Overview

Much of July was close to normal, but in the fourth week of the month Oregon experienced a very significant heat wave – record-setting, in many locations. Notable were exceptionally high dewpoint temperatures, leading to uncharacteristically high nighttime temperatures. Many places set all-time “high minimum” temperatures.

July is Oregon’s driest month statewide, but this month was dry even for July.

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 July 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 July 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)

Forecasts

The Climate Prediction Center forecast for August-October (see maps below) suggests a higher-than average chance of above-normal temperatures for the southern half of Oregon. Precipitation odds show equal chances of above-, near-, and below-normal totals. Oregon Climate Service predicts average temperatures and below-average precipitation during those months

ENSO Outlook
Australian Bureau of Meteorology, August 9, 2006

Summary: A persistence of neutral ENSO conditions

The overall ENSO status remains neutral. Generally weak trends have been observed in the main Pacific climate indicators during the past few weeks, and the potential for an El Niño event to develop this year is still relatively low.

At odds with the mainly neutral setting is the Southern Oscillation Index (SOI), which has dipped below -10 during the past week as a result of well above average air pressure at Darwin, and consistently below normal air pressure at Tahiti. This indicates a general weakness in the Pacific Walker Circulation. However, the Trade Winds have increased to somewhat above average strength during the past week after being suppressed for much of July. There was little response to the weakened July Trades in ocean temperatures, and these remain somewhat above average, both on and below the surface. Therefore there is only a slight risk that the Pacific will warm to levels high enough for an El Niño event to develop.

Most of the ENSO prediction models surveyed at the end of July showed a continuation of neutral conditions, although on the warm side of normal. Two of the twelve models suggested the Pacific may warm to El Niño levels by the end of the southern spring. However, a sustained warming beginning in August or September would be unusual, given that ENSO events typically begin to evolve between March and June.

July Heat Wave
George H. Taylor and Cadee Hale
July 24, 2006

A record-setting heat wave affected Oregon and most of the West in late July. A strong ridge of high pressure, accompanied by an influx of warm, moist air from the south, caused temperatures to rise rapidly on the 21st and 22nd and remain high through the 24th.

A number of daily maximum temperature records were set. Most remarkable, however, were the overnight minimum temperatures, some of which were all-time records.

Friday, July 21, was a very warm day, mostly clear in the morning (except in southern Oregon, where clouds had persisted most of the day, keeping temperatures down). In the afternoon, the clouds moved into the Willamette Valley. A little light rain fell.

The clouds persisted overnight. The 7:30 a.m. visible and infrared satellite pictures shows the clouds covering most of Oregon as well as parts of Washington and British Columbia. As a result of the cloud cover, minimum temperatures were very high. Eugene, Medford, Portland, and Salem recorded their highest “high minimum” ever. Many local residents compared July 22 to weather in the eastern U.S.
The table below is a synopsis of observed and record daily maximum and minimum temperatures.

Eugene Airport

Portland Airport

Salem Airport

Medford Airport

Pendleton Airport

Corvallis Hyslop (date shifted due to 8 a.m. observation time)

* daily record high temperature
** daily record high minimum temperature

Several stations set all-time high minimum records for any day of the year:

Other notable records: