intense winds over the coastal waters helped to generate a fresh swell that produced very dangerous surf with large breaking waves. The Big Sur Coast buoy reported a peak wave of 19.7 feet.

The event was followed by several weaker storm systems that week that brought additional rainfall, continued flooding and mudslide concerns to the area.