[5] Early Forecast Calls For Below-Average Water Year
Government forecasters and scientists from the University of Washington are both pointing to a warmer, probably drier winter this year in the Pacific Northwest.
At a daylong presentation last week in Kelso, Wash., they showed strong evidence for a mild winter ahead with below-average snowpack, after what turned out to be the hottest May-through-July period in the Northwest since 1895. The June-through-August timeframe was the third hottest on record, according to Nate Mantua, climate researcher from the UW's Climate Impacts Group.
They pointed to a mild El Niño brewing in the tropical Pacific, where water temperatures have been above normal for several months.
Steve King from NOAA's Northwest River Forecast Center said that his group's forecasting tools estimate a 96 percent of normal water year at both Grand Coulee and The Dalles.
The weak-to-moderate El Niño that federal forecasters announced last month comes on the heels of a mild cooling event in the tropical Pacific last winter that shifted quickly into a warming trend during spring.
But some experts have questioned the robustness of associating El Niño events with seasonal weather conditions compared to past years, since the official definition of El Niño has changed from the early 2000s. Less warming in the equatorial Pacific will now garner an event a full-blown El Niño designation that it wouldn't have gotten five years ago.
The change has made some scientists question the value of the moniker. "Our core point is that the new NOAA definition has identified as El Niño a number of seasons that have substantially different U.S. seasonal weather impacts from those conventionally identified," said two federal researchers in the peer-reviewed journal Geophysical Research Letters in July 2005 (Larkin et al., 2005).
They said lumping these new periods in with the older, conventional El Niños not only reduces the statistical power of the seasonal associations, but obscures more robust impacts as well. They cautioned that, "it is best not to expect the conventional associations to apply" until all familiar regional associations with El Niño are reexamined using the NOAA definition, "both globally as well as in the U.S."
The new definition says an El Niño occurs when sea surface temperatures rise in the equatorial Pacific by a least 0.5 degrees C from normal for three consecutive months. It has been adopted by the World Meteorological Organization Region IV to replace a fuzzier definition that most researchers had agreed upon in general terms--that 11 of these warming events had occurred since 1950.
NOAA's Climate Prediction Center has modified the new definition to extend the anomalies to five consecutive months before it officially dubs a warming event an El Niño.
The latest El Niño Advisory, updated Oct. 5, said the latest sea-surface temperature anomalies are all above 0.5 degrees C. The advisory also stated that more than two-thirds of the other predictive models used by El Niño researchers are also indicating El Niño conditions through next spring. -B. R.
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