Report hints at impact of El Nino on snowfall for ski season

El Nino is gathering strength in the equatorial Pacific.

Will skiing and riding for winter 2009-10 be good? An updated seasonal weather outlook is released by the Climate Prediction Center (CPC) every third Thursday of the month.

 The CPC is part of the National Weather Service, the forecast arm of the National Oceanic and Atmospheric Administration. A month ago the CPC’s Seasonal Outlook Discussion reported that temperatures in the eastern and central equatorial Pacific Ocean were well above normal. An El Nino was officially declared in effect.

The seasonal outlook released last Thursday, August 20 declared that confidence in the impact of a predicted El Nino event has increased since then.

John Livingston, Meteorologist in Charge at the National Weather Service in Spokane, said there is no clear signal that ties El Nino with an increase or decrease in precipitation. But there will be a shift toward slightly warmer temperatures and higher snow levels.

The Seasonal Outlook Discussion gives you a summary of current oceanic and atmospheric conditions, weather predictions from three to six months out, and an inventory of the tools used to make those predictions.

The report is written by meteorologists for meteorologists. It’s rather long. The small text, all capitals, is a challenging read. The following summary makes it easy to understand:

Temperature and precipitation forecasts through May 2010 give significant weight to the impacts of El Nino. Shorter-term temps for September through November are likely to be above normal. Accumulated precipitation (snow?) is most likely during this time for parts of the Pacific Northwest and northern Rocky Mountains.

El Nino’s influence on the climate over North America is expected to increase from having little or no impact through November to weak/moderate during winter, though a stronger event is possible.

El Nino and his sister La Nina comprise ENSO, the El Nino Southern Oscillation. Another factor in the seasonal outlook is the Pacific Decadal Oscillation (PDO). The PDO is an ENSO-like pattern of climate variability on a much longer time scale. The developing El Nino may enhance a warming trend in this 10-year cycle.

Historically, above normal temperatures in the tropical Pacific precede the onset and development of El Nino. Right now the tropical Pacific is unusually warm at the surface and below—exceeding .9 degrees Fahrenheit above mean across most of the equatorial ocean and from 1.8 to 2.7 degrees in the eastern half of the basin.

Beneath the waves 100 meters and below, temperatures exceed 3.6 degrees Fahrenheit above mean west of about 160 degrees longitude, and just below the surface in the eastern Pacific.

Above the waves, atmospheric convection is suppressed over the Maritime Continent and enhanced across the western Pacific to the International Date Line, as well as north of the equator to the east.

This is important because the Maritime Continent is what meteorologists call the region of Southeast Asia where there are many islands, peninsulas and shallow seas. The region is significant in meteorology because it is considered the most important energy source in the entire global circulation system.

The name combines the terms “maritime” and “continent” normally used as opposites in the description of climate. Maritime air is humid, and continental air is dry.

In the Southeast Asia region, land masses and bodies of water are about evenly distributed. The land masses have high mountains, and the seas are among the warmest on earth. This produces a widespread area of consistent thunderstorms pumping huge quantities of moisture and heat high up into the atmosphere that stoke the upper level winds circling the planet.

What all this means is that when convection is suppressed over the Maritime Continent and enhanced across the western pacific, westerly wind bursts often occur over the western equatorial Pacific. This is part of the developing El Nino signal that kicked in at the end of July.

El Nino is expected to strengthen and persist into the winter and spring 2010. However, uncertainty in predicting the future strength and duration of El Nino increases the further ahead the forecast.

Temperatures will definitely be warmer. But Livingston said there is no clear signal that ties El Nino with an increase or decrease in precipitation. Higher snow levels will translate to rain in the city, while the ski areas, although they may endure more rain events than in the past few years, will still have their powder days.