Overview of the Upcoming Season
All in all we expect generally average fall-winter temperatures, precipitation that is a little above normal, and above-average mountain snowfall. There's about a 50-50 chance of one or more big snow events in western valleys. There is also an increased possibility of flooding in western Oregon.
Fall is expected to arrive a bit early, with cool temperatures and average precipitation. Winter storms will begin in earnest in the first half of October. November and January are expected to be quite wet, with December and February-March near average.
Last winter surprised us in several ways. Although fall was mild and dry, as predicted, winter temperatures were close to average (we thought it would get colder). Precipitation was a little above average, as were mountain snows -- we did okay there. The big snows, even at valley locations, never occurred. But we never REALLY said they would, either! Here's what we said last year: "there is an increased likelihood of significant snowfall in the western valleys, especially in late January and early February." No guarantees! But it's amazing how many complaints we've heard (mostly from frustrated schoolchildren) when the "predicted" snow storm never materialized.
In any given year, there's about a 50% chance of "significant snow" (enough to close the schools) in the Willamette Valley. Last year, I thought the probability was more like 75%, and said so. Unfortunately, statements like "we'll PROBABLY get at least one snow storm" somehow became "the state climatologist said we're going to have a lot of snow this winter!"
The second half of the winter was actually fairly mild (we thought it would be a lot colder). And Central Oregon residents complained long and hard about an increased frequency of fog (especially in December), which we said nothing about last year.
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. The three-season forecast predicts that weak La Niña conditions will continue throughout the winter, until at least next spring.
The Multi-variate ENSO Index (MEI) shows the 2-year La Niña , as well as current "near normal" conditions.
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 thoguh 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 which resembled 1999 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 have reentered a wet phase again. Thus we considered similar years in previous wet phases as being more representative of the coming winter than those that occurred during dry 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 which most closely resemble 2000 are 1963, 1971, 1976, 1985, 1989, 1996, and 1999. The single closest analog year to 2000 is probably 1989. In terms of SOI alone, the closest matches are 1943, 1946, 1963, 1971, 1972, 1976 and 1985. 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 slightly above average this winter. There is about an even chance of significant snowfall in the western valleys, especially between mid-December and late January.
The most likely time for valley fog (Rogue, Umpqua, Willamette, and Deschutes Valleys in particular) will be in December.
Climate Prediction Center's forecasts for temperature and precipitation are shown.
Western Oregon
| Period | Precipitation | Temperature |
| September- mid October | Generally average precipitation totals, with a few really wet days | Cooler than average temperatures |
| mid-October-November | Winter storms begin to arrive. November much wetter than average, with chance of local flooding. | Generally cool days and mild nights. |
| December-January | Generally average or below-average precipitation in December, but wetter than average in January. Best chance for flooding in southern portions. | Generally seasonable in December and early January, becoming cooler than average in late January. |
| Ferbuary-March | Average throughout the period. | Slightly cooler than average in February, and near-average in March. |
Eastern Oregon
| Period | Precipitation | Temperature |
| September - mid-October | Average or drier than average, with several very wet days. | Cooler than average. |
| Mid-October-November | Generally dry first half of October, wet second half. November generally drier than average. | Continued cooler than average in October, with November generally mild. |
| December-January | Generally average precipitation in December, wetter than average in January, especially in southern counties. | Generally seasonable in December and early January, becoming cooler than average in late January. |
| Ferbuary-March | Average in February, wetter than average in March | Generally average temperatures. |
