Overview

The second half of July, 2005 was sunny, warm and dry in most of Oregon. That trend continued throughout most of August, except along the coast. Coastal temperatures were below normal, while inland temperatures were above normal. However, the entire state was drier than normal.

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.

Basin Summary

Here is a summary of water indicators at the end of the month, by river basin:

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 August stream flow, from U.S. Geological Survey (USGS)
(5) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(6) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Fall & Winter Forecast 2005-2006
Overview of the Upcoming Season
George H. Taylor, State Climatologist
Cadee Hale, Publication Manager
August 31, 2005

Last Year's Forecast

Last year we said, "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."

We didn't anticipate the very dry mid-winter (Nov-Feb). Fall was quite wet, however, and spring was even wetter. But we sure didn't expect the dry winter.

On the other hand, in February we predicted a wet spring, at the same time NOAA was predicting a dry one, so we count that a success.

This Year's Forecast

Below are the factors we used to generate this year's forecast, along with the bottom line for this fall and winter: mild and wet.

Sea Surface Temperatures

Sea surface temperature (SST) anomalies reported by FNMOC on August 29, 2005 show slightly negative values in most of the eastern Tropical Pacific. There has been a steady decline in SST anomalies since spring, 2005.

Analog Years

In addition to analyzing El Niño and La Niña conditions from past years, we looked for ways in which years in the past resembled this year. Using the months January - July, we compared past years with 2005 and identified the closest matches. These are listed below by year. 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 Pacific Decadal Oscillation (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). The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,...,Nov/Dec). The closest analog years to 2005 are 1953, 1959, 1978 and 1990.

3. Other indices. In addition to MEI, OCS examined sea surface temperatures, and the Eastern Pacific (EP), 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 1952, 1957, 1969, 1973 and 1993.

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 approaching a solar minimum period. The closest analog years are 1952, 1974, 1987, and 1994.

5. Hurricanes. Years ago we noticed a strong correlation between the number of Atlantic hurricanes and Pacific Northwest climate the following winter. The best match for this year's very busy hurricane season is 1995. A graph of hurricane days vs.Portland winter precipitation appears here. The best match for this year's very busy hurricane season is 1995.

6. Regional climate patterns. Past climate trends (for the calendar year so far) were studied to try to find analogs. The best matches were 1957, 1962, 1988 and 1995.

Based on a composite of those analyses, the analog years which most closely resemble 2005 (considering the months Jan - Aug) are (in descending order of similarity, beginning with the most similar): 1995, 1969, 1952, 1954, 1957 and 1973. 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 temperature and precipitation anomaly maps that indicated:

Temperature

Oct-Dec: above normal west of the Rockies, below normal in the east.
Jan-Mar: mostly above normal in the western U.S. except for extreme northern states

Precipitation

Oct-Dec: near to slightly above normal in the Northwest, below normal in the southwestern U.S.
Jan-Mar: above normal along the west coast and the northern intermountain states; dry in the center of the country

A breakdown for Oregon by climate division is as follows:

Temperature:

Precipitation:

Extreme Weather Events in Analog Years

Extreme events were fairly common during the analog years identified above (1995, 1969, 1954, 1957, 1973); these included wind storms, floods, and ice storms. For example, major events included:

Wind storms: 1952-53, 1995-96

Floods: 1952-53, 1973-74, 1995-96

Ice storms: 1969-70, 1995-96

1995, the best overall match with this year (2005), had an abundance of extreme events:

Floods in November and February
Ice storms in December and February
Extreme wind storm (biggest in at least 15 years) in December

With that in mind, it would appear that the chances for an extreme event are quite high this year compared with an average year. An active year seems likely!

Forecast Discussion

It is likely that this year will see relatively mild temperatures, and above-average precipitation, especially in the second half of the year. We expect early fall conditions to remain mild and dry well into October before winter storms begin, probably some time in November.

Every year, the most common question we hear is, "will we see low elevation snow?" we guess (which is all we can do in good faith -- predicting snow is really tough!) is that low elevation snow will not occur. If snow DOES occur, it is more likely after January 1. Sorry, that's as exact as we can be!

Watch for extreme events. The Oregon coast will be especially susceptible to wind storms and flooding.

Oregon Climate Service

George H. Taylor, Oregon State Climatologist

Wayne P. Gibson, Programmer/GIS mngr.

Eileen Kaspar, Research Assistant/Manager of Data Services

Melanie Mitchell, Undergraduate Assistant

Wolf Read, Undergraduate Assistant

Emily Gibson, Student Assistant

Cadee Hale, Publications Assistant

Sara Joos, Student Assistant

Sean Daly, Student Assistant

Kirk Borgerding, Undergraduate Assistant

Kelsey Kuykendall, Undergraduate 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