Overview of the Upcoming Season
All in all we expect generally average fall-winter temperatures, precipitation that is a little above normal, and average mountain snowfall. There's about a 50-50 chance of one or more big snow events in western valleys; historically we receive snow storms about 50% of the time, so we're considering this an "average" year when it comes to snow possibilities at low elevations.
Fall is expected to arrive just about on schedule. In Oregon, October is the biggest transition month of the year, with the month usually beginning mild and dry and ending cool and wet. Often the transition to the wet season is quite abrupt. This year we expect the transition some time in mid- to late-October.
Last year's forecast bombed. It is as simple as that. But we were in good company, because no one foresaw a dry winter, let alone one of the driest of the last century. And we are still puzzled by what happened. There is no good explanation for the extreme dryness that we experienced. We can't blame it on El Nino or La Nina, we can't blame it on other known causative factors -- and if we saw last August's conditions again we would probably predict near-normal precipitation again, and be wrong again.
The only explanation that seems to make sense involves the strong trough of low pressure that persisted over the Midwest for much of last winter, bringing very cold temperatures to the eastern half of the country. Even though the trough was "downstream" of us, it may have played a factor in causing the strong, persistent ridge of high pressure that diverted the storm track from our area and led to the very dry winter.
Drought conditions continue across the Northwest. The August 21 Drought Monitor shows the extreme dryness across the region, the result of last year's dryness. Significant rains in late August helped firefighters, but did little for overall water supply. We expect little general change in drought conditions until fall rains begin.
Latest sea surface temperature anomalies (bottom) show the cool tongue of water extending westward along the Equator from the South American coast, and the warmer than average water in the north Pacific. The ongoing "La Niña" conditions continue, although they are closer to normal than they were several months ago. These conditions are very similar to what occurred a year ago.
The Multi-variate ENSO Index (MEI) shows the 2-year La Niña , as well as current "near normal" conditions.
A variety of predictions for the tropical Pacific are available:
National Centers for Environmental Prediction (NCEP) coupled ocean/atmosphere model shows a weak El Nino developing in the next six months.
Predictions from the Scripps / Max Planck Institute (MPI) hybrid coupled model include a moderate El Nino developing this fall, followed by a return to near-normal conditions in mid-winter.
The Climate Diagnostics Center (CDC) predictive model takes a contrary viewpoint, indicating the development of much stronger La Nina conditions in the next several months, continuing throughout the winter.
Precipitation and Temperature During "Average" ENSO Years
Oregon Climate Service has examined precipitation and temperature during similar years in the past. Years with "average" tropical Pacific conditions (neither El Niño nor La Niña) produce generally average precipitation and temperature conditions throughout the region. On the other hand, many of our biggest floods and deepest snowfalls occur during such years. In much of Oregon, in fact, the snowiest winters occur in "average" years, even though not all average years are snowy ones.
In addition to analyzing El Niño and La Niña conditions from past years, we looked for past years that resembled this year in other ways. Some of the criteria we used are:
1. Multi-decadal phase. OCS has identified periods of 20-25 years with generally warm-dry or cool-wet conditions (see article). 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. Although dry years can occur during wet phases of the cycle, it is highly unusual to receive consecutive very dry years during these phases.
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). These observations have been collected and published in COADS for many years. The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,...,Nov/Dec). A comparison of this year with similar previous years is shown.
3. Other indices. In addition to MEI, OCS examined sea surface temperatures, the West Pacific (WP), North Pacific (NP) and Pacific-North America (PNA) indices and compared this year's observations with those of previous years.
Based on a composite of those analyses, the analog years that most closely resemble 2000 are 1953, 1970, 1975, 1978, 1980, and 1985. The single closest analog year to 2000 is probably 1975. In terms of SOI alone, the closest matches are 1956, 1966, 1975, and 1990. Below is our forecast based on a composite of the identified analog years.
Assuming that the La Niña conditions slowly wane in the next several seasons, as predicted here, it is likely that Oregon will experience near-normal precipitation and temperatures, with both slightly higher than average.
We expect fall to arrive relatively early, with cool temperatures and average precipitation. By mid-October, winter storms will be well established, with very wet conditions on and off for the next 4 months. November and January are expected to be especially wet, with subtropical "Pineapple Express" moisture abundant. Southern portions of Oregon are especially likely to see high precipitation amounts.
Snow pack in the mountains is expected to be generally average this winter. There is about an even chance of significant snowfall in the western valleys, especially in late January and early February.
Climate Prediction Center's forecasts for October-December for temperature and precipitation show normal temperatures and precipitation. For January-March, the predictions for temperature and precipitation indicate above-normal temperatures and normal precipitation.
If CDC is correct and La Nina conditions reintensify, we can expect cooler and wetter weather, with more snow, than described above.
Western Oregon
| Period | Precipitation | Temperature |
| September- mid October | Generally below-average precipitation totals, with only a few really wet days | Warmer than average temperatures |
| mid-October-November | Winter storms begin to arrive. Overall precipitation close to average. | Generally cool days and mild nights. |
| December-January | Generally average precipitation in December, but wetter than average in January. | Seasonable temperatures. The best chance for low elevation snows occurs in late January or early February. |
| February-March | Generally average. | Slightly cooler than average in February, and near-average in March. |
Eastern Oregon
| Period | Precipitation | Temperature |
| September - mid-October | Generally below-average precipitation totals | Warmer than average. |
| Mid-October-November | Average. | Much cooler in the second half of October, with November generally mild. |
| December-January | Generally average precipitation in December, but wetter than average in January. | Generally seasonable in December and early January, becoming cooler than average in late January. |
| February-March | Average in February, wetter than average in March | Generally average temperatures. |
