The San Francisco Bar – just offshore from the Golden Gate Bridge – serves up treacherous conditions just about any day of the year. Large swells, generated by thousands of miles of fetch from across the Pacific, roll into the narrow channel and quickly meet shallow water. The inevitable result is enormous seas that barrel through the Golden Gate.
The Coast Guard recently requested that the National Weather Service generate a specific forecast for conditions on the San Francisco Bar. The forecast offers a 24-hour snapshot of the expected conditions.
The San Francisco Bar Forecast is transmitted over NOAA Weather Radio on its marine transmitter. The forecast can be accessed on NOAA’s homepage, www.wrh.noaa.gov, and by following the links to local forecasts.
“Each year, the U.S. Coast Guard Station Golden Gate receives numerous distress calls from mariners caught unaware by the pounding surf generated over the San Francisco Bar,” said forecaster Mark Strobin of the NWS San Francisco Bay Area Forecast Office. “This has made the Coast Guard eager to work with the National Weather Service to develop a specialized forecast for the Bar. In addition, the San Francisco Bar Pilots and the Offshore Yacht Racing Association of San Francisco Bay have also played a major role in the development of the Bar Forecast.”
The conditions at the Bar are unusual and pose a risk to all mariners, not just small recreational vessels, according to Strobin. “During major winter storms, powerful deep water swells are generated over the Gulf of Alaska and travel to the California coast,” Strobin said. “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 largest ships that transit the coastal waters. This forecast will alert mariners to where and when these hazardous sea conditions are expected to occur.”
The waters of the San Francisco Bar are as shallow as 25 feet, even as far as 6 miles from shore. The shallows are the result of silt and soil runoff into local creeks and streams that are tributaries of the San Joaquin and Sacramento Rivers. This silt and soil is 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, over time, it is deposited onto the ocean floor.