Overview

August was generally dry throughout Oregon, but much milder than July had been. In contrast with record-setting heat during July, August’s temperatures were close to average overall. Minimum temperatures were generally below average statewide. Coastal temperatures were also cooler then normal due to strong sea breeze winds and active ocean upwelling.

Table 1 is a summary of monthly averages and totals at selected stations throughout the state. Table 2 lists daily temperatures and precipitation for most of the locations listed in Table 1. In Table 3, monthly and seasonal precipitation totals throughout the state are listed.

The annual fall-winter forecast is included on pages 3 and 4.

Basin Summary

Here is a summary of water indicators at the end of the month, by river basin:

n.a. Not available
(1) Percent of normal August precipitation, from NOAA Cooperative sites
(2) Percent of normal seasonal precipitation (since Oct. 1), from NOAA Cooperative sites
(3) Percent of normal seasonal precipitation, from Natural Resources Conservation Service (NRCS) SNOTEL sites
(4) Percent of normal August stream flow, from U.S. Geological Survey (USGS)
(5) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(6) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Forecasts

The Climate Prediction Center forecast for September-November (see maps below) suggests near-normal temperatures for most of Oregon. Precipitation odds favor below-normal totals. Oregon Climate Service predicts above-average temperatures, with precipitation above average west of the Cascades and near-average in central and eastern Oregon.

ENSO Outlook
Australian Bureau of Meteorology, September 13, 2006

Summary: Strengthening signs of an El Niño

Since early August, key ENSO indicators have been consistent with the development phase of an El Niño event. These include sustained negative values of the Southern Oscillation Index (SOI), warmer than normal tropical Pacific Ocean temperatures and a marked decrease in the strength of the Trade Winds. During the past fortnight, weekly surface temperatures have risen above El Niño thresholds in the western and eastern Pacific, while reaching a level just marginally below in the central Pacific.

However, to qualify as an El Niño event, tropical Pacific temperatures would need to persist at their present weekly levels for around another four months, accompanied by a continued weakening of the Trade Winds and above average central Pacific cloudiness. Most computer models indicate continued warming in the Pacific, which implies a probable strengthening of the developing El Niño pattern.

The observed below average rainfall during late autumn and winter across Australia's southern half, especially in the southeast and southwest, is also consistent with the early stages of an El Niño event. Furthermore, even if El Niño thresholds are not reached across the range of indicators, a warming Pacific and a low SOI bias the climate towards being drier and warmer than average across eastern and northern Australia for the remainder of 2006. This is reflected in the spring outlooks for rainfall and temperature.

Fall and Winter Forecast for 2006-07
George H. Taylor and Cadee Hale
August 28, 2006

Last Year's Forecast

Last year (September, 2005) we said, “it would appear that the chances for an extreme event are quite high this year compared with an average year. An active year seems likely!”

Also, “It is likely that this year will see relatively mild temperatures, and above-average precipitation, especially in the second half of the year. We expect early fall conditions to remain mild and dry well into October before winter storms begin, probably some time in November.

“Every year, the most common question we hear is, "will we see low elevation snow?" we guess (which is all we can do in good faith—predicting snow is really tough!) that low elevation snow will not occur. If snow DOES occur, it is more likely after January 1st. Sorry, that's as exact as we can be! Watch for extreme events. The Oregon coast will be especially susceptible to wind storms and flooding.”

Sea Surface Temperatures

Sea surface temperature (SST) anomalies reported by FNMOC on August 23, 2006 show slightly positive values in most of the Tropical Pacific east of the Date Line (180 degrees longitude). West of the Date Line temperatures are slightly cooler than average. All in all, conditions are in the “neutral to slight El Niño” category.

ENSO is the single biggest indicator of future weather-climate conditions over the next several seasons. Unfortunately, it is difficult to predict and there are often wide variations in forecasts from different agencies and models. For example:

Climate Prediction Center: continued slightly warmer than average in the east-central Pacific (weak El Niño).
Climate Diagnostics Center (Canonical Correlation Analysis): moderate El Niño event developing.
Climate Diagnostics Center (Linear Inverse Modeling): Neutral conditions throughout the winter.

Meanwhile, the Australian Bureau of Meteorology says:

“The overall ENSO status remains neutral. Generally weak trends have been observed in the main Pacific climate indicators during the past few weeks, and the potential for an El Niño event to develop this year is still relatively low.

“At odds with the mainly neutral setting is the Southern Oscillation Index (SOI), which has dipped below -10 during the past week as a result of well above average air pressure at Darwin, and consistently below normal air pressure at Tahiti. This indicates a general weakness in the Pacific Walker Circulation. However, the Trade Winds have increased to somewhat above average strength during the past week after being suppressed for much of July. There was little response to the weakened July Trades in ocean temperatures, and these remain somewhat above average, both on and below the surface. Therefore there is only a slight risk that the Pacific will warm to levels high enough for an El Niño event to develop.”

Most of the ENSO prediction models surveyed at the end of July showed a continuation of neutral conditions, although on the warm side of normal. Two of the twelve models suggested the Pacific may warm to El Niño levels by the end of the southern spring. However, a sustained warming beginning in August or September would be unusual, given that ENSO events typically begin to evolve between March and June. Our forecast favors a continuation of the current neutral-to-slightly-El Niño conditions. If the Pacific reverts back to weak La Niña conditions, a much wetter winter is likely – similar to last year. We’ll see!

Zone Forecasts

Temperatures

Precipitation

Forecast Discussion

It is likely that this year will see warmer than average temperatures, and average to somewhat above-average precipitation.
Every year, the most common question we hear is, "will we see low elevation snow?" Our guess (which is all we can do in good faith -- predicting snow is really tough!) is that “widespread” low elevation snow—over all the Willamette Valley and much if the rest of Oregon—will not occur. However, we believe that at least one large snow event (4 inches or more) is likely for the Portland area (especially in those areas close to the Gorge).

Further, most of our biggest snow events have occurred during ENSO-neutral years, such as this one. It could be a really big snow year for Portland.

Mountain snows during analog years were a little deeper than average, especially in southern Oregon. For skiers, this winter may not be as good as last year’s (few are!), but it should be decent at least.
For more details see http://www.ocs.oregonstate.edu/Winter_06-07/forecast.html