Overview

April was a “mixed bag” month. The first half was generally cool and rather wet, but the last two weeks were considerably warmer and drier. About 60 percent of Oregon stations had below-average precipitation for the month, and about 65% ended up with above-normal mean monthly temperatures.

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 snow water equivalent, from NRCS SNOTEL sites
(5) Percent of normal April 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 forecast for May-July (see maps below) suggests a higher-than average chance of above-normal temperatures. Precipitation odds slightly favor drier than normal conditions in eastern Oregon. Oregon Climate Service continues to predict a warmer and drier than average period during those months.

ENSO Update
Summary: Pacific Neutral as La Niña signals weaken
Australia Bureau of Meteorology, May 3, 2006

After briefly approaching La Niña-like conditions in the first quarter of 2006, the Pacific Ocean has warmed steadily throughout April, resulting in surface and sub-surface temperatures close to average. The large body of cooler than normal water below the surface of the eastern Pacific, which was associated with the cool ocean surface temperatures earlier in the year, has now dissipated.

The atmosphere has responded slowly to these ocean changes. The 30-day SOI peaked at +20 on April 14, easing to +14 on May 1. These high values have been sustained by low pressures over Darwin, in part associated with Tropical Cyclone Monica. Cloudiness remains suppressed near the dateline, though eastern Pacific cloudiness has shifted towards normal. Trade winds in the western Pacific remain slightly enhanced. As large-scale coupling between the Pacific atmosphere and ocean is generally weak in the southern autumn, such lingering La Niña-like patterns in the atmosphere are to be expected.

Predictions of Pacific Ocean temperatures from Australian and international computer models suggest a continued warming over the coming seasons, with neutral conditions in the southern winter and spring. It should be noted that March to June is the period when the ability to predict future ENSO conditions is at its lowest.

Below is an abstract published in 2002 which describes an important new index for evaluating ocean and atmosphere conditions in the North Pacific.

The Northern Oscillation Index (NOI): A new climate index for the northeast Pacific
By F.B. Schwing, T. Murphree, P.M. Green
Progress in Oceanography 53: 115-139

We introduce the Northern Oscillation Index (NOI), a new index of climate variability based on the difference in sea level pressure (SLP) anomalies at the North Pacific High (NPH) in the northeast Pacific (NEP) and near Darwin, Australia, in a climatologically low SLP region. These two locations are centers of action for the north Pacific Hadley–Walker atmospheric circulation. SLPs at these sites have a strong negative correlation that reflects their roles in this circulation. Global atmospheric circulation anomaly patterns indicate that the NEP is linked to the western tropical Pacific and southeast Asia via atmospheric wave trains associated with fluctuations in this circulation. Thus the NOI represents a wide range of tropical and extratropical climate events impacting the north Pacific on intraseasonal, interannual, and decadal scales. The NOI is roughly the north Pacific equivalent of the Southern Oscillation Index (SOI), but extends between the tropics and extratropics. Because the NOI is partially based in the NEP, it provides a more direct indication of the mechanisms by which global-scale climate events affect the north Pacific and North America. The NOI is dominated by interannual variations associated with El Niño and La Niña (EN/LN) events. Large positive (negative) index values are usually associated with LN (EN) and negative (positive) upper ocean temperature anomalies in the NEP, particularly along the North American west coast.

The NOI and SOI are highly correlated, but are clearly different in several respects. EN/LN variations tend to be represented by larger swings in the NOI. Forty percent of the interannual moderate and strong interannual NOI events are seen by the SOI as events that are either weak or opposite in sign. The NOI appears to be a better index of environmental variability in the NEP than the SOI, and NPH SLP alone, suggesting the NOI is more effective at incorporating the influences of regional and remotely teleconnected climate processes. The NOI contains alternating decadal-scale periods dominated by positive and negative values, suggesting substantial climate shifts on a roughly 14-year ‘cycle’.

The NOI was predominantly positive prior to 1965, during 1970–1976 and 1984–1991, and since 1998. Negative values predominated in 1965–1970, 1977–1983, and 1991–1998. In the NEP, interannual and decadal-scale negative NOI periods (e.g. EN events) are generally associated with weaker trade winds, weaker coastal upwelling-favorable winds, warmer upper ocean temperatures, lower Pacific Northwest salmon catch, higher Alaska salmon catch, and generally decreased macrozooplankton biomass off southern California. The opposite physical and biological patterns generally occur when the index is positive. Simultaneous correlations of the NOI with north Pacific upper ocean temperature anomalies are greatest during the boreal winter and spring. Lagged correlations of the winter and spring NOI with subsequent upper ocean temperatures are high for several seasons. The relationships between the NOI and atmospheric and physical and biological oceanic anomalies in the NEP indicate this index is a useful diagnostic of climate change in the NEP, and suggest mechanisms linking variations in the physical environment to marine resources on interannual to decadal climate scales. The NOI time series is available online at: http://www.pfeg.noaa.gov.

Manuscript available at http://www.pfeg.noaa.gov/research/publications/publications.html

Lightning Terminology

Lightning season is upon us. Below are definitions of lightning terms from the American Meteorological Society’s Glossary of Meteorology, Second Edition, 2000.

Lightning
Lightning is a transient, high-current electric discharge whose path length is measured in kilometers. The most common sources of lightning is the electric charge separated in ordinary thunderstorm clouds (cumulonimbus). Well over half of all lightning discharges occur within the thunderstorm cloud and are called intracloud discharges. The usual cloud-to-ground lightning (sometimes called streaked or forked lightning) has been studied more extensively than other lightning forms because of its practical interest (i.e., as the cause of injuries and death, disturbances in power and communicating systems, and the ignition of forest fires) and because lightning channels below cloud level are more easily photographed and studied with optical instruments. Cloud-to-cloud and cloud-to-air discharges are less common than intracloud or cloud-to-ground lightning. All discharges other than cloud-to-ground are often lumped together and called cloud discharges. Lightning is a self-propagating and electrodeless atmospheric discharge that transfers through the induction process the electrical energy of an electrified cloud into electrical charges and current in its ionized and thus conducting channel. Positive and negative leaders are essential components of the lightning. Only when a leader reaches the ground, the ground potential wave (return stroke) affects the lightning process. Natural lightning starts as a bi-directional leader although at different stages of the process uni-directional leader development can occur. Artificially triggered lightning starts on a tall structure or from a rocket with a trailing wire. Most of the lightning energy goes into heat, with smaller amounts transformed into sonic energy (thunder), radiation, and light. (See also cloud-to-ground, intracloud, and air discharges) Lightning, in its various forms, is known by many names such as the common streak lightning, forked lightning, sheet lightning, heat lightning, and the less common air discharge; also, the rare and mysterious ball lightning and rocket lightning . (For some detailed explanation of lightning processes, see lightning discharge and related terms.) An important effect of world-wide lightning activity is the net transfer of negative charge from the atmosphere to the earth. This fact is of great importance in one problem of atmospheric electricity, the question of the source of the supply current . Existing evidence suggests that lightning discharges occurring sporadically at all times in various parts of the earth, perhaps 100 per second, may be the principal source of negative charge that maintains the earth-ionosphere potential difference of several hundred thousand volts in spite of the steady transfer of charge produced by the air-earth current. However, there also is evidence that point discharge currents may contribute to this more significantly than lightning.

Lightning Discharge
The series of electrical processes taking place within one second by which charge is transferred along a discharge channel between electric charge centers of opposite sign within a thundercloud (intracloud discharge) between a cloud charge center and the earth's surface (cloud-to-ground discharge or ground-to-cloud discharge), within two different clouds (intercloud or cloud-to-cloud discharge), or between a cloud charge and the air (air discharge). It is a very large-scale form of the common spark discharge. A single lightning discharge is called a lightning flash.

Lightning Flash
The total observed lightning discharge generally has a duration less than one second. A single flash is usually composed of many distinct luminous events that often occur in such rapid succession that the human eye cannot resolve them.