Bar Forecasting for Bay Area Boaters

The National Weather Service San Francisco Bay Area Forecast Office is working together with the Coast Guard and the local marine community to make the waters outside the Golden Gate safer for mariners.

A new marine forecast has been developed for the San Francisco Bar, an area of shallow water just outside the Golden Gate, where deep water swells interact with the shallow water across the Bar to produce dangerous seas that are much higher and steeper than other portions of the coastal waters.

Every year, the Coast Guard receives numerous distress calls from mariners caught unaware by the pounding surf. Compounding the problem is the proximity of the Bar to the San Francisco Bay Area, attracting numerous recreational boaters who may not be familiar with the locally dangerous conditions just outside the San Francisco Bay.

Small recreational vessels are not the only ones impacted by the hazardous bar conditions. Large cargo ships entering through the Golden Gate to reach Oakland and other major ports can be delayed by the rough seas, as tugboats may have difficulty navigating the massive commercial vessels into San Francisco Bay. Fishing operations off San Francisco and the San Mateo coast can be impacted by rough bar conditions as well.

During rain events, silt and soil run off into local creeks and streams that are tributaries of the San Joaquin and Sacramento Rivers. This silt and soil are then transported into the San Francisco Bay via the Carquinez Strait and San Pablo Bay. Currents and tides then carry the silt and soil out of the San Francisco Bay through the Golden Gate, where it is deposited onto the ocean floor over time creating the shallow waters.

The waters of the San Francisco Bar are as shallow as 25 feet, even as far as six miles from shore. During major winter storms, powerful deep water swells are generated over the Gulf of Alaska and travel to all the way to the California coast. A modest storm will provide swells 15 to 20 feet high across the coastal waters. However, over the San Francisco Bar, these swells can interact with the shallow water to produce breakers as high as 30 feet; large enough to easily overcome or capsize all but the biggest ships that transit the coastal waters.

The forecast, in effect since Mar. 2006, is a specialized forecast for the Bar, which includes Potatopatch Shoal and Fourfathom Bank near Point Bonita, and the South Shoal off of Ocean Beach in San Francisco. For more information: www.noaa.gov