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Seasonal to Interannual Forecasts

Climate Outlook

August 2010 - Archive Copy

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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. Another important climate variable for Pacific Northwest climate is the Pacific Decadal Oscillation (PDO). 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?

August 2010
Updated 25 August 2010 (posted 27 August)

The climate outlook is reviewed monthly and updated as needed.

The 5 August NOAA El Niño Southern Oscillation (ENSO) diagnostic discussion states that "La Niña [cold ENSO] conditions" developed in July and "are expected to strengthen and last through the Northern Hemisphere winter 2010-11." The forecasts of the eventual strength of the episode exhibit considerable range, from moderate to strong, and many of the models suggest that the episode will peak during October through January, slightly earlier than the typical ENSO episode. The analysis of historical ENSO variability and the NOAA Climate Prediction Center (CPC) seasonal forecasts, to be described next, both indicate that the PNW will see the influence of the cold ENSO in the coming months. A review of tropical Pacific observations and more recent ENSO forecasts is found below.

The CPC forecast for September-October-November seasonal mean precipitation is for a greater than 33 percent chance of above normal precipitation in northern Idaho, all of Washington, and all but the southeast corner of Oregon; with the chances of the same exceeding 40% to the west of the Cascades in both Washington and Oregon. The remainder of the PNW is forecast to to have an equal chance of below, near, and above normal precipitation. The seasonal mean temperature forecast for the same period is for an equal chance of below, near, and above normal temperatures across the region.

The seasonal forecasts should be interpreted as the tilting of odds towards general categories of conditions, and should not be viewed as a guarantee that the specified conditions will be realized. In general, seasonal precipitation forecasts only have skill during periods of significant ENSO variability like the present (CPC tools discussion).

The ENSO models, described below, are predicting cold ENSO conditions through March-April-May (MAM) of 2011, and the CPC temperature and precipitation forecasts are consistent with ENSO influencing the PNW climate during this period. For comparison with the forecasts, an analysis of typical ENSO-related November through April mean temperatures and precipitation is also linked below. The sign of the historical analyses should be reversed to described cold ENSO conditions.

Forecasts
DJF temperature | precipitation
MAM temperature | precipitation

Historical analysis
NDJFMA temperature | precipitation

Storms during October, November, and December can bring heavy precipitation amounts and cause significant flooding in western Washington and northwest Oregon. The tendency for these storms is enhanced in ENSO neutral and cold years and, for this reason, it makes sense to examine specifically the CPC October-November-December seasonal precipitation forecast. The forecast is for a greater than 33% chance of above normal precipitation throughout the PNW, with the chances of the same exceeding 40% in north and central Idaho, western and northern Oregon, and all of Washington state. The temperature forecast for this season (not shown) is for equal chances of below, near, and above normal temperatures in the region.

For More Information

Recent Pacific Northwest Climate

The Pacific Northwest experienced slightly above normal temperatures during the 30 days ending 22 August, with the exception of the coast, which was slightly cooler than the 1971-2000 mean (WRCC). The temperature departure magnitudes were, at largest, 2-4 °F (1-2 °C). The near- to warmer-than normal temperatures ended a period of generally cooler than normal temperatures that began in March (July Outlook). The 30-day precipitation totals (departures, percent of normals) were below normal to the west of the Cascades and in portions of the Columbia Basin. The northern Washington coast normally receives 5 cm of rain during this period, and this year received about 1 cm (Quillayute, NCEP).

The Northwest Interagency Coordination Center (NWCC) maps the locations of "large wildland fires" (timber > 100 acres, grassland > 300 acres; 100 acres = 0.16 square miles) for Oregon and Washington, and the 24 August analysis (today's analysis) documented one fire in the southern Washington Cascdes, three fires on the eastern flank of the Oregon Cascades, and one in the Klamath Mountains of southwest Oregon (geography). Inciweb documents fires in Idaho, Oregon, and Washington and, on 24 August, Idaho had 5 fires that would be included in the "large wildland fire" category (today's analysis for Oregon, Washington, and Idaho).

The 17 August Drought Monitor identifies drought in south central Oregon and southeast Idaho. Drought conditions have diminished in recent months throughout the PNW (July, June, May).

July coastal sea surface temperatures (SSTs) were cold and colder than normal (SSTs < 58 °F (14.4 °C)) along Washington and from central Oregon to central California (9km resolution, PFEL). The SST departures, shown in the upper panel, indicate departures in excess of -1°C from Vancouver Island to central California (1985-97 mean). The unseasonably cold conditions have been associated with an enhancement and greater northern extent of the climatological subtropical high (June-July average 2010, 1971-2000 climatology, ESRL composites). The associated enhanced coastal northerly winds and cloud cover both contribute to the colder temperatures (James Johnstone, UW). The cooler than normal coastal SSTs are consistent with cooler coastal land temperatures described above.

On the broader scale of the northeast Pacific, SST departures for the 30 days ending 21 August represent an intensification of the cold anomalies within 500 km of the North American coastline that first developed in April (ESRL maproom). Typical cold departures are in excess of -1 °C (1982-96 mean). Warm SST departures of comparable magnitude are observed in the central Gulf of Alaska.

Sources:

  • Western Regional Climate Center (WRCC)
  • National Centers for Environmental Prediction (NCEP)
  • Northwest Interagency Coordination Center (NWCC)
  • Incident Information System (InciWeb)
  • Drought Monitor
  • Pacific Fisheries Environmental Laboratory (PFEL)
  • Earth System Research Laboratory (ESRL composites)
  • James Johnstone (UW)
  • Earth System Research Laboratory (ESRL maproom)

  • Recent and Projected Changes in Key Indicators for Pacific Climate

    El Niño/Southern Oscillation (ENSO). The 5 August NOAA El Niño Southern Oscillation (ENSO) diagnostic discussion summarized that "La Niña [cold ENSO] conditions" developed in July and "are expected to strengthen and last through the Northern Hemisphere winter 2010-11." The average SST anomaly averaged over 5°N-5°S, 170-120°W in the equatorial Pacific, in what is called the "Niño 3.4" region, is employed as a key indicator of tropical Pacific climate in the discussion. The July and May-June-July Nino 3.4 SST departures were -0.94 and -0.52 °C, respectively (1971-2000 mean). The 3-month mean SST departure captures the seasonal variability of the index.

    NOAA employs mechanistic and statistical models to forecast how ENSO will evolve over the next several seasons. Mechanistic models solve equations for ocean and atmosphere motions, and precipitation and radiative processes to forecast the future from present conditions. Statistical models, on the otherhand, are constructed from observations of past climate, and apply regression coefficients to present climate conditions to forecast the future. Agreement of the forecasts from these two types of models increases our confidence in the forecast.

    The Nino 3.4 SST forecasts of twenty-three ENSO models initialized with ocean and atmosphere data through July are summarized by the International Research Institute for Climate and Society. Nineteen of the models forecast mean September-October-November (SON) Nino 3.4 SST anomalies in excess of -1.0 °C, six of the forecasts are for Nino 3.4 < -1.5 °C, and the mean forecast is -1.3 °C. There is considerable spread in the SST predictions for the SON season and throughout the forecast period. The mean forecast Nino 3.4 for December-January-February and March-April-May 2011 are -1.2 and -0.7 °C, respectively.

    Pacific Decadal Oscillation (PDO). The intensification of cold SST anomalies along the west coast of North America, described above, contributed to a drop in the PDO value from -0.22 to -1.05 standard deviations from June to July (1900-90 reference period, PDO values). [For a normally distributed variable, only 32% of the values exceed one standard deviation in magnitude.] These two months of negative values follow positive PDO values that were observed last winter and spring. ENSO variabilty is one of the mechanisms that can produce changes in the PDO, and the present cold ENSO can be expected to contribute to negative PDO values in the coming seasons.

    NOAA employs both statistical and mechanistic models to forecast the PDO and coastal ocean conditions. The statistical linear inverse model forecasts the PDO to be moderate and negative through the end of the forecast period in summer 2011. The NCEP coupled forecast system, a mechanistic ocean-atmosphere model, predicts a continuation of the present pattern of cold SST anomalies along the eastern boundary and warm anomalies in the central north Pacific through December-January-February (DJF), and continued cold SST anomalies along the eastern boundary to the north of 40°N (Cape Mendocino in northern California) through next spring (25 August forecast, more recent forecasts). The model has no skill in the winter and spring forecast for the central and southern California coast.

    For More Information


    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 and North Pacific Conditions

    The Current State of the Globe

    Current and Predicted U.S. Conditions

    Pacific Northwest Conditions

    State Climatologist Offices

    Special Areas