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Seasonal to Interannual Forecasts

Climate Outlook

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The Climate Impacts Group (CIG) translates global-scale climate forecasts and conditions into regional-scale climate forecasts for Pacific Northwest (PNW) resource managers and the general public. The El Niño/Southern Oscillation (ENSO) is the most important factor for seasonal forecasting, changing the odds for different types of winter and spring weather (e.g. warmer/drier, cooler/wetter) in the PNW. The climate outlook also provides the basis for natural resource forecasts, including the CIG's annual streamflow forecasts.

What's Next for the Pacific Northwest?

 

Winter 2005
Updated January 11, 2005
The climate outlook is reviewed monthly and updated as needed.

Current indicators for Pacific climate:

Sea surface temperatures (SSTs) continue to be substantially warmer than average across most of the western and central tropical Pacific, while SSTs in the eastern tropical Pacific that are typically associated with El Niño events have been only slightly above average for most of the past 6 months. The most recent 5-month running average for the Southern Oscillation Index (-0.6) indicates that a weak El Niño episode has been underway. However, because of the relative strength of warm SSTs in the west, and relative weakness of warm SST anomalies in the east, many climate scientists have questioned whether this warm period deserves the standard "El Niño" label, or whether it should be characterized as something more indicative of the persistent warmth of the central and western equatorial Pacific.

The latest forecasts from leading climate prediction centers indicate a high likelihood for continued above average SSTs in the tropical Pacific; only 1 of 19 models forecast an intensification of El Niño conditions (as indexed by warm SST anomalies in the eastern equatorial Pacific). For more information on the current ENSO state and forecast, see the forecast summaries provided by the NOAA Climate Prediction Center and the International Research Institute for Climate Prediction.

Seasonal to interannual forecasts for the state of the Pacific Decadal Oscillation (PDO) index (based on a pattern of North Pacific SSTs) are an emerging science. A major source of uncertainty in developing PDO forecasts is our lack of understanding of what causes the observed multi-year persistence in the PDO index and, more importantly, what triggers PDO regime shifts. However, a strong tendency for year-to-year persistence of the PDO index along with a well-established statistical relationship with the state of ENSO provides a means for making skillful 1-year projections of the PDO index.

Using that simple statistical method with the observed PDO index values from July 2003-June 2004, combined with a prediction that SSTs in the NINO3.4 region of the tropical Pacific would be in the range of +0.4 °C to +1.2 °C deg C, yields a prediction for a July 2004-June 2005 PDO index value ranging from ~ +0.5 to +1. Observed PDO index values for July-December 2004 are: 0.44, 0.85, 0.75, -0.11, -0.63, and -0.17 (averaging out to ~0.19). The recent trend to negative PDO index values may be explained by the anomalous persistence of a negative phase "Pacific North America" (PNA) circulation pattern over the North Pacific and North America. If the PNA phase continues to stay negative, the PDO index is likely to stay negative. If the PNA phase returns to a mostly positive phase, as observed in past El Niño years, the PDO index could change again to a positive phase (CPC PNA forecast).

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What will it mean for the PNW in coming months?

There remains high confidence that the tropical Pacific Ocean will remain warmer than average for the next few months, and that SST anomalies in the eastern equatorial Pacific will remain relatively small yet positive. The latest U.S. climate forecasts issued by NOAA's Climate Prediction Center call for increased odds for above average temperatures and below average precipitation for most of the Pacific Northwest for the rest of this winter and much of the coming spring (CPC precipitation, temperature outlooks).

Likewise, the latest NCEP forecasts predict lower than average odds for below average temperatures and above average precipitation for the next 2 seasons. An additional factor informing the current one year lead time climate forecast for the U.S. is the clear trend to warmer fall and winter temperatures in the western part of the country (e.g., 8 of the last 10 winters have been at least 1 ° F warmer than "normal" in Washington state).

At this time it seems unlikely that the PDO pattern has played a major role in influencing North Pacific climate in recent months, but instead recent changes in the PDO index reflect a strong SST response to recent atmospheric circulation patterns. Additionally, circulation patterns for the past few months have not resembled those most often observed in "classic" El Niño fall/winter periods. For these reason, there seems to be little reason to expect that the PDO index forecast for 2004-05 will be accurate.

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Summary

Even though the Pacific Northwest has experienced the warmer and drier fall (September-December) conditions typically associated with an El Niño" event, the atmospheric circulation patterns that brought these warmer/drier conditions have been quite unlike those most often observed in "classic" El Niño fall/winter periods. Consequently, the odds for experiencing a "classic" El Niño circulation pattern over the North Pacific and Pacific Northwest appear to be relatively low. However, given the expectations that weak El Niño-like conditions will persist for the next few months in much of the tropical Pacific and the strong climate trends observed in the past decade, the NOAA/NCEP forecast calls for an increased likelihood that the Northwest will experience below average precipitation and above average temperatures for the next few months (January-April).

It is critical to note that these climate forecasts indicate relatively subtle shifts in the odds for warmer/cooler temperatures and more/less precipitation in the PNW rather than a deterministic (or exact) climate prediction for the next 2 seasons. Simply stated, expectations for continued El Niño conditions in the tropics and trends for warmer winter PNW temperatures yield a climate outlook for the PNW that has higher than average odds for a warm and dry winter, and lower than average (but not zero!) odds for a cool and wet winter.

Pacific Northwest Resource Outlooks

Climate Prediction Resources

The links below provide access to the latest information on the current state of global and regional climate, as well as links to global and regional climate predictions.

The Current State of the Tropical Pacific

Predictions of Tropical Pacific Conditions

The Current State of the Globe

Current and Predicted U.S. Conditions

Pacific Northwest Conditions

State Climatologist Offices

Special Areas