Overview

In Oregon in the winter, wet months are usually warmer than average, while dry months are likely to be cooler than average. January of 2006 was very wet and very mild, but February was mostly dry and mostly cool. A strong outbreak of Arctic air brought very cold temperatures (and some daily temperature records) to the state during the middle of the month. Though mild and somewhat wet conditions occurred both early and late in the month, the dominance of the cold, dry period was adequate to leave the monthly averages of precipitation and temperature below normal throughout the state.

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 February 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 February 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 March-May (see maps below) suggests an equal chance of above-, near-, and below- temperatures and precipitation. Oregon Climate Service continues to predict a warmer and drier than average spring following the end of winter (say, as of the end of March), but that March will be cooler than average with average precipitation.

ENSO Wrap-Up (Australian Bureau of Meteorology, March 8)

Summary: La Niña indicators remain mixed
Key ENSO indicators in the equatorial Pacific region remain mixed, with some suggesting neutral conditions and others typical of a weak La Niña event.

Neutral indicators include Trade Winds near the Date Line; central Pacific Sea Surface Temperatures (SSTs), which remain within the neutral range despite a small cooling in the past fortnight, and the SOI, which is near zero. Indicators which favour La Niña conditions include the equatorial ocean subsurface, which is warmer than normal in the west and cooler than normal in the east, and the areas in the eastern Pacific where this cool subsurface water has penetrated to the surface, resulting in significantly cooler than normal SST values.

While Pacific sea surface and atmospheric conditions have undergone a number of changes since the start of the year, it is the persistence of cool subsurface waters that have maintained the possibility of basin wide La Niña conditions developing. However, most Australian and international computer models indicate an ocean warming, with a neutral ENSO situation prevailing by the Austral winter. It should be noted that March to June is the period when the ability to predict future ENSO conditions is at its lowest. Summary:

* Though sea surface temperatures in the eastern Pacific experienced a slight warming during early February, they have cooled again during the past fortnight.
* Subsurface temperatures in the eastern half of the Pacific show conditions typical of a La Niña.
* The SOI has a current (6th March) 30-day value of zero and a 90-day value of +6.
* The Trade Winds have been generally above average in the region of the dateline since December.
* The pattern of below to much below average cloudiness in the region around the equatorial dateline since November continued until a brief period of above average cloudiness in early February. Conditions have since returned to below average. This situation is typical of a weak La Niña.
* Most computer models predict neutral ENSO conditions in July 2006, with two predicting weak La Niña conditions over the next few months and one predicting a development into warm conditions.

Some Facts About Lightning

Lightning? Well, summer approaches, and lightning season along with it. Although lightning does occur here in winter, it is much more common during the warm season.

Lightning is a form of electricity that results from the buildup of electrostatic charge in clouds. Positive and negative charges within a cloud separate, usually because of vertical motion. Generally, negative charges collect near the bottom of the cloud, while positive charges go to the top. At some point, the negative charge can leap to another cloud or to the ground, producing lightning. The stronger the electric field, the more likely that lightning will be attracted to the ground.

The electrical discharge from a cloud will travel the path of least resistance. A ground strike requires a series of steps to develop. At the bottom of the cloud, where the negatively charged parts are found, a "stepped ladder" (sometimes called “stepped leader”) forms. This is what gives lightning its forked look. As this charge starts to head down towards the ground, it branches out, like a tree branch. As it gets closer to the ground, the negative charge in the ladder begins to attract positive charges from the object. When these negative and positive charges connect, a "return stroke" occurs. The result is a bright, flickering flash of light, which is usually followed by a rumble of thunder. The return stroke, from the ground to the cloud, is the one we usually see and hear.