Bay Area Storm Index

Recent storms have prompted anecdotal comments that this "is this the strongest storm I have ever seen" and questions like "how does this storm compare to other past storms" or "was this even a storm?" But there is no handy methodology for rating the relative strength of winter wind and rain events that roll through the San Francisco Bay Area. Tornadoes are ranked on the Fujita Scale, hurricanes by the Saffir-Simpson Scale and even earthquakes can be ranked on the Richter Scale. In an effort to rank the storms that affect the San Francisco Bay Area there is presented here an evaluation framework called the Bay Area Storm Index (BASI).

There are dozens of ways that an index like BASI could be calculated but the overriding principle in this case was to produce a product that is both straightforward to calculate and simple to understand. It is also a given that everyone has their own ways and means of evaluating the strength 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 weather elements. The first is the 24-hour rainfall for downtown San Francisco and which is 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 below 1500 feet elevation. These two wind factors each account for 30% of BASI. See below for a BASI calculator.

Retroactively I have determined BASI values for several memorable storms (see Table 1) to see where they would rank. In the past 50 years I only found 1 events that rated a BASI of 10. This was the storm on December 12, 1995 that is the benchmark of strong windstorms among Bay Area meteorologists. The three storms which rated a 9.5 were on December 22, 1955, March 31, 1982 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 9.0 and then the October 13, 2009 (day after Columbus day) was a 9.0. There is an archive of ongoing storm events with a BASI ≥ 6.5 is located at http://ggweather.com/basi_archive.htm.

Table 1	Dec 22, 1955	Oct 12, 1962	Mar 31, 1982	Dec 22, 1982	Dec 12, 1995	Dec 16, 2002	Jan 4, 2008	Oct 13, 2009
SF City 24hr Rain	2.57"	3.11"	2.57"	2.00"	3.27"	2.07"	2.01"	2.48"
SFO Max Sustained Wind	42 mph	43 mph	47 mph	47 mph	54 mph	43 mph	553 mph	41 mph
Peak Bay Area Wind	90 mph	86 mph	81 mph	100 mph	103 mph	91 mph	87 mph	77 mph
Bay Area Storm Index	9.5	9.5	9.5	9.0	10	9.0	9.0	8.5

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.

Bay Area Storm Index Calculator [w/ thanks to Scott Archer for his programming wizardry]

San Francisco Airport Sustained Wind (mph)

Bay Area Peak Wind Below 1500 ft (mph)