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home : current news : current news October 23, 2018

10/10/2018 8:11:00 AM
Will winter restore snowpack?
photo by Ron Thorkildson
+ click to enlarge
photo by Ron Thorkildson
By Ron Thorkildson

After a hot, dry, and smoky summer here in Central Oregon, autumn finally arrived at 6:54 p.m. PDT on Saturday, September 22, bringing with it the prospect of cooler temperatures and cleaner air as firefighters continue to battle the numerous wildfires still burning in the western U.S.

While summers here in the Pacific Northwest are normally warm and dry, conditions this year have bordered on the extreme.

For the five-month period May through September, both Sisters and Bend totaled just over a third of their normal precipitation. Neither station recorded any rain in July and September. Redmond received about half of its normal moisture.

And the period May through August here in Sisters was very warm. The maximum daily temperatures in May averaged more than 6 degrees F above normal. Although no rain fell in September, temperatures dipped below normal levels.

The prolonged spell of dry weather was primarily the result of a persistent quasi-stationary bubble of very warm air aloft that circulated over the western states. In the northern hemisphere, the clockwise rotation of an upper level ridge causes air to sink, promoting further warming and drying. Because these conditions were so widespread, nearly every western state, including British Columbia and Alberta in Canada, were forced to deal with multiple wildfires, some of them severe.

In Central Oregon, current water resources are a bit of a mixed bag. According to the U.S. Bureau of Reclamation, Wickiup reservoir near La Pine is nearly empty, standing at just one percent of capacity on October 2.

"We haven't seen this level in Wickiup Reservoir in about 50 years," said Deschutes Basin Watermaster Jeremy Giffin. The Ochoco Reservoir east of Prineville is 13 percent full, while the Prineville Reservoir is fairing somewhat better at 39 percent of capacity. Crane Prairie and Crescent Lake are more than half full. The wide disparity in water levels at these sites has more to do with how the resource is managed and less to do with the amount of moisture each receives throughout the year.

As of October 2, the National Integrated Drought Information System has declared the severity of the drought in Deschutes County to be extreme.

There's no question that our region is suffering from a serious lack of water; the real question now becomes will winter bring with it any significant relief?

In an attempt to find an answer, the first order of business is to take a peek at sea-surface temperatures in the equatorial region of the Pacific Ocean. The tool used most often to determine this is the Oceanic Nino Index (ONI), which considers surface water temperature only. The current value is +0.1, putting the El Niño Southern Oscillation (ENSO) in a neutral phase. At this time, most forecasters think the neutral phase will carry into early winter. Beyond that point, however, prognosticators' opinions begin to diverge. Some believe the neutral phase will continue throughout the winter, while others think the ONI will exceed a value of +0.5, ushering in an El Niño.

The best chance for the Pacific Northwest to experience normal temperatures and precipitation is for the ENSO phase to remain neutral. An El Niño would likely result in warmer temperatures, less rainfall and smaller mountain snowpacks.

Pete Parsons, meteorologist for the Oregon Department of Agriculture in Salem, issued his seasonal forecast for the three-month period, October through December, on September 20. He's calling for relatively placid conditions early in October that will likely turn stormy by the end of the month and continue through December. Though he hasn't made specific forecasts beyond December at this time, he is leaning toward the ENSO phase remaining neutral throughout the winter.

By contrast, the Climate Prediction Center (CPC) favors the development of an El Niño later in the winter (50-75 percent chance). Because of this, their three-month forecast is quite different. Temperatures should average above normal and precipitation below normal. An important factor in the CPC's analysis is the fact that although surface temperatures in the tropical Pacific are near normal, the water temperature at a greater depth is above normal.

It should be pointed out that despite the presence of a moderately strong La Niña last winter, the snow and cold didn't kick in until mid-February. But by that time it was too late to catch up and the springtime snowpack in the Central Oregon Cascades came in at just 65 percent of normal.

Making useful seasonal weather forecasts is difficult at best, and not for the faint of heart. But is global climate change interfering with some of the methods currently being used? While there's not yet a definitive answer to this question, certain high-profile failed forecasts (such as the super El Niño that was supposed to inundate Southern California with rain - but didn't) suggest that maybe this is so. If this is the case, perhaps atmospheric scientists can learn to identify what some of these influences are and adjust for them to improve forecasting skill. Until that happens, seasonal prognosticating may continue to be a game of hit and miss.


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