Overview

May was a relatively warm month, and generally drier than average. There were several very warm periods, notable the 30th-31st, when several sites reported 90+ temperatures (Drain was 92 on the 30th, Ashland and Forest Grove 91 on the 31st, and Drain and Aurora 90 on the 30th; Medford was over 90 for three days – the 29th through the 31st).

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 May 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 May 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 June-August appear below. There is a higher likelihood of warmer than average temperatures and below-average precipitation in Oregon.

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

ENSO Update
Australia Bureau of Meteorology (BOM), April 9, 2007

Summary: Pacific Basin remains primed for La Niña
Although computer models show a La Niña event is likely in 2007, there hasn't been any consistent development in that direction across the range of ENSO indicators during May. Current conditions in the equatorial Pacific remain neutral.

The precursors for the development of a La Niña event are still evident. The most important is the presence of substantially 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 eastern Pacific. These surface waters are a little cooler than they were three weeks ago, although during the past fortnight there has been some slight warming along the equator in response to a basin-wide weakening of the Trade Winds. La Niña events are characterized by stronger than average Trade Winds.

There is no apparent trend as yet in the amount of convection (high cloud) near the date-line, and the SOI remains slightly negative. A switch to consistently reduced convection and positive SOI values would be favorable indications for La Niña development.

The fact that all major international coupled models show further cooling of the equatorial Pacific Ocean over the coming months, suggests that there is an elevated chance of a La Niña event occurring in 2007. Conversely, this suggests that the El Niño risk is very low. Historically, La Niña events bring wetter than normal conditions across much of the eastern half of Australia from autumn onwards.

ENSO Summary: Climate Prediction Center (CPC), May 20, 2007
Synopsis: A transition from ENSO-neutral to La Niña conditions is possible within the next 1-3 months.

ENSO-neutral conditions continued in the tropical Pacific during May 2007, with average to below-average sea surface temperatures (SSTs) extending from the date line to the west coast of South America. The latest weekly SST departures are negative in the Niño 1+2 (-1.9°C) and Niño 3 (-0.6°C) regions, and remain near zero in the Niño 3.4 (0.0°C) and Niño 4 (+0.4°C) regions .

The upper-ocean heat content (average temperatures in the upper 300 m of the ocean) remained below average across the central and east-central equatorial Pacific, with temperatures at thermocline depth ranging from 1°-4°C below average. Consistent with the surface and sub-surface ocean temperature patterns, stronger than-average low-level easterly winds continued over the central equatorial Pacific. Also, convection was generally enhanced over the western equatorial Pacific and suppressed east of the date line. Collectively, these atmospheric and oceanic conditions continue to indicate that La Niña conditions could develop over the next 1-3 months.

Nearly all of the model forecasts predict below-average SSTs in the Niño 3.4 region (5°N-5°S, 120-170°W) during the remainder of the year. Most statistical models show ENSO-neutral conditions persisting through August 2007, while most dynamical models indicate La Niña will develop within the next three months. Some forecast models, especially the NCEP Climate Forecast System (CFS), continue to predict a rapid transition to La Niña by 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 few months are a favorable period for the development of La Niña.

Contrails – or Chemtrails?
George H. Taylor, June, 2007

Recently I was a guest on a radio talk show which highlighted weather and climate. The host was bright and clever and I had a good time. There were quite a few callers who wanted to ask questions, and the most common question by far concerned aircraft contrails – or as many listeners declared, “chemtrails.”

It’s clear that there are more contrails nowadays than there used to be. Realizing that there is also a lot more air traffic, I assumed that more aircraft equals more contrails. But I was wrong.

At least, I was wrong in the minds of the “chemtrail” people. Some folks accuse the government/military/aircraft industry of intentionally emitting materials that cause contrails to last longer. Furthermore, these same materials contain chemicals which cause illness in many people, according to chemtrail lore.

H. Appleman wrote the definitive paper on contrails in 1953 – “The formation of exhaust condensation trails by jet aircraft” for the Bulletin of the American meteorological Society. It’s still widely used for forecasting contrails. According to Appleman, “Contrails are clouds that form when a mixture of warm, unsaturated, engine exhaust gases and cold ambient air reaches saturation with respect to water, forming liquid drops, which quickly freeze.” Formation requires a particular combination of air temperature, exhaust temperature, humidity, water content of the exhaust, and particles in the exhaust.

Forecasting of contrails is usually done by the military. The advent of stealth technology has made radar much less effective, but a contrail emanating from a stealth plane is a real giveaway!

Contrails can also affect temperatures in the lower atmosphere. In general, high clouds such as contrails cause a slight warming effect near the surface by trapping a portion of Earth's outbound infrared radiation. Studies in the aircraft-free days following September 11, 2001 indicated a greater daily temperature range than would have been expected had contrails been present.

We can agree that contrails are important. But are they being controlled? Or might there be another explanation for their apparent increase (aside from aircraft increases)?

A caller, Ken, provided some useful information. He had been an aircraft mechanic for many years and told me about some changes in aircraft that can account for “most of the changes in contrails.” The most significant involved a switch to turbofans.

All of the jet-engines that power currently-manufactured commercial jet aircraft are turbofans. They are mainly used commercially because they are highly efficient and relatively quiet in operation. They are cleaner burning, and emit fewer particles. According to Ken, the adopting of turbofans helped increase contrail and make them last longer.

There were also changes in fuel, mostly involving additives that made engines last longer and burn cleaner. They also reduce icing and prevent sparking. But they have caused the exhaust stream to change a bit, promoting longer-lasting contrails.
Which sometimes turn into legitimate, long-lasting clouds.

Jet engines and jet fuel? Or chemicals? Before you decide, perhaps you should read about the “chemtrail conspiracy.” A good place to begin is wikipedia:
http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory