The recent storm that impacted the San
Francisco Bay Area has prompted
anecdotal comments that this "was the strongest storm since
2009" and questions like "how does this storm compare to other past
storms". Typically 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 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.
See below for a BASI calculator.
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 three 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. An archive of
storm events with a BASI ≥ 6.5 is located at
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.
Top Ranked BASI Storms (1950-present)
BASI ≥ 9.0
San Francisco Airport Sustained Wind (mph)
Bay Area Peak Wind Below 1500 ft (mph)
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