August, 2004

 

Overview of the Upcoming Season
We expect generally below-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 above-average temperatures statewide and average or above-average precipitation (with western Oregon more likely to be above average).

There appears to be a good chance of one or more extreme events this winter, particularly wind storms and rain storms.

Last Year
Last year we said "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. It would appear that there is a strong possibility of at least one severe weather event this winter."

For the most part, that was accurate. The October transition was a sudden increase in precipitation. For the second year in a row, we experienced record cold in late October and early November. January and February were quite wet, but March was extremely mild and dry, to everyone's surprise.

We certainly had some extreme events! The most significant was the record-setting ice storm in early January, which was accompanied by the biggest snow storm in many years.

Sea Surface Temperatures
Latest sea surface temperature anomalies from FNMOC show cooler than average water near the South American coast, and warmer than average temperatures in the central tropical Pacific. The Climate Prediction Center (CPC) analysis for August 25 showed a similar pattern, though the magnitude of the cool anomalies was smaller than FNMOC's.

NOAA's Climate Prediction Center released the following statement on August 5:

"The Climate Prediction Center's (CPC) forecasts for August-October appear below. Temperatures for all of Oregon are likely to be above normal. Precipitation probabilities suggest an increased chance of below-normal precipitation in southeast Oregon. CPC also says:

"The SSTs in the central Pacific near the Date Line have drifted upward and are now over 1 degree C above normal along the Equator from 125W to just west of the Date Line. Equatorial SSTs in the eastern Pacific continue to average below normal from the South American coast to about 120 W. Tropical intraseasonal activity appears to be weaker than in early July. With a mixture of warmer and cooler than normal SSTs in the equatorial Pacific and weakening intraseasonal activity... there are no clear signals from the tropical Pacific influencing temperature and precipitation over the CONUS and Alaska."

A variety of predictions for the tropical Pacific are available:
  • The Climate Diagnostics Center (CDC) model shows neutral conditions persisting until next summer.

Most forecasts suggest very little change in the tropical Pacific over the next year or so.

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. 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.
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 2004 are 1994, 1987, 1999 and 1971.
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 1956, 1959, 1962, 1987, 1992 and 1994.
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 1952, 1962, 1987, 1992 and 1994.

Based on a composite of those analyses, the analog years which most closely resemble 2004 are (in descending order, beginning with the most similar): 1994, 1987, 1999, and 1971. 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 normal or below normal west of the Rockies, above normal elsewhere.
precipitation Oct-Dec above normal in the coastal Northwest, below normal in the eastern U.S.
temperature Jan-Mar mostly above normal in the western U.S.
precipitation Jan-Mar
wet along the west coast, normal or below-normal elsewhere.

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.

A breakdown by climate division is as follows:

Temperatures
    
Climate Division October-December January-March
1 - Oregon Coast Slightly below normal Slightly above normal
2 - Willamette Valley Near normal Slightly above normal
3 - Southwest Interior Slightly below normal Above normal
6 - Northeast Oregon Slightly above normal Above normal
7 - Southeast Oregon Near normal Above normal

 

Precipitation
    
Climate Division October-December January-March
1 - Oregon Coast Slightly above normal Slightly above normal
2 - Willamette Valley Slightly above normal Slightly above normal
3 - Southwest Interior Slightly below normal Slightly above normal
6 - Northeast Oregon Slightly above normal Slightly above normal
7 - Southeast Oregon Near normal Near normal

Extreme Weather Events in Analog Years

The highest-rated analog years were characterized by numerous extreme events, including wind storms, high rainfall events, snow storms, and even a few tornadoes. Some examples:

1971—A flood struck northwestern Oregon in January, 1971, leading to two deaths and a number of injuries. The most remarkable event was the tornado which began in Oregon and crossed the Columbia into Washington on April 5, 1972:

The tornado touched down near Portland at the south shore of the Columbia River damaging 4 pleasure boat morages on Marine Drive, 50 cabin cruisers, boathouses and dock shelters. It then crossed the Columbia drawing water up with it. The tornado continued on its 9 mile long path in Washington, near Vancouver where it caused 6 deaths, 300 injuries and 25 million dollars in damage. This was the deadliest tornado in the U.S. in 1972!

1987—While not as consistently "exciting" as the other years, this winter produced several extreme events, including a big wind storm in December with winds of 50-60 mph in the Willamette Valley; three people were killed. An unusual later season snow storm affected most of state in March, accompanied by high winds.

1994—This was truly a wild year for weather. After a peaceful early fall, storms began in late October and continued on and off all winter. In the midst of the first week of storms was the wind/rain storm of October 31, 1994:

The second major Pacific storm in 5 days hit northwestern Oregon brought more extremely heavy precipitation, but somewhat less wind, than the previous storm. Portland airport received 2.44 inches of rain, with 3-4 inches in the Coast Mountains and Cascade foothills. Some minor flooding occurred but no significant damages were reported. Over a foot of new snow fell in most of the North Cascades above pass levels and 6 inches or more were reported in the South Cascades. Wind gusts of 67 mph at Astoria, 70 mph at Tillamook, and 72 mph at Sea Lion Caves were reported along the coast. Sustained winds of 74 mph with gusts to 100 mph were recorded at 7000 feet on Mt. Hood, while at Condon, in the Columbia Basin, winds gusted to 62 mph.

Later, a major snow storm affected western Oregon, all the way to the valley floors, on December 6, 1994:
A wintry storm with cold air from the Gulf of Alaska produced snow down to the valley bottoms in Western Oregon. Dallas, west of Salem in the foothills of the Coast Range, received 5 inches, while most Willamette Valley locations got 2 to 3 inches. Passes in the Coast Range reported 6 inches of new snow, but the Cascades only had 6 to 10 inches.

1999—Wind storms occurred allwinter, beginning in November. The strongest was the one which arrived January 16, 2000:

An intense Pacific low pressure center moving across Northwest Oregon brought very strong winds to the area. Along the coast, Cannon Beach reported a gust to 115 mph, Netarts 85 mph, Newport had sustained 60 to 70 mph winds with gusts to 87 mph, Newport Jetty had sustained winds 56 mph with gusts to 80 mph, and Arch Cape reported gusts to 80 mph. In the Cascades, Timberline Lodge on Mt Hood reported winds to 94 mph and Mt Hood Meadows 86 mph. Portland International Airport reported gusts to 59 mph and Salem reported 60 mph gusts. Reported damages included a house blown off its foundation at Netarts, and widespread downed trees near Florence and Lincoln City. Clatsop County reported roof damages, power outages and brief road closures due to downed trees.

Forecast Discussion

All in all, we expect generally below-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 above-average temperatures statewide and average or above-average precipitation (with western Oregon more likely to be above average). And watch for one or more extreme events this winter, particularly wind storms and rain storms.The best chance for extreme cold (and snow) should occur in December.

Other Forecasts
Wolf Read of Oregon Climate Service provided his own forecast here.

Steve Pierce, a scientist from Vancouver, WA, provided his own forecast here.

The Farmers' Almanac forecast map for the upcoming winter appears here.

The Old Farmer's Almanac
Region 15 Summary
November through March temperatures will be normal, on average , with near or below-normal snowfall. Rainfall will be above normal in the north and below normal in the south. The stormiest periods will be mid-Novermber, mid-December, early and late January, Late February, and mid- to late March. Expect snow in early to mid-January and late February. The coldest temperatures will occur in early November, late December, early January, and mid-February.

April and May will be cool and wet, with rain in early and late April and mid-May.

Summer will be cooler and drier than normal, on average. The hottest periods will be late June, mid-July, and early August.

September will be warmer and drier than normal, with especially nice weather to start the month. October will be warmer than normal, with above-normal rainfall in the north and below-normal in the south (86).

The Harris' Farmers' Almanac
Region 1
November-04. Near normal temperatures and slightly below normal precipitation
December-04. Temperatures slightly below nornal and precipitation slightly above normal
January-05. Below normal temperatues and slightly above normal precipitation.
February-05. Temperatures slightly below normal and precipitation near normal
March-05. Slightly above normal temperatures and slightly below normal precipitation
April-05. Temperatures below normal and precipitation slightly above normal.