Published Tuesday, May 22,  2001 in the San Jose Mercury News


Weather plays important role in fire season

Special to the Mercury News

The brief hot spell during the second week of May and a 2,000-acre wildfire in the Klamath National Forest were a fitting kickoff for the California Department of Forestry and Fire Protection's Wildfire Awareness Week last week. The fire season officially has already started in several Southern California counties, and during the next month fire facilities across the remainder of the state will be staffed.

Even though most lowland areas around Northern and central California had near-normal rainfall, most of the mountainous areas had less than three-quarters of their usual rainfall. Fire officials fear the vegetation that grew so abundantly from last year's heavy rains is now drier than usual and points toward an above-average number of fires.

Long-range forecasts for this summer are calling for near-normal temperatures for California, but devastating fires from even a brief hot and dry episode of offshore winds can spell disaster. For example, the summer of 1991 preceding the deadly Oakland hills firestorm was cooler than normal.

When there is a major fire, an important part of the team that fights it is a meteorologist. Working with fire-behavior specialists, the meteorologist provides valuable information about conditions likely to affect the blaze. This includes everything from large-scale changes such as an approaching weather front to how winds will interact with the terrain and change throughout the day.

The CDF and local fire agencies urge people who live in or near undeveloped areas to take precautions now. Clear brush for at least 30 feet around all structures. Approximately two-thirds of the wildfires in California last year were the result of careless or destructive human activities, ranging from vehicle and equipment use, which caused 39 percent of fires; to campfires and burning debris at 16 percent; arson at 6 percent; and children playing with fire at 2 percent.

I was interested in your column on May 8 about 100-year floods. I lived in a house in New Jersey in the 1970s and '80s. I used to joke I had lived there 450 years, because I experienced one 250-year flood, one 100-year flood and two 50-year floods. I'm not too impressed by these designations.

In New Jersey, increasing urbanization, including filling in lowlands and building next to rivers, removing forested hillsides and replacing them with houses, streets and lawns, and building bridges just above the water level all contributed to the situation. The latter situation meant that even small increases in water level caused the bridge to act like a dam, making upstream flooding worse. Also, communities would channel streams, exporting their flood to the next town downstream. The end result was that a rainfall that would have caused no problem in 1930 caused 50- or 100-year floods in 1980.

I'm happy to say that many communities learned from these problems and banned construction near rivers, using the land for parks or nature preserves where the river could spread out at flood time and not do significant damage.   Steve Johnson - Los Gatos

You are absolutely right. A flood plain is not a static part of the landscape, but one that is constantly changing. This is largely responsible for the ongoing confusion about who needs flood insurance.

When 100-year floods are mentioned, the speakers seem to be assuming that year-to-year weather is statistically independent. Have any decent studies been done to investigate year-to-year correlations? Certainly the El Niņo years were not independent, and it does not make sense that nature breaks its behavior into neat 365-day chunks. Bernhard Hoyt - Santa Clara

The statistics look at the composite picture for an extended period and do include El Niņo and La Niņa years. However, it is interesting to note that the majority of California's big flood events have NOT occurred during El Niņo and La Niņa seasons. These tend to be seasonal events, while most major floods occur in periods of heavy rain that last from three to 10 days.

As far as breaking flood statistics into 365-day chunks, two factors make this insignificant here in California. First, most floods occur over a much smaller time frame, and a 100-year designation is merely a statistical way of expressing the occurrence and has nothing to do with a ``real'' calendar. Second, rainfall and flooding statistics in the West are usually defined by the rainfall ``season,'' which runs July 1 until June 30. Nature conveniently puts about four months of dry weather in between to make a logical dividing point.

Whenever we have a heat wave in Northern California, it seems to correlate to the full moon cycle. Is this true?  Nora Lynch - Palo Alto

Yours is an interesting question; however, I can find no evidence to support your supposition. For example, I have looked at all the days that San Jose recorded 100 degrees or more since 1997, and none has fallen within two days of a full moon. I think it's probably more a combination that on warm evenings people are outside more, and that during a full moon it rises at sunset and is very prominent and memorable in the eastern sky.

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 or e-mail them to Please indicate in your e-mail what city you live in.