Mild winter anticipated for the Sierra Nevada |

Mild winter anticipated for the Sierra Nevada

John Cobourn and Heather Segale

Forecasters from the National Weather Service are predicting a mild El Niño weather pattern for North America this winter.

El Niño is a disruption of the ocean-atmosphere system in the Tropical Pacific having important consequences for weather and climate around the globe. The National Oceanic and Atmospheric Administration is monitoring a weak El Niño in the tropical Pacific, which they expect to continue into early 2005. The water temperature near the equator, west of South America, is a little more than half a degree Celsius above normal for this time of year. This creates conditions that barely qualify as an El Niño pattern. In contrast, if the water temperature in the equatorial Pacific Ocean gets colder than normal, a La Niña weather pattern is triggered. A La Niña weather pattern is one that can cause winter temperatures that are warmer than normal in the Southeast, and cooler than normal in the Northwest.

Forecasters are not sure if this year’s mild El Niño will affect the weather in our part of the Sierra. The Lake Tahoe area is on the dividing line for how such events affect the West Coast. During a strong El Niño, the area south of here is wetter than normal and the area north of here is drier. Since the Tahoe basin is right on the line, the El Niño usually must be very strong to give us wetter than average weather.

Gary Barbato, National Weather Service in Reno, says that he is expecting close to average precipitation for the first part of this winter. He says that NOAA’s Climate Prediction Center is predicting wetter than average conditions just south of the Reno/Tahoe area between January and May 2005.

NOAA has also announced that the winter outlook for December 2004 through February 2005 calls for warmer-than-normal conditions in the West and Alaska, and cooler-than-normal conditions in the South and sections of the Mid-Atlantic States. The Sierra should still accumulate its deep snowpack at higher elevations, since warmer-than-normal conditions should still be cold enough for most Sierra precipitation to arrive as snow. However, climate change experts predict that by about 2090, global warming will greatly reduce the average depths of the Sierra’s April 1 snowpack, possibly to half of today’s levels.

NOAA scientists say the leading climate patterns expected to affect this winter’s weather are long-term climate trends and features, such as the El Niño/Southern Oscillation and the Pacific/North American pattern, which influence the jet stream and the track storms take across the eastern Pacific and North America. A well-known example of a severe weather storm track is the “Pineapple Express,” which occasionally brings a succession of warm, wet storms from near Hawaii to the Sierra, creating snow accumulations measured in feet over a period of multiple days. When such a pattern includes an extremely warm storm after 2 feet or more of snow has accumulated, a “rain-on-snow” event can cause rapid melting of the new snow, leading to flooding downstream.

Interestingly, the largest winter floods in our area and on the Truckee River in Reno have been during La Niña or ENSO-neutral years (when neither El Niño nor La Niña is present). The largest flood ever recorded in Reno was on Dec. 23, 1955, during a strong La Niña event. The second largest flood, caused by intense Sierra rains for several days in mid-November 1950, was during a moderate La Niña pattern. Our most recent large flood, which affected both the Truckee and Carson Rivers on Jan. 1, 1997, was a rain-on-snow event during an ENSO-neutral year.

Though weather forecasters have the benefit of continual research into the climate trends that produce our weather, they still cannot predict the timing or nature of the storms we will receive this winter. Many skiers and snowboarders are hoping that this year’s mild El Niño will lead to an ample snowpack building up about now, and being replenished every week or two until April. We’ll have to wait and see.

– Contact Heather Segale, University of Nevada Cooperative Extension, at (775) 832-4138.

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