Overview

August started warm and dry, like July, but things really changed by the end of the month. Several unseasonably cool storms (called “early fall storms” by local forecasters) brought significant amounts of rain to most of the state. Many sites had more than a month’s worth of rain in a few days. The vast majority of Oregon stations had above-normal precipitation for the month, and some were more than three times the August normals. Temperatures, by and large, were above normal, reflecting the warmer temperatures which characterized the first three weeks of August.

Table 1 is a summary of monthly averages and totals at selected stations throughout the state. Table 2 lists daily temperatures and precipitation for most of the locations listed in Table 1. In Table 3, monthly and seasonal precipitation totals throughout the state are listed. Figure 1 shows the percentage of seasonal precipitation statewide.

Basin Summary

Here is a summary of precipitation, water supply, and snow pack as of the end of the month, by river basin:

	Precipitation			Stream Flow		SWSI
BASIN	(1)	(2)	(3)	(4)	(5)	(6)
OWYHEE	221	83	105	66	63	-1.4
MALHEUR	161	104	104	145	41	-0.6
GRAND RONDE, POWDER, BURNT	155	114	103	80	84	-1.2
UMATILLA, WALLA WALLA, WILLOW	274	112	109	108	113	2.1
UPPER JOHN DAY	189	115	103	76	90	0.6
UPPER DESCHUTES, CROOKED	195	111	96	65	57	-0.1
LOWER DESCHUTES, HOOD RIVER	321	104	95	118	76	-0.2
WILLAMETTE	281	95	96	99	85	-0.2
ROGUE, UMPQUA	132	97	94	94	83	0.0
KLAMATH	187	98	91	79	64	-2.6
LAKE COUNTY, GOOSE LAKE	159	91	85	61	59	-0.8
HARNEY	169	84	105	80	74	-0.6
NORTH COAST	331	92	n.a	242	82	0.0
SOUTH COAST	175	99	n.a	93	86	-1.5

n.a. Not available
(1) Percent of normal August precipitation, from NOAA Cooperative sites
(2) Percent of normal seasonal precipitation (since Oct. 1), from NOAA Cooperative sites
(3) Percent of normal seasonal precipitation, from Natural Resources Conservation Service (NRCS) SNOTEL sites
(4) Percent of normal snow water equivalent, from NRCS SNOTEL sites
(5) Percent of normal August stream flow, from U.S. Geological Survey (USGS)
(6) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(7) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Forecasts

The Climate Prediction Center’s (CPC) forecasts for October-December appear below. Temperatures for all of Oregon are likely to be above normal. Precipitation probabilities suggest an equal probability of below-, near-, or above-normal precipitation. CPC also says:

“ The outlook for OND (Oct.-Dec.) 2004 is based on indications from warming trends in the Southwestern U.S... with above normal temperatures anticipated along the U.S. West coast and in coastal Alaska primarily due to abnormally warm SSTs in the eastern north Pacific Ocean and Gulf of Alaska. As the seasons progress into winter... the temperature forecasts reflect El Niño composites in combination with trends and indications from other tools... especially the newly operational NCEP CFS model and the IRI multi-model forecasts.”

Oregon Climate Service’s fall-winter forecast follows.

Regarding ENSO, CPC says:

“ Current observations indicate that we are in the early stages of a warm episode (El Niño) over the equatorial Pacific and a majority of sea surface temperature (SST) prediction tools indicate that at least weak El Niño conditions will continue from this fall through at least the first few months of 2005. There is a slight chance that the El Niño could reach moderate intensity or weaken back to neutral status... but the consensus among statistical and dynamical model predictions indicates that three-month averaged SST anomalies in the east central equatorial Pacific averaged over the region bounded by 120W to 170W and within 5 degrees of the equator (Niño 3.4 region) will most likely remain in the range of +0.5 to +1.0 deg C from OND (Oct.-Dec.) 2004 through MAM (Mar.-May) 2005. After MAM 2005 the outlook becomes more uncertain... but most models indicate a weakening and eventual end of the warm epsiode later in 2005.”

Fall & Winter Forecast 2004 – 2005
George H. Taylor and Cadee Hale
August, 2004

Overview of the Upcoming Season
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 and rain storms. The best chance for extreme cold (and snow) should occur in December.

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.”

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.

Analog Years
In developing the forecast, 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:

A breakdown by climate division is as follows:

Temperatures

Precipitation

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 moorages 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!

For the full Fall and Winter forecast go here.

Oregon Climate Service

George H. Taylor, Oregon State Climatologist

Wayne P. Gibson, Programmer/GIS mngr.

Eileen Kasper, Research Assistant/Manager of Data Services

Melanie Mitchell, Undergraduate Assistant

Wolf Read, Undergraduate Assistant

Emily Gibson, Student Assistant

Cadee Hale, Publications Assistant

Kelsey Kuykendall, Undergraduate Assistant

Sara Joos, Student Assistant

Oregon Climate Service, Strand 326,                             	  Phone: (541) 737-5705  
Oregon State University                                                      Fax: (541) 737-5710   
Corvallis, Oregon  97331                                                     E-mail: oregon@coas.orst.edu   
                                                                             Web: http://www.ocs.oregonstate.edu