Fall and Winter Weather Forecast - 2003-2004

George H. Taylor, State Climatologist

Overview of the Upcoming Season

We expect generally above-average temperatures during the first half of the season (Oct-Dec), with normal or somewhat above-average precipitation statewide. The second half of the year (Jan-Mar) should see average or somewhat below-temperatures statewide and average or above-average precipitation (with western Oregon more likely to be above average).

Fall is expected to arrive a bit early. 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 late September or the first half of October.

Last Year

Last year we said "We expect generally average temperatures during the first half of the season (Oct-Dec), with above-normal precipitation west of the Cascades and below-normal amounts east of the Cascades. The second half of the year (Jan-Mar) should see cooler temperatures and the best chance for low-elevation snow. Precipitation statewide is expected to be above average.

"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."

For the most part, that was accurate. The October transition was in terms of temperatures rather than precipitation: we experienced record cold in late October and early November. The precipitation pattern turned out to be the same as predicted, with the second half of winter (through spring) quite wet. We missed on the late-season temperature forecast, though -- January was very mild, and February and March pretty close to normal.

Sea Surface Temperatures

Latest sea surface temperature anomalies from FNMOC show a rather La Niña-like pattern along the Equator, with cooler than average water near the South American coast, and warmer than average temperatures in the western tropical Pacific. The Climate Prediction Center (CPC) analysis for August 20 showed a similar pattern, though the magnitude of the cool anomalies was smaller than FNMOC's.

Typical Oregon winters during weak La Niña winters are slightly wetter than normal, with near-normal temperatures.

NOAA's Climate Prediction Center released the following statement on August 7:

"Current atmospheric and oceanic conditions in the tropical Pacific are near average and do not support the development of either La Niña or El Niño in the next few months. Equatorial sea-surface temperature anomalies greater than +0.5°C persisted in the region west of the date line, while negative anomalies remained in the eastern Pacific, near the South American coast. During July very little net change was observed in the SST anomalies in the Niño regions.

"Since late May positive equatorial upper-ocean temperature departures have spread eastward into the central and eastern Pacific. This evolving subsurface pattern is associated with an eastward propagating oceanic Kelvin wave, resulting from a period of weaker-than-average easterlies in the central equatorial Pacific that occurred during late May and early June. SST anomalies in the Niño 3.4 and Niño 3 regions increased during early-June through early July, but then decreased during the last half of July, as the equatorial easterlies strengthened."

A variety of predictions for the tropical Pacific are available:

The Climate Prediction Center consolidated model shows the current near-neutral conditions continuing through the winter, with a return of El Niño conditions developing by next summer.

The Climate Diagnostics Center (CDC) CCA model shows weak El Niño conditions developing and persisting until next summer. CDC's Linear Inverse Model shows the current weak La Niña conditions strengthening slightly and continuing through the winter.

Forecasts from the Experimental Climate Forecast Division of the International Research Institute for Climate Prediction show mostly neutral conditions for the December-February period.

Dynamic Predictables' SST model suggests development of a weak El Niño in the next 6-12 months.

Others are listed at http://www.pmel.noaa.gov/tao/elnino/forecasts.html.

So many predictions, so many differences. But the models seem to agree that there will be no sweeping changes in Pacific SSTs in the next 12 months -- a generally "neutral," or "La Nada" year lies ahead.

Analog Years

In addition to analyzing El Niño and La Niña conditions from past years, we looked for past years which 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, making a wetter than average year much more likely than a dry one. Probably the best indicator of these cycles is the PDO parameter. Note how closely the PDO cycle mathces wet and dry cycles on the Oregon coast.

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). The closest analog years to 2003 are 1964, 1958, 1954, 1973, 1978, 1995, 1955 and 1959.

3. Other indices. In addition to MEI, OCS examined sea surface temperatures, the Eastern Pacific (EP), North Pacific (NP) 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 1958, 1960, 1962, 1970, and 1995.

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 emerging from a very active solar period, which may be at least partly responsible for the rather mild winters in the last several years. A plot of sunspot numbers since 1950 appears here. The closest analog years are 1951, 1961, 1971, 1973, and 1993.

Based on a composite of those analyses, the analog years which most closely resemble 2003 are (in descending order, beginning with the most similar): 1995, 1970, 1988, 1958, 1966, 1959, 1973, 1964, and 1978. Using a map composite tool provided by the NOAA-CIRES Climate Diagnostics Center, Boulder Colorado from their Web site at http://www.cdc.noaa.gov, we created the following temperature and precipitation anomaly maps:

Element	Period	Discussion
Temperature	Oct-Dec	Mild temperatures statewide (and regionally)
Precipitation	Oct-Dec	Somewhat wetter than average in most of Oregon and the Northwest.
Temperature	Jan-Mar	Near normal or slightly below average.
Precipitation	Jan-Mar	Wet in western Oregon, average east of the Cascades.

The analog years were equally weighted for creation of the maps above; in our analysis we weighted individual years based on their similarity with 2003.

Forecast Discussion

All in all we expect generally above-average temperatures during the first half of the season (Oct-Dec), with normal or somewhat above-average precipitation statewide. The second half of the year (Jan-Mar) should see average or somewhat below-temperatures statewide and average or above-average precipitation (with western Oregon more likely to be above average).

My colleague Wolf Read and I have had several discussions about occurrences of extreme historical weather events. We have found that most of the analog years deemed similar to 2003 experienced one or more severe events, mostly wind storms or floods. For example:

It would appear that there is a strong possibility of at least one severe weather event this winter.

Climate Prediction Center's forecasts for October-December for temperature and precipitation indicate equal chances of above-, below- and near-normal temperatures and precipitation. For January-March, the predictions for temperature and precipitation indicate similar conditions.

The latest Farmer's Almanac forecast for winter appears here.

Winter 2003-2004 Forecast

Western Oregon

Period	Precipitation	Temperature
October-December	Generally average or somewhat above-precipitation totals	Warmer than average temperatures
January-March	Above average	Generally near average

Central and Eastern Oregon