Jan Null, CCM
Golden Gate Weather Services
updated 2/7/2024

There are dozens of ways that an index like BASI could be calculated but the overriding principle used here was to produce a product that is both straightforward to calculate and simple to understand.  It is fully understood that everyone has their own ways and means of evaluating the strength (and impact) of a particular storm and BASI is just one of many possible methodologies.

Historically it has been the combination of strong winds and heavy rain that have the largest impact on the Bay Area and a maximum BASI event is predicated on significant amounts of both and would get a ranking of a perfect "10".  The index is based on only three weather elements.  The first is the 24-hour rainfall for downtown San Francisco and this makes up 40% of the BASI.  The wind component of BASI is composed of two elements, the sustained wind speed at San Francisco International Airport and the peak wind gust in the Bay Area at elevations below 1500 feet.  These two wind factors each account for 30% of BASI. 

Retroactively, I have determined BASI values for significant storms (see adjacent table) dating back to 1950 to see where they would rank.  In the past 64 years I only found one events that rated a BASI of 10.  This was the storm on December 12, 1995 that is the benchmark of strong windstorms among most Bay Area meteorologists. The four storms which rated a 9.7 or greater were on December 22, 1955, December 11, 1995  and the infamous Columbus Day Storm (October 12) of 1962. In the more recent past the gusty storm that blew through the region on December 16, 2002 had a BASI of 9.03 and then the October 13, 2009 (day after Columbus day)  was a 9.3.  On October 24, 2021, a 942.5 mb low off the Washington coast entrained a long plume of mositure for record and near record rainfall. An archive of past storm events with a BASI ≥ 6.5 is located at http://ggweather.com/basi_archive.htm.

Of course the actual impacts upon the Bay Area by a pair of storms that had identical BASI values would vary significantly due to a wide variety of antecedent conditions that are not a part of the index.  For example, is the ground saturated, are rivers and streams running high from previous rains, are there leaves on the trees to produce more wind stress than a bare tree or are there extra high tides or high surf?   Any of these could be incorporated into a more comprehensive index, but for the time being the Bay Area Storm Index in its present form will give a basis for evaluating and ranking storms as well as the genesis for further discussion and study.