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Weird weather: a sign of global warming?

Picture

Lightning strikes San Clemente on Oct. 19.

Khristain Snyder
11/05/2010
Pat Brennan
The Orange County Register

More lightning and thunder in a single day than we typically see in a year. Drenching rain in what was supposed to be a dry fall. An oddly cool summer, then record heat in November.

Is Orange County's wild weather somehow linked to planetary warming? Or is asking such a question falling prey to a very human tendency: magnifying local conditions to global proportions?

Climate experts say it's mostly a case of the latter.

No single weather event can be linked directly to global warming -- a statistical measure of changes over decades.

In other words, it's risky to draw conclusions about long-term climate patterns from a few weeks of weird weather.

"The simple answer is that this is weather," said Bill Patzert, a climatologist at the Jet Propulsion Laboratory in Pasadena. "So don't confuse it too much with global warming."

Fall in coastal Southern California also is typically a time of transition; it's not unusual to have rain and cool weather one week, heat and wind the next.

And we can get a distorted picture by focusing on too small a slice of space as well as time.

Hotter summers and more drought do seem to be long-term trends in Southern California, Patzert said.

But he believes the "heat island" effect -- when alterations to the land surface including cities, roads and agricultural areas help drive up temperatures -- might swamp the global-warming signal in places like Orange and Los Angeles counties when it comes to temperatures.
He sees no such long-term trend for the region in terms of rainfall.

And October's lightning, thunder and locally strong rainfall, while unusual, was the product of a "cutoff low" -- when a low pressure system stalls over land, as happened over Southern California in October.

"The thing that always kills us are these cutoff lows," Patzert said. "The cutoff low is the forecaster's woe."

At the moment, we appear to be ahead of the game in terms of rainfall. In Irvine, for example, the rainfall total since July 1 is 2.59 inches -- more than 300 percent of normal.

But Patzert is betting we'll end up below normal by the time the rainy season is over.

That is largely because of the La Niña phenomenon, which brings cooler ocean temperatures to the eastern equatorial Pacific and is having a powerful effect on Southern California.

The November assessment by the Predictive Services branch of the U.S. Forest Service says La Niña will likely mean a drier spring -- even though recent rainfall patterns seem more like El Niño, a periodic warming of the same ocean waters that is famous for bringing rain to Southern California.

La Niña might sometimes bring extra rain to the region early in the season before drying things out later, said meteorologist Tom Rolinski, a Forest Service predictive services manager.

"It's looking like we could get a few showers Saturday, but by and large, with a few exceptions, I think we are heading into a drier pattern -- which is what you would expect with La Niña," Rolinski said.

The rainfall might have been enough to dampen wildfire danger regionwide, he said, although large wildfires in Southern California are still possible.

But even if things dry out, recent moisture plus fewer hours of sunlight as we move toward winter should mean less chance of wildfire, he said.
"Once you get that kind of condition to set in this time of year, it's kind of hard to undo that, even if you get some dry weather," Rolinski said.
In Orange County, the rains brought moisture in wild vegetation up slightly, but not enough to take us out of the "high" fire-danger category, said George Ewan, wildland fire defense planner for the Orange County Fire Authority.

That might at first seem hard to believe; the hills around Orange County have turned green earlier than usual in November because of the rains.
"It'll fool you," Ewan said. "There's lots of green grass out there, green weeds, but it can be of short duration. It depends on what the rest of the season brings us."

For the short-term, another big swing is in store. The high-pressure air bringing all the heat should give way to low pressure by Saturday, cooling things down and possibly adding light rain to the mix.

But while a variety of long-term measurements show an unmistakeable warming trend for the planet as a whole, there is no evidence that the global trend is driving Orange County's crazy weather -- including this week's heat wave.

"The models predict California is going to get warmer," said UC Irvine Earth System Science professor Michael Prather, who specializes in climate modeling. "Is this particular warming a result of that? No."