Published Tuesday, December 31, 2002 in the San Jose Mercury News

Weather Corner
Index helps to calculate storm intensity, ranks events in the Bay Area

By Jan Null
Special to the Mercury News

Recent storms have prompted comments such as, ``This is the strongest storm I have ever seen,'' and questions like, ``How does this storm compare with past storms?'' Those may be timely observations, but there hasn't been a way to provide a quick response -- until now.

There is no handy methodology for rating the relative strength of winter wind and rain events that roll through the Bay Area. Tornadoes are ranked on the Fujita scale, hurricanes by the Saffir-Simpson scale, and even earthquakes can be ranked with the Richter scale. In an effort to rank the storms that affect the Bay Area, I have put together a 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 an index that is straightforward to calculate and simple to understand. It is also a given that everyone has his or her own way of evaluating the strength of a particular storm, and BASI is just one of many possible methodologies.

Historically, the combination of strong winds and heavy rain have the largest impact on the Bay Area. A maximum BASI weather event is predicated on significant amounts of both and would get a ranking of 10.

The index is based on only weather elements. The first is the 24-hour rainfall for downtown San Francisco, counting toward 40 percent of the BASI. The wind component of the 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 1,500 feet elevation. These two wind factors account for 30 percent each of the BASI.

I have retroactively determined BASI values for several memorable storms to see where they would rank. In the past 50 years, I found only two events that rated a BASI of 10. These were the storm on Dec. 12, 1995, that is the benchmark of strong wind storms among Bay Area meteorologists, and the infamous Columbus Day storm of 1962.

More recently, the gusty storm that blew through the region on Dec. 16 had a BASI of 9.0, and the event on Nov. 7 was a 7.5. Saturday's storm rated a 6.0.

BASI, of course, can't tell whole story. The actual effects upon the Bay Area from a pair of storms that had identical BASI values would vary significantly due to a wide variety of conditions that are not a part of the index. For example, is the ground saturated, are rivers and streams running high from previous rains, 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 help form a basis for evaluating and ranking storms. To find out more about the Bay Area Storm Index and to try out an online BASI calculator, go to basi.htm.

Q. What is the National Temperature Index that is featured on late-night ``ABC World News Now''? I have been seeing numbers like 450 degrees, and these seem way too hot. Tom Robinson - Redwood City

A. This is a question I have been asked many times, and a definitive answer has been elusive until now. According to Gerard McNiff, the behind-the-scenes meteorologist for ``Good Morning America,'' the NTI is a number that was devised to make the weather segment of ``World News Now'' ``more interesting'' and is simply the sum of the forecast high temperatures for Boston; Casper, Wyo.; Dallas; Denver; Fargo, N.D.; Las Vegas; Miami; New Orleans; Raleigh, N.C.; and Seattle.

While the NTI might make the weather more interesting for some, its value as a scientific tool is probably limited to being able to tell the viewer if it's winter or summer.

Q. On a sunny morning as we were traveling recently along Highway 1 a few miles south of Capitola, an airplane flew parallel high above us. As it progressed, we saw a dark line extending from the craft's nose to the horizon. We stopped the car to watch (and wonder). Is there any possibility of the phenomenon being weather-related?
Malcolm Sheehan - Los Gatos

A. I recently saw this same phenomenon, and I'm confident that what you saw was the shadow of the high-flying jet. When there is a contrail or vapor trail from an aircraft flying above a thin layer of high cirrus clouds and the sun's angle is just right, the shadow of the contrail is projected onto these underlying clouds and appears as a black line.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015, e-mail them to or fill out a form online at Please indicate in your e-mail what city you live in.<