Nautical Terminator – Small Craft Advisory

The forecast calls for a “small craft advisory.” Latitude 38 once made the comment that these don’t really mean anything. This is false, but I can understand how one might come to that conclusion.

First, there’s no legal definition of “small craft.” However, this means you.

Secondly, none of the many websites of the National Weather Service offers an explanation specifically applicable to our unique San Francisco Bay conditions. But a phone call or two to the NWS elicited the following thresholds for the three types of small craft advisory for our local waters:

1) A small craft advisory is issued if current or forecast sustained winds are 21-33 knots, or frequent gusts of 21-33 knots lasting more than two hours are predicted.

Outside the Gate:

2) A small craft advisory for hazardous seas is issued if the period of the combined seas measured in seconds is less than or equal to the wave height in feet, except when the wave height is less than 5 feet.

3) A small craft advisory for rough bar conditions is issued if breaking waves and/or combined seas of 10 feet or greater are expected on the San Francisco Bar.

The confusion over this advisory results partly from the difference between our summer conditions and winter conditions. In summer, a daily thermal low in California’s central valley pulls wind from the sea through the path of least resistance, which is the Golden Gate. A venturi effect amplifies the velocity in the “slot,” and from there the wind fans out and its speed diminishes. So in the summer, a sailor in the Oakland Estuary may see 10 mellow knots while a boat near Alcatraz may be well reefed down in 30 knots.

In the winter, however, wind velocity is generated by Pacific weather systems that can have massive fronts of over 1000 miles in length. These sweep over the entire bay and may generate sizable waves, so there will be no place to hide.

Since the NWS makes predictions for the highest wind anywhere within the forecast area on a given day, and does not slice our area any smaller than “San Francisco, San Pablo, Suisun Bay and the West Delta,” you need to know summer from winter patterns to interpret this advisory. In fall and spring, take a closer look at the local weather reports, as winds can be generated by either dynamic.