March 2008

Overview

March was another cool month in what has been a significant run of cooler-than-average months. Precipitation totals were fairly evenly divided between above- and below-normal.

Unusually late cold, snowy weather brought snow to low elevations, including the Willamette Valley, during the last week in March. Some stations set new records “latest snow date.”

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 precipitation totals throughout the state are listed. Figure 1 is a map showing the percentage of normal precipitation statewide for the Water Year.

Basin Summary

Here is a summary of water indicators at the end of the month, by river basin:

* Last month’s numbers; current data unavailable at press time

n.a. Not available
(1) Percent of normal November 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 November 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 April-June appear below. Temperature probabilities show equal chances of above-, near-, and below-normal for Oregon, and precipitation probabilities also show equal chances.

Oregon Climate Service predicts below-normal temperatures and normal precipitation for the next three months, as La Niña conditions continue.

ENSO Update

Australian Bureau of Meteorology, April 16, 2008

The La Niña event has shown further signs of weakening over the past two weeks, especially in the far eastern Pacific where weekly NINO3 SST anomalies are now positive for the first time since early 2007. Further signs that the event is decaying include the weakening of negative anomalies in the subsurface in the eastern Pacific, and the strengthening of positive subsurface anomalies in the western to central Pacific.

However, colder than normal temperatures remain at La Niña levels at the surface in the central equatorial Pacific. In addition, the Pacific Trade Winds continue to be enhanced and below average cloudiness is evident near the dateline. The SOI also remains at La Niña levels.

Computer models continue to predict the return to neutral conditions across the Pacific as the most likely scenario for 2008, with central to western Pacific temperatures warming back to the neutral range during the next one to two months. Such an evolution suggests that La Niña is likely to play a reducing influence on Australian weather patterns.

CPC ENSO Forecast (April 10, 2008)

La Niña declined to moderate-strength during March 2008 as negative sea surface temperature (SST) anomalies weakened across the central and east-central equatorial Pacific. The latest weekly SSTs are more than 1.0°C below average in areas between 160°E and 120°W. All of the Niño indices warmed during March, with only the westernmost Niño-4 and Niño-3.4 regions having values nearly 1.0°C below average. Above-average SSTs remained restricted to the far eastern equatorial Pacific in association with a significant warming trend that began in mid-December. In the central Pacific, the subsurface temperature anomalies also lessened, and became increasingly confined to the surface region. This evolution led to a significant weakening of the negative ocean heat content anomalies (average temperatures in the upper 300m of the ocean). Despite this oceanic trend, the atmospheric conditions continue to strongly reflect La Niña. Enhanced low-level easterly winds and upper-level westerly winds persisted across the central equatorial Pacific, convection remained suppressed throughout the central equatorial Pacific, and enhanced convection covered the far western Pacific. Collectively, these atmospheric and oceanic conditions indicate an ongoing, but weaker, La Niña.

Looking Back

George H. Taylor

April, 2008

As my May 1 retirement date approaches, I realize that this is the last Oregon Weather Summary I will manage. My associate Cadee Hale will try to keep this going as long as possible – but it will be different. The first issue of the Summary was published in January, 1990; by my calculations, this is issue number 219! Wow!! I had no idea there had been that many until I did the calculation.

For this final essay I decided to reminisce about significant weather-climate events that have occurred in the last 18 years.

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Wind-rain storm, January 1990. I moved here with my family from Santa Barbara in the summer of 1989. Frankly, I was feeling intimidated about the Oregon winter which was approaching, but the weather that fall was quite benign – and gloomy. December 1989 was especially foggy, cold and clammy. The Taylors escaped to Southern Cal for Christmas, without a single “big storm” story to relate.

And then came a very large, multi-day storm (actually three storms back-to-back), January 6 through 9. In the first, coastal areas experienced winds up to 65 mph. The second was even windier, with a highest gust of 130 mph in eastern Washington. The third and final storm brought wind gusts exceeding 100 mph on the northern Oregon coast.

Unlike most big wind storms (which tend to be rather dry), these storms produced a lot of rain and some local flooding. They also knocked down a lot of trees, especially along the coast and in the Coast Range. The southwest side of Mary’s Peak west of Corvallis really got hammered.
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1993. Wow, what a strange year – not just here, but all over.

Summer of 1992 had been a devastating drought year. Fall of that year was relatively dry, as was the winter. So water problems were expected the following summer.
But the winter was also a very cold one. Snow and ice remained on the ground in the Valley for nearly the entire month of January. Then, on February 19-20, a big snow storm arrived, dumping 12 inches on Corvallis in 24 hours. After that things warmed up, and then the rains came.

March was wet, and wet conditions continued through July. It was the wettest spring western Oregon had ever seen. May brought lots of thunderstorms as well. The wet spring erased any water supply worries and took away the “drought” status.

Meanwhile, the East Coast was experiencing the “Storm of the Century” in mid-March. In the summer, the Mississippi Valley was beset by the “Flood of the Century.” Some scientists believed that the eruption of Mt. Pinatubo a year earlier may have been responsible for the crazy weather, but who knows?
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1996 Floods. Not since 1964 had the Northwest experienced a flood like the one in February, 1996. A four-day “Pineapple Express” dumped a lot of rain (28 inches in the Coast Range, more than 8 in the Willamette Valley) and melted most of the snow. The media went crazy (well, let’s face it – floods are very photogenic!). It was quite exciting. One of the Portland TV stations sent a truck to my house and interviewed me live for the 11 o’clock news.

And the excitement continued for awhile. I was besieged by media requests for interviews. I was asked to give presentations on flooding. I developed a slide show I called “Recipe for a Flood” in which I compared the ingredients of a flood with those of a cake – get all the ingredients, mix them, and the “cake” will rise. For a winter flood, the ingredients are (1) a wet winter, to saturate the soil and fill the rivers and streams; (2) a moderate or deeper snow pack; (3) a period of cold weather to freeze the soil surface; and (4) a period of heavy, warm rain. All of those happened in February, 1996.

Ironically, there were two other major floods that year, but they were somewhat overlooked. On November 18-19, Oregon experienced what may have been the wettest single day ever, with all-time 1-day records set over much of western and central Oregon – including 11.65 inches at the Elk River Fish Hatchery near Port Orford, at that time the wettest day in Oregon’s history (a new record was set in November, 2006).

And then over New Year’s a big, wet, windy, multi-day storm affected the entire West Coast. California and southern Oregon were especially hard hit, but flooding and wind damage were very widespread.

Nonetheless, media coverage was much less extensive than in February – we’d already “been there, done that,” I guess.

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February 2002 wind storm. On the afternoon of February 7, I opened up the West Coast satellite image and said to my student assistant, Melanie, “Yow! Where did THAT come from?” A menacing-looking storm was just off the central Oregon coast, and it looked ominous.

In Corvallis, the day was very weird. Warm, windy conditions alternated with heavy snow. South winds were followed by north winds. It seemed as if the weather couldn’t figure out what it wanted to do.

South of here, very strong winds caused major damage in Florence, Eugene, Lebanon and Sweet Home.

My friend and colleague Wolf Read, the Northwest’s premier wind expert, wrote a nice piece on the storm. Among his comments was the fact that the intensity of the storm surprised all the forecasters. He wrote:

This was the modern era of meteorology, with the most sophisticated forecasting models yet put in use, a variety of satellite tools, new and expensive automated reporting stations, and a fairly dense network of Skywarn spotters. Even PacJet, an experimental method for measuring low to mid-level winds in the atmosphere across a large chunk of the eastern Pacific, was operating at the time. And yet, the magnitude of the winds in the South Willamette valley was not recognized beforehand. The cyclone, it should be noted, wasn't a surprise. Its effects were.

Wolf’s report on the 2002 wind storm is contained in his very thorough online paper at

http://www.ocs.oregonstate.edu/storm_king_site/February2002.html