Overview of the Upcoming Season
George H. Taylor, State Climatologist
Cadee Hale, Publications
Oregon Climate Service
Oregon State University

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.”

All in all, it was probably our most successful forecast ever.

This Year's Forecast

Below are the factors we used to generate this year's forecast, along with the bottom line for this fall and winter:

• warmer than average temperatures,
• average to somewhat above-average precipitation
• good chance of one or more significant snow events in the Portland area (less likely elsewhere in western Oregon)
• average snowpack in the mountains (possibly higher than average in southern Oregon)

We have also included forecasts from a government agency, the Old Farmer's Almanac, and two friends, Steve and Teague.

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 Niño 3.4 (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.
Dynamic Predictables: weak El Nino returning to neutral in 2007.

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!

Analog Years

We identified El Niño and La Niña conditions from past years, as well as other physical parameters for which long-term data exist. Using the months January - July, we compared past years with 2006 and identified the closest matches (analogs). These are listed below. The main criteria we used are:

  1. Multi-decadal phase. OCS has identified periods of 20-25 years with generally warm-dry or cool-wet conditions. In each phase, about 75% of all years have been above (wet phase) or below (dry phase) average. The last wet phase was from the late 1940's until the mid-1970's. We believe that we reentered a wet phase in the mid-1990's, making a wetter than average year much more likely than a dry one. Probably the best indicator of these cycles is the Pacific Decadal Oscillation (PDO) parameter. The closest analog years to 2006 are 1935, 1939, 1942, 1960 and 1977.

  2. The Multivariate ENSO Index (MEI) is based on six main observed variables over the tropical Pacific. These six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C). The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,...,Nov/Dec). The closest analog years to 2006 are 1951, 1963, 1986 and 1996.

  3. Other indices. In addition to MEI, OCS examined sea surface temperatures, and the Eastern Pacific (EP), and Pacific-North America (PNA) indices and compared this year's observations with those of previous years. The closest analog years using these indices are 1960, 1976, 1986, 1991, 1996 and 2003.

  4. Solar cycle. Solar radiation changes are known to have effects on climate, although there is still debate within the climate community regarding the degree and character of those effects. Currently we are approaching a solar minimum period. The closest analog years are 1945, 1953, 1966, 1974, 1987, and 1997.

  5. Hurricanes. Years ago we noticed a strong correlation between the number of Atlantic hurricanes and Pacific Northwest climate the following winter. A graph of hurricane days vs. Portland winter precipitation appears here. Last year's very busy hurricane season followed by an extreme, active winter in the Northwest is consistent with that pattern. The 2006 hurricane season (so far) has been unremarkable (on the quiet side), suggesting a much less active winter.

  6. Regional climate patterns. Past climate trends (for the calendar year so far) were studied to try to find analogs. The best match for 2006 is 1996, but only if the Pacific moves back to neutral or weak La Nina. If current weak El Nino conditions continue or strengthen (our guess), then the best matches are 1963, 1986, and 2003.

Based on a composite of those analyses, the analog years which most closely resemble 2006 (considering the months Jan - July) are 1951, 1960, 1986, 1996 and 2003. Using a map composite tool provided by the NOAA-CIRES Climate Diagnostics Center, Boulder Colorado from their website at http://www.cdc.noaa.gov, we created the following temperature and precipitation anomaly maps:

Extreme Weather Events in Analog Years

1951-52: major wind storm in November; two major wind storms in December; tornado in December near Eugene; large, widespread snowstorm in March
1960-61: statewide flooding in February
1986-87: record-setting rainfall event (coast) in November; strong coastal wind storm in January; flooding in northwest Oregon in February
1996-97: extreme floods in November and late December-early January; ice storm in Portland area in December
2003-04: record cold in November; Arctic outbreak with freezing rain and snow in January

Zone Forecasts

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.

Other Forecasts

• Climate Prediction Center

CPC's forecasts include monthly and seasonal (3 months) predictions for precipitation and temperature anomalies. For the upcoming winter they are as follows:

• Old Farmer's Almanac Forecast

General Weather Forecast and Report: Our study of solar activity suggests that a weak El Nino will develop this winter, resulting in below-normal temperatures, on average, in much of the country. Snowfall will be above normal in most areas, especially in the Upper Midwest and Heartland. But, with the Atlantic warmer than normal (and getting warmer still) and the Pacific also relatively warm, most regions will have at least one mild month. (If the El Nino fails to develop as expected, the very cold periods will be brief and most of the country will experience a mild winter overall—perhaps even a very mild winter.)

Spring will come relatively early in most regions.

Pacific Northwest Summary: Winter will be a bit colder than normal, on average, with above-normal snowfall. Precipitation will be below normal in Washington and above normal in Oregon and California. The heaviest snow will fall in mid- to late February, with other snowfalls in mid-November, early to mid-December, and early to mid-January. The stormiest periods will be in early and mid-November, mid-December, and mid-March. The coldest temperatures will be in early December, mid-January, and mid-February.

• Steve Pierce's Forecast

• Teague Rakoz's Forecast