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home : columns : columns May 25, 2016

4/22/2014 2:21:00 PM
Moisture was too little, too late
By Ron Thorkildson

Last fall three meteorologists from Oregon offered their opinions about what the upcoming 2013-14 winter had in store. The event was the Oregon Chapter of the American Meteorological Society's winter weather forecast conference held at the Oregon Museum of Science and Industry in Portland.

The most reliable tool the forecasters employed to arrive at their prognostications was the El Niño/La Niña Southern Oscillation (ENSO) index that is determined by atmospheric and oceanic conditions in the central tropical Pacific Ocean. Cooler-than-normal sea-surface temperatures and strong trade-winds are associated with La Niñas, while warmer ocean temperatures and weak trade-winds, or even westerly surface winds, define an El Niño event. As had been the case for the previous two winters, however, the ENSO index was again neutral, meaning that neither La Niña nor El Niño would be in effect. When faced with a weak ENSO signal, forecasters usually lean toward "normalcy" for lack of anything better to go on.

As a group, the weathermen settled on temperatures averaging about normal, with precipitation ranging from slightly below normal to slightly above normal, for the winter as a whole. One forecaster prophesied "ample snow in the Cascades." Although the winters of 2011-12 and 2012-13 were both devoid of a true cold-snap, opinion ran high that the Pacific Northwest would experience at least one significant arctic-air outbreak this time around. Finally, it was generally agreed that the upcoming winter would feature more variability than the past two or three.

These forecasts are mainly aimed at western sections of Washington and Oregon, but because weather regimes are caused by broad-scale atmospheric circulations, they can generally be applied to the Northwest as a whole.

So, with the benefit of 20-20 hindsight, let's review last winter's weather and compare it to what the atmospheric analysts had predicted.

The winter season got off to a dry start as a ridge aloft dominated the Pacific Northwest in October and November. Salem came in with a precipitation deficit of almost four inches in November alone. But then things got active in a hurry.

On December 1 a fierce windstorm toppled trees throughout Central Oregon, sending one of them crashing into the Ski Inn Restaurant here in Sisters. This was followed four days later by three to four inches of snow ahead of an arctic front that sagged slowly southward from the Canadian Yukon.

This was the first true arctic-air outbreak into the Northwest in three years. The cold air reached its greatest intensity on the morning of December 8, when Sisters recorded a low temperature of minus 28 degrees Fahrenheit. The lowest recorded temperature in Oregon that morning was minus 41 degrees Fahrenheit, measured at the Horse Ridge ODOT weather station near Brothers. Although this was the coldest air Central Oregon residents had experienced in many years, it didn't last very long as the maximum daily temperature rose to above the freezing mark in Sisters on December 12.

The ridge aloft rebuilt strongly into the Northwest and persisted from mid-December through the entire month of January, deflecting Pacific storms far to the north into British Columbia and Alaska. During this time nearly all of Washington and Oregon continued to develop significant precipitation shortfalls. Salem's dearth of moisture totaled just 3.15 inches for December and January, nearly 10 inches short of what is normally expected for those two months.

By late January the atmospheric circulation in the eastern Pacific Ocean showed signs of change as the blocking ridge began to weaken. Riding the newly configured jet stream, a series of Pacific storms finally brought much-needed precipitation to the region in February.

On February 8 much of Sisters Country was dealing with three-foot snow depths. Subsequent storms kept the precious moisture coming throughout February and March, though moderating temperatures turned the precipitation to mostly rain at lower elevations.

In Sisters the precipitation for February totaled 5.62 inches, 4.15 inches above normal. March also received greater-than-normal moisture. But for Oregon, it was too little, too late.

As of April 12 the snow water equivalent in the Cascade mountains was only about 50 percent of normal in Central Oregon. That figure improved to 70 percent in northern Oregon, and generally 90 to 100 percent in Washington.

So, how accurately did the meteorologists foretell of last winter's events? Biggest hit: The strong arctic air outbreak in December. Biggest miss: The long stretches of extremely dry weather. Partial credit is earned for the variability forecast; dry early, active weather in December, dry again through January, wet conditions in February and March.

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