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Seasonal to Interannual Forecasts

Climate Outlook

ARCHIVE COPY - MAY 2010

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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?

May 2010
Updated 25 May 2010 (posted May 25)

The climate outlook is reviewed monthly and updated as needed.

The 6 May NOAA El Niño Southern Oscillation (ENSO) diagnostic discussion expects "a transition to ENSO-neutral conditions ... by June 2010, which will continue into the Northern Hemisphere summer 2010." This discussion is updated early in each month. A review of tropical Pacific observations and more recent ENSO forecasts is found below.

The NOAA Climate Prediction Center (CPC) June-July-August temperature forecast is for a greater than 33 percent chance of above normal temperatures in southeast Oregon and southern Idaho, and an equal chance of below, near, and above normal temperatures in the remainder of the Pacific Northwest. The CPC precipitation forecast is for a greater than 33 percent chance of below normal precipitation in Oregon and central Idaho, with the probability of the same exceeding 40 percent in the northern panhandle of Idaho and throughout Washington state. The precipitation forecasts have only marginal skill during ENSO neutral periods like the present (CPC tools discussion).

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.

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Recent Pacific Northwest Climate

The 30 days ending 17 May saw a continuation of the cooler than 1971-2000 normal PNW temperatures that characterized the preceding 30 day average (20 March through 28 April, WRCC; April Climate Outlook). The coldest temperature departures, between 2 and 4 °F (1 and 2 °C), were again observed on the eastern slopes of the Oregon Cascades, southeast Oregon, and the southern half of Idaho. Additional detail is provided in daily records for Burns OR, Boise, and Pocatello (NCEP).

Precipitation summed over the 30-days ending 17 May was near the 1971-2000 normal over the Pacific Northwest (total, departure, percent normal). The Idaho panhandle experienced slightly above normal precipitation (Lewiston) as did southwest Oregon (Medford). The long-lived drought conditions to the east of Cascades remain unchanged (18 May, Drought Monitor).

Late season snow, particularly in the first 2 weeks of May, has significantly increased the snowpack throughout the region (20 April analysis, 25 May analysis, legend, today's analysis, NWRFC). The 25 May snow pack, as measured by snow water equivalent (SWE), is at or above normal in the Oregon Cascades, southern Washington Cascades, and in the Olympic Penninsula. The SWE on the eastern side of the Columbia Basin also increased during this period, but remains significantly below normal. The snowpack can also be viewed as snow water content aggregrated by basin (25 May, WRCC).

Based on late April observations of significantly below normal snowpack and a forecast of warmer, drier May through August conditions, the National Interagency Coordination Center predicted above-normal "significant fire potential" for May through August in a region that includes northeastern Washington, the northern half of Idaho, and southcentral Oregon (analysis, 22 April report, NICC). The authors of this report expressed moderate confidence in this forecast.

The April coastal sea surface temperatures (SSTs) are near to slightly below the 1985-97 normal along the Washington, Oregon, and California coasts, with significant cold SSTs (<-1 °C) at least 150km off of the southern Oregon and northern California coast (PFEL). The NCEP - NCAR reanalysis daily meridional wind for a gridpoint centered along the Oregon coast indicates near-normal monthly-average winds in April (analysis; James Johnstone, UW). On a broader scale, SSTs over the entire northeast Pacific basin were were cooler than the than the 1971-2000 mean, with the largest anomalies, in excess of -0.25 °C, to the north and east of Hawaii (UW).

Sources:

  • Western Regional Climate Center (WRCC)
  • National Centers for Environmental Prediction (NCEP)
  • Drought Monitor
  • Northwest River Forecast Center (NWRFC)
  • Western Regional Climate Center (WRCC)
  • National Interagency Coordination Center (NICC)
  • Pacific Fisheries Environmental Laboratory (PFEL)

  • Recent and Projected Changes in Key Indicators for Pacific Climate

    El Niño/Southern Oscillation (ENSO). The NOAA El Niño/Southern Oscillation (ENSO) Diagnostic Discussion employs the average sea surface temperature (SST) anomaly over 5°N-5°S, 170-120°W in the equatorial Pacific, in what is called the "Niño 3.4" region, as a key indicator of tropical Pacific climate. The Nino 3.4 SST anomaly peaked in December at 1.78°C, and declined in subsequent months: 1.58, 1.26, and 1.12 °C, with an April anomaly of 0.75°C.

    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 ENSO model forecasts are summarized by the International Research Institute for Climate and Society. The February-March-April mean Nino 3.4 SST was 1.03°C, and the models are initialized with ocean and atmosphere data through April. The 23-model average is for a zero May-June-July Nino 3.4 anomaly (ENSO neutral) and there is some divergence of the model forecasts for later in the year. For the August-September-October season, 13 of the models forecast a continuation of ENSO neutral conditions (Nino3.4 magnitude less than 0.5°C); 8 models forecast a moderate cold ENSO (Nino3.4 between -0.5 and -1°C); and 2 of the models predict a stronger cold ENSO. Both dynamical and statistical models exhibit their smallest skill when initialized at this time of year: "forecasts ... are better when made between June and December" (IRI), and a much better assessment of next winter's ENSO condition will be available in the coming months.

    Pacific Decadal Oscillation (PDO). The PDO index has been small and positive beginning with December 2009 and continuing through April (monthly values of 0.08, 0.83, 0.82, 0.44, and 0.78 standard deviations, 1900-90 reference period; PDO values). Positive PDO values are consistent with above normal SST anomalies along the eastern portion of the north Pacific and negative SST anomalies in the central north Pacific. [For a normally distributed variable, only 32% of the values exceed one standard deviation in magnitude.] The existence of warm ENSO in the equatorial Pacific during the last half-year is consistent with the development of a PDO warm phase.

    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 weak and negative by next winter, December-January-February. The NCEP coupled forecast system, a mechanistic ocean-atmosphere model, predicts small, negative July-August-September SST anomalies (magnitudes < 0.5 °C) along the Alaska, British Columbia, Washington, and Oregon coasts; and anomalies between -0.5 and -1°C along the California coast (1981-2006 reference period). This pattern is consistent with a weak, negative PDO later this year (24 May forecast, more recent forecasts).

    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