Forecast calls of 'normal,' active winter

The fine summer-like weather that Central Oregon residents had been enjoying during much of October came to an abrupt end last week when an upper-level low dropped south from Canada, bringing with it chilly temperatures and some low-elevation snow.

The sudden change is a reminder that another winter season has the Pacific Northwest squarely in its sights. But will our region incur the full wrath of Old Man Winter this time around, or will he be gentle? This is precisely the question that a group of atmospheric scientists tried to answer on Saturday, October 26. The event was the Oregon Chapter of the American Meteorological Society's 21st annual winter weather forecast conference held at the Oregon Museum of Science and Industry in Portland.

Speakers included Andy Bryant, hydrologist at the National Weather Service office in Portland; Kyle Dittmer, hydrologist/meteorologist for the Columbia River Inter-Tribal Fish Commission; and Jim Little, meteorologist of NorthWest Weather Consulting.

These long-range weather prognosticators have devised their own methods for selecting and evaluating various pieces of available data in which to develop their forecasts. But all of them start with the El Niño/La Niña Southern Oscillation (ENSO) signal in the tropical Pacific Ocean. This is because it is the single most reliable indicator when it comes to forecasting our winter weather. When an El Niño is in effect winters likely will be warmer and drier than normal, while a La Niña tends to produce cooler and wetter conditions.

Currently the ENSO signal is neutral, meaning that neither is in effect. Sometimes referred to La Nada, this condition often results in "normal" weather over the entire winter season, but also produces more variability.

Last autumn the ENSO signal was also neutral. So does that mean a repeat of last winter's weather? While that outcome is unlikely, a look back to what transpired last winter will provide a basis for comparison.

Although November 2012 came in a touch warmer and drier than normal, the central Cascades did receive a good blanket of snow. But it was December that took winter's best punch. Sisters and Redmond picked up greater than 150 percent more precipitation than normal as storm after storm rolled through the region. It was during this period that the Cascades picked up most of its snowpack for the entire winter. Temperatures were also a bit cooler than normal in December.

By January, however, the storm track began to shift northward as fewer and weaker storms moved through northern Washington and southern British Columbia. Most stations in Oregon recorded far less precipitation than normal. Sisters reported no rain or snow for the entire month.

The inactive weather continued through February. Rain and snow began to return to the region in March, but didn't reach normal levels until April. The mid-elevation central Cascade snowpack was at 83 percent of normal. The 2012-13 winter was free of any cold air outbreaks.

This winter seems to have gotten off to a fast start as two low-pressure systems in October brought snow levels down to nearly pass level, providing the Cascades with an early coating of snow. And computer models currently indicate that the first two weeks in November will be active with occasional storms moving into the Northwest.

Bryant explained that the Climate Prediction Center is going with near-normal temperatures and precipitation levels in November. The same holds true for the three-month period November through January. For the winter as a whole, he anticipates normal temperatures and normal to slightly below normal precipitation. Bryant told the audience that some of the Northwest's greatest snowfalls occur during ENSO-neutral winters.

Dittmer uses the ENSO signal, sea-surface temperatures in the tropical Pacific Ocean, and the Pacific Decadal Oscillation (PDO) to arrive at his forecasts. The PDO is detected as warm or cool surface waters in the Pacific Ocean north of 20 degrees north latitude. These abnormally warm/cool waters shift position across Pacific on a time scale of about 20 to 30 years. He also incorporates sunspot cycles into his forecasts. Dittmer believes the winter will get off to a slow start then become more active as the season progresses. Precipitation levels should come in slightly above normal.

Jim Little foresees temperatures and precipitation averaging fairly close to normal values, though it may be a shade drier in January and February. The best chance for western valley snow appears to be mid-December. Little believes the Cascades will receive ample snowfall.

One interesting climate forecasting chart showed the possibility of significantly colder than normal temperatures developing in western Canada and Alaska during December and January. If this situation does develop, this pool of cold air can impact our weather in a couple of important ways, depending on the configuration of the upper level wind flow.

If low pressure persists in the Gulf of Alaska during this time, the cold air will be drawn into the eastern Pacific, spawning frequent storm systems aimed at the Pacific Northwest. But if a strong, high-amplitude blocking ridge forms in the eastern Pacific, this frigid air might be directed southward through British Columbia and into our region. Over the next two to three months, forecasters will be watching to see if this particular set of meteorological factors shows signs of emerging.

Weather experts foresee a generally normal winter with regard to temperature and precipitation levels, based largely on the neutral state of the ENSO. They also expect more variability than was experienced last winter.