January 2008

Overview

January was a cool month throughout Oregon. January was wetter than average in most of the state, the exceptions being southeast Oregon and some sections of the coast. January was also a very snowy month, even at low elevations. Weather typical of La Niña conditions in the Pacific prevailed; not coincidentally, La Niña continues do persist (see Page 2).

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 January 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 January 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 February-April appear below. Temperature probabilities show equal chances of above-, near-, and below-normal for Oregon, while precipitation probabilities show above-normal probabilities.

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

ENSO Update

Australian Bureau of Meteorology, February 13, 2008

The La Niña event in the Pacific basin is mature, and continues to influence the climate of eastern Australia.

While weak warming has occurred in the far eastern Pacific Ocean, cooler than average sea surface temperatures (SSTs) continue to extend across the central equatorial Pacific. When combined with enhanced Trade Winds, suppressed cloudiness in the central Pacific and a strongly positive (+12) Southern Oscillation Index (SOI), it is clear that the atmosphere and ocean are firmly reinforcing each other, sustaining the La Niña event.

Some warming has occurred in the western Pacific sub-surface in response to a weakening of the Trade Winds during January. While this warming has shown some signs of eastward propagation, it has not yet had any noticeable impact on central Pacific SSTs, which have in fact continued to cool. However, a gradual weakening of these cool anomalies would be consistent with the latest outlooks from computer models. These show Pacific temperatures gradually warming over the next few months, although remaining below La Niña thresholds until at least the end of the southern autumn.

CPC ENSO Forecast (January 10, 2008)

Current atmospheric and oceanic conditions indicate that La Niña has continued to strengthen in the tropical Pacific. By the end of January 2008, equatorial SST anomalies were more than 2.0°C below average across parts of the central and east-central equatorial Pacific. Other than the far eastern Niño-1+2 region, the magnitude of the cold anomalies in the Niño region indices increased during the past month with the latest weekly values near -1.5°C. The upper-ocean heat content (average temperatures in the upper 300m of the oceans) also decreased further during January, and negative subsurface anomalies between -2°C to -5°C expanded westward towards the Date Line. Consistent with these oceanic conditions, stronger-than-average low-level easterly 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 oceanic and atmospheric conditions are similar to those accompanying the last strong La Niña episode in 1998-2000.

The recent dynamical and statistical SST forecasts for the Niño 3.4 region indicate a moderate-to-strong La Niña through the rest of the Northern Hemisphere winter, with the likely continuation of a weaker La Niña through April-May-June. Thereafter, there is considerable spread in the models, with approximately one-half indicating La Niña could continue well into the Northern Hemisphere summer. Current atmospheric and oceanic conditions and recent trends are consistent with the likely continuation of La Niña through the Northern Hemisphere spring 2008.

Flood in the Making?

Twelve years ago this month we had a memorable flood. Heavy rains from warm, subtropical air fell on a deep snowpack, causing a big “rain on snow” flood, in which runoff from the rains was augmented by melting snow. The result: big-time flooding, the biggest since 1964.

Following that flood, I gave a lot of presentations in which I discussed the flooding. One popular topic was what I called “recipe for a flood.” Like a food recipe, my flood recipe had certain ingredients. Think of a cake: start with flour, add leavening, a little shortening and then liquid, and the cake rises! And maybe tastes good as well…

In the case of the flood, the ingredients are:

1. a wet winter (to saturate the soils and fill the streams)

2. a moderate-to-deep snowpack

3. a period of cold weather (to freeze the soil surface) – not a necessity, just think of this as “spice”

4. several days of very wet, mild weather

The first ingredient has happened. Between October 1 and February 8, our local area received 29.28 inches, which is almost 4 inches above the average for that period, 25.45”.

Ingredient two: deep snowpack. Check! In fact, this is one of the deepest ever in western Oregon. See Idanha story below.

Ingredient three: January and February have been considerably colder than average.

Ingredient four, the wet, mild weather: not yet, and nothing is imminent, but we’re watching closely. Usually the weather prediction models will start warning us 4-5 days in advance.

You may have seen the news coverage of Idanha, a little town east of Detroit on Highway 22. They had 10 foot snow drifts and had assistance from the National Guard to dig them out. And that at a place that’s only 1700 feet in elevation.

I was asked recently to put Idanha’s snow depth in perspective. Unfortunately, there is no weather station there, so I looked for the most applicable nearby station. I chose Marion Forks, a little higher in elevation (2600 feet) but not far away. I compared the current readings with historical data for Marion Forks, which go back to 1941. “Snow water equivalent” (the amount of water in the snow pack) is measured twice monthly by USDA.

On February 1 of this year, total snow water was 22.1 inches (this represents a depth of about 80-100 inches). I looked back at February 1 values for previous years, and found that this year’s total was the highest since the very snowy 1949-52 period – which was the snowiest period Oregon has seen in the last 100 years. See chart below.

And it’s the low-elevation snow that is really remarkable this winter. For elevations above 4000 feet, the snow pack is running about 150% of average for this time of year. But below 4000 feet, the pack is about 250-300% of average.

The latter is the area of biggest concern when it comes to flooding. Because of the lower elevations, these areas will be the first to rise above freezing if the weather warms. Also, there is a lot more area in low elevations than high ones, and thus a lot more water is sitting there, waiting to melt.

Chances are, warmer temperatures will arrive gradually, and heavy rains will not come. That’s the most likely scenario. But we have to be prepared for worst-case, so weather watchers will continue to monitor the ocean to the southwest of us, watching for big slugs of moisture moving our way. Bringing warm air, high humidity, and heavy rains.

And we’ll be hoping they never arrive…

Marion Forks Snow Water Equivalent, as reported by USDA Natural Resources Conservation Service