Overview

April had generally below normal precipitation and near-normal temperatures, with more above- than below-normal temperatures. Most of the precipitation fell in the middle of the month, and the warmest weather occurred in the first week and the last week.

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 April 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 April 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’s (CPC) forecasts for May-July appear below. Temperatures and precipitation for Oregon equal chances of above-, near-, and below-normal.

Oregon Climate Service predicts normal temperatures and below-normal precipitation for the next three months.

ENSO Update
Climate Prediction Center (CPC), April 4, 2007

Synopsis: A transition from ENSO-neutral to La Niña conditions is possible within the next 2-3 months.

The pattern of anomalous sea surface temperatures (SSTs) during April 2007 was consistent with ENSO-neutral conditions in the tropical Pacific, with average to slightly below-average SSTs extending from the date line to the west coast of South America . The latest weekly SST departures in the Niño regions are -1.2†C in Niño 1+2, -0.3†C in the Niño 3, zero in Niño 3.4, and +0.1†C in Niño 4.

The upper-ocean heat content (average temperature departures in the upper 300 m of the ocean) remained below average across the central and east-central equatorial Pacific, with temperatures at thermocline depth generally 2-5°C below average. Consistent with the surface and sub-surface ocean temperature patterns, stronger than-average low-level easterly winds persisted over the central equatorial Pacific. Also, convection was enhanced over the western equatorial Pacific and suppressed east of the date line. Collectively, these atmospheric and oceanic conditions continue to indicate the possibility that La Niña conditions will develop over the next 2-3 months.

Most of the statistical and coupled model forecasts, including those from the NCEP Climate Forecast System (CFS), indicate below-average SSTs during the next several months. Some forecast models, especially the CFS, predict a rapid transition to La Niña during May-July 2007. However, for the past few months the CFS forecasts have been predicting a stronger and more rapid cooling than has actually occurred. Historically, the next couple of months are a critical time period for the possible emergence of La Niña.

ENSO Summary: Australia Bureau of Meteorology (BOM), May 9, 2007

Summary: Pacific Basin remains primed for La Niña

Current conditions in the equatorial Pacific remain neutral. However the chance of a La Niña developing during 2007, and in particular over the coming 2 to 3 months, continues to be significantly elevated above the long-term likelihood of around 20%.

Although neutral, current conditions continue to display the precursors required for the development of a La Niña event. These include cooler than normal conditions in the Pacific Ocean subsurface - a situation that has persisted since mid-January - which have led to cooler than average surface waters in the far eastern Pacific. Over the past two weeks, these surface waters have undergone further slow and steady cooling, though they remain largely confined to the eastern edge of the basin: a La Niña event occurs when this cooling is more extensive along the equator. Furthermore, Trade Winds remain enhanced near the dateline, and convection (high cloud) has continued to be reduced in the east and slightly enhanced in the west. The SOI, which dropped below -10 during April, has returned to near-zero. Such variations are not uncommon during the Austral autumn, and are not necessarily an indicator of the longer term climatic situation. The equatorial SOI, which is calculated using observations on the equator, has remained positive since February.

These conditions, combined with the fact that all major international coupled models show further cooling of the equatorial Pacific Ocean over the coming months, suggest that there is an elevated chance of a La Niña event occurring during 2007. Conversely, they suggest that the El Niño risk is very low.

Air Pollution Trends in the U.S. and Oregon
George H. Taylor, May, 2007

A couple weeks ago there was an editorial in the local Sunday paper that included a mention about “our deteriorating environment.” It got my attention, but it’s not the first time I have heard that. Sometimes when I give talks to school kids I ask them if they think the air has gotten cleaner, dirtier, or stayed the same over the last 30 years. Most of them say “dirtier.”

Before moving to Oregon in 1989, I lived in Southern California. I worked mostly in air quality studies, so I know a thing or two about air pollution. Most of my work involved working on “environmental impact studies,” required by the state and the U.S. Whenever a new factory, power plant, highway, or other facility is planned it must first undergo an impact study to estimate how it will impact the air, the water, the biosphere, and so on.

In the 50s, 60s and 70s, Southern California air quality was really bad. I lived about 100 miles from L.A., and when I’d go there my eyes would burn and I’d have trouble breathing. I always felt kind of sick when I was there.

In 1970, Congress passed the “Clean Air Act Amendments. “ This mandated “Ambient Air Quality Standards” for various pollutants known to affect human health or damage property.

Among the pollutants for which standards were established were ozone, sulfur dioxide, and carbon monoxide.

Emission controls on vehicles and stationary sources followed soon after. As time has passed, the standards have become much tighter, so that today’s vehicles emit far less than they did before 1970. One prominent manufacturer claims that the exhaust from its car is cleaner than the ambient air in L.A.

And that’s saying something, because L.A.’s air is getting much cleaner, despite increasing numbers of people and vehicles. In 1975, for instance, L.A. 1-hour ozone exceeded the standard in 192 days out of the year – more than half the time. But that number began to fall, and continued to do so. By 2005 it was down to only 27 days.

And this trend actually understates the magnitude of improvement. Under EPA rules, a violation at just one (out of dozens) of ozone monitors in the large Los Angeles air basin is enough to qualify as a violation for the whole basin. Actually, there are large areas of the Los Angeles air basin where there have been no violations of the ozone standard for the last several years, which suggests that many residents have had no exposure to high levels of ozone.

Other pollutants have seen similar improvements: carbon monoxide, particulates, nitrogen oxides, sulfur dioxide, and lead have seen big declines in emissions. The L.A. air is WAY better than it used to be! And as older, dirtier cars are taken out of circulation and replaced by newer, cleaner ones, this trend will continue.

But that’s L.A. What about Oregon?

Same story. According to the Oregon Department of Environmental Quality (DEQ), “In 1980, only 30% of Oregonians lived in clean air areas that met national health standards for air pollution. Today, all areas in Oregon meet these standards.” At the same time, DEQ is addressing another group of pollutants, air toxics, that has emerged as a serious concern. Not much is known about these, so ambient air standards don’t exist yet, but now that Oregon is fully in attainment of national air quality standards DEQ can concentrate on the other compounds. Doubtless they will.