Do wet winters mean bad summer fire seasons in California? Not usually, according to history

Do wet winters mean bad summer fire seasons in California? Not usually, according to history

01 June 2010

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USA — Regardless of how wet the winter, fire officials often broadcast the same warning as summer begins: “Prepare for a bad fire season.”

If it didn’t rain much, they say conditions are bone dry. If it rained a lot, they often say the tall grass translates to lots of fuel.

So what’s the real story?

A Mercury News analysis of rainfall and fire records over 40 years shows the worst fire seasons come after dry winters, not wetter ones like the one we’ve just had.

Ranked by acres burned, 12 of California’s 15 worst fire seasons back to 1970 — including the four very worst — all occurred in years after drier-than-normal winters, according to the analysis.

“This is one of my pet peeves. I hear it every year and I grind my teeth,” said meteorologist Jan Null of Saratoga, a former lead forecaster with the National Weather Service.

“Have they ever said it’s going to be a good year? I understand why they don’t. They want funding and they want people to be careful. But they shouldn’t make it sound like they are basing it on science.”

The science is fairly simple, as most weekend campers know. Wet wood doesn’t burn as well as dry wood.

When California experiences a rainy winter or spring — as it has this year — more grasses and other plants grow. Those plants, or “fuels,” dry out by July when summer heats up, increasing the risk of grass fires.

But trees and big shrubs have some insurance. They draw in more moisture during the rainy winter and spring months. That moisture content makes it more difficult for grass fires to spread to larger vegetation, reducing the risk of major conflagrations that can rage out of control.

“After wet springs, it’s easier for grass fires to start, but harder for them to carry. Wet winters and springs buy us more time,” said Craig Clements, an assistant professor of meteorology at San Jose State University who specializes in fire science.

So far, much of the state remains damp. Sierra snowpack was the deepest since 2006. Most reservoirs are full or near full. And nearly every major city in California had rainfall exceed historic averages.

On Friday, Cal Fire state director Del Walters warned Californians to beware of fire danger over the Memorial Day weekend.

“The heavy rainfall this past winter and spring has led to abundant growth of grass and brush,” he said in a news release. “Despite the recent wet weather, this weekend’s warmer temperatures will dry out the vegetation, contributing to California’s elevated risk of wildfires.”

Yet, the Mercury News analysis showed wet weather is more ally than enemy.

Over the past 40 years, the number of acres burned

in California has varied widely, from a high of 1.3 million acres in 2008 to a low of 33,870 acres in 1991, according to totals from the U.S. Forest Service and Cal Fire.

Although there are anomalies — years when large amounts of the state burned even after a wetter than normal winter, the overall pattern is clear.

Most of the years with the fewest acres burned occurred after winters with higher-than-normal precipitation, like 1983 and 1998, both El Niño years with lots of flooding. Years following multiple below-normal rainfall winters, like 2008, or very dry winters, like 2007 and 1977, end up having more acres burn in the summer.

State fire officials concede the point. But they say there are lots of shades of gray.

“Even after wet winters, hundreds of thousands of acres still burn in California,” said Daniel Berlant, a spokesman for Cal Fire.

“We don’t want people to become complacent because of the rain. Hopefully this year we’ll have fewer fires and fewer acres burn, but even if it’s a slower year, we are still going to have wildfires and we have to be prepared.”

Berlant noted also that in some years, fewer acres burn, but more homes burn. In October 1991 — the year the least acres burned in the past four decades — the wind-whipped Oakland Hills fire killed 25 people and burned 3,354 homes.

This year because of the wet weather, Cal Fire has delayed fully staffing many of its fire stations in Northern California by two weeks. Last year, most stations opened by June 1, but this year, it will be mid-June, Berlant said.

After terrible fire years in 2003 — when huge sections of San Diego and Los Angeles counties burned — and 2008, when a freak lightning storm on June 21 set off more than 2,000 simultaneous wildfires from Chico to Big Sur, lawmakers put in place tougher new state rules.

Starting Jan. 1, every new home and townhouse built in California will be required to contain fire sprinklers. Also, five years ago, a new state law required homeowners in rural areas to thin brush and trees 100 feet around their homes, up from the previous requirement of 30 feet.

And finally, last year, a host of new building codes took effect requiring more fire-resistant shingles, siding and eaves on new construction in rural areas statewide, from the Los Gatos hills to forest homes in Redding to ranches in San Diego County.

What’s the fire danger outlook right now?

A report released Tuesday by the National Interagency Fire Center in Boise, Idaho, projected that California’s central coast and Central Sierra will have below normal fire risk this summer because of all the rain. Some areas, like the northeastern part of the state, which received less rain, will have higher risk, while most of the rest of California will have average risk.

“By and large in Northern California, we think it will be a normal fire season,” said David Olson, a Forest Service spokesman. “We’ve had pretty good snow, and the cooler temperatures have helped to slow the drying of grasses, shrubs and trees.”

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