Overview

Following the very wet conditions prevailing in November, a dry first week in December seemed somehow out of place. But that changed quickly. A wet second week, punctuated by a major wind storm, brought a sense of normalcy. The fourth week was also quite wet. By month’s end, the majority of Oregon stations were reporting above-average precipitation. Temperatures were mostly above average, but not by much.

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 December 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 December 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’s (CPC) forecasts for January-March appear below. Temperatures for Oregon (and much of the northern half of the U.S.) are likely to be above normal, while precipitation probabilities show equal chances of above-, near-, and below-normal.

Oregon Climate Service predicts above-normal temperatures and normal precipitation for the next three months ---basically agreeing with CPC.

ENSO Update
Australian Bureau of Meteorology, January 10, 2007
All the main ENSO indicators show that the El Niño event has begun to weaken. This bodes well for a switch towards wetter conditions across Australia sometime in the late summer or autumn. The timing of the weakening also fits in well with that observed during previous events, although it is still possible for there to be renewed strengthening of the El Niño event for a month or two before it finally dissipates.

Since late November, near equatorial sea-surface temperatures have cooled by about 0.1 to 0.4°C, cooler-than-normal sub-surface water has extended well into the eastern equatorial Pacific Ocean, with significant weakening of warm anomalies occurring as a result, and the Trade Winds have been stronger than normal across the western and central Pacific. Furthermore, monthly SOI values were neutral for both November and December and central Pacific cloudiness has been near to or slightly below average for almost a month.

However, a moderate to strong Madden-Julian Oscillation (MJO), with an associated westerly wind burst, is currently affecting the area between northern Australia and Indonesia. If the MJO maintains its intensity as it propagates into the western Pacific, a fall in the SOI, a decrease in the Trade Wind strength and an increase in cloudiness may occur as a result. This may cause temporary strengthening of the El Niño event, but given the recent trends and the predictions from computer models, neutral ENSO conditions are expected to return during the southern autumn.

Wind Storm Rips Through NW Oregon
George H. Taylor and Cadee Hale
The strongest wind storm in over a decade plowed through the Northwest on December 14, causing widespread damage, power outages, and three deaths. Most of the damage was due to fallen trees, weakened by earlier wet weather. Last month was one of the wettest Novembers ever, and last week prior to the wind storm several inches of rain fell.

The storm developed as it moved across the Pacific, reaching its full strength just offshore. Just prior to landfall, it curved northward, its center passing over Vancouver Island. But in a mid-latitude storm, the most dangerous area is not the center – it’s the area south of the center, in association with the cold front. That’s the part that passed over Washington and northern Oregon.

A strong storm, in just the right place – and at just the right time. That’s a recipe for disaster. Every storm goes through a “life cycle,” from its formative stages to its strengthening, a “mature” stage where it reaches its greatest intensity, through dissipation. This storm reached its peak just as it arrived at the coast.
Extreme gusts at the coast exceeded 100 mph in places, with Newport hitting 106 mph. Lincoln City wasn’t far behind at 91 mph, while Rockaway Beach and Garibaldi, farther north, reached 97 and 93 mph, respectively. Mt. Hebo, in the Coast Range, topped out at 114 mph. These are slightly below the peaks recorded in the December 12, 1995 storm, the last “really big” storm of similar size to hit our area.

In the Willamette Valley, Salem had the highest gusts with an 80 mph reading. Dallas reached 70, Portland’s West Hills were 65-70, and Portland Airport had a 53 mph gust (over 30 flights were cancelled). Locally we reached 55 mph. Most of these were slightly higher than the gusts in the 1995 storm, which reached the low 60s in the Valley
Severe winds knocked down utility poles, brought down wire and caused trees to tangle with lines and transformers. About 158,000 Portland General Electric (PGE) customers and 77,000 Pacific Power users still lacked power at noon Friday. Impassable roads in some areas hampered restoration efforts, and utility officials warned powerless customers that some might not get service for several days.

At the height of the storm on Friday night, nearly 250,000 PGE customers, or one in three users, were without power. That makes this the largest storm-related restoration effort since the Dec. 12, 1995 storm, which affected about 300,000 customers.

In Seattle, over a million people were without power and the damage caused by the wind storm prompted comparisons to another memorable storm: the Inaugural Day storm of 1993 when Bill Clinton was sworn in as president.

Four people are confirmed dead so far, a total matching those killed in the 1995 storm.

My hat is off to weather forecasters and their forecast models. They recognized the dangerous nature of the system well, in advance, with warnings beginning 4-5 days before the storm arrived. This prevented even bigger problems by allowing utilities and municipalities time to prepare. Dave Willson and Ira Kosovitz of the National Weather Service received awards for successfully predicting the timing and intensity of the December, 1995 storm. Forecasters deserve similar recognition for the 2006 wind storm.

Some peak observed wind speeds as reported by the National Weather Service:

Coast

Coast Range

SW Washington Lower Columbia Lowlands

Willamette Valley & Clark County

Columbia Gorge

Cascades & Foothills

A complete report, with pictures, can be found at http://www.ocs.oregonstate.edu/page_links/whats_new/dec_windstorm.html