La Nina May Bring Winter Floods To Sierra

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Experts say cooling ocean temperatures in the east Pacific indicate development of a La Nina weather phenomenon and the possibility of floods in the Reno area next winter.

Even as this year's unimpressive El Nino dwindles into obscurity, its meteorological opposite appears to be taking shape.

Kelly Redmond of the Western Regional Climate Center in Reno said that could mean trouble.

"It seems to raise the odds in the Sierra Nevada of wintertime floods," Redmond told the Reno Gazette-Journal. "La Nina can bring some dramatic weather of its own to areas like Reno."

El NiIno, associated with warming of eastern Pacific waters, and La NiIna, associated with cooling, both can dramatically affect the weather and produce either wet or dry years.

This year, a moderate El NiIno condition prevailed, but it appears "almost imminent" that it's going to be replaced by La NiIna, said Vernon Kousky, a forecaster at the federal Climate Prediction Center in Maryland.

La NiIna years appear more likely to tap the so-called Pineapple Express, a freight trainlike series of tropical storms that can funnel warm and heavy rain into the Sierra.

When that rain falls on snow, trouble in the form of brimming rivers and flooded streets can result. That's what happened during Reno's last major flood on Jan. 1, 1997, during what was a weak La NiIna, said Redmond.

The flood caused $700 million in damage to the Reno-Sparks area.

Some of Reno's other major floods in recent history occurred during La NiIna years, Redmond said. Major floods of November 1950 and December 1955 both occurred during strong La NiIna years. Another flood in December 1964 occurred during a moderate La Nina and one in February 1963 occurred during a year with neutral conditions leaning toward a La NiIna event, Redmond said.

Reno floods in December 1937 and February 1986 occurred during solid neutral years when neither La NiIna nor El Nino were in place, Redmond said.

It wasn't until about the 1970s that meteorologists began suspecting the profound impact of El Nino and La NiIna on global weather patterns, with the profoundly strong El Nino of 1982-83 cementing those suspicions.

Scientists now can guess which condition prevailed on a given year over much of the last century based on historical climate data.

The term El Nino means "Christ child" or "the boy" and was first used by Peruvian fishermen in the late 1800s to describe the warm current appearing off the western coast of Peru around Christmas. La Nina is a play on words for "the girl" to describe the opposite weather phenomenon.

For La NiIna years, roughly the same flooding pattern exists within the American River drainage on the opposite side of the Sierra from Reno, Redmond said.

"It looks like those Pineapple events have higher odds of materializing in La NiIna winters than during El Nino winters," Redmond said.

Although the rainfall alone might not be enough to cause major flooding, it's the rain-on-snow scenario that's most worrisome.

"Every year that there's snow on the mountain there's potential for something to happen," said Paul Urban, Washoe County's flood control manager.

"But whenever I hear of the potential for Pineapple Express, I get concerned. That's when you get rain on snow."

(Copyright 2003 by The Associated Press. All Rights Reserved.)