10 Feb 2008, 9:18pm
Climate and Weather
by admin

Another 1996 Flood May Be On the Way

by George Taylor, Certified Consulting Meteorologist and head of the Oregon Climate Service at OSU.

Re-posted from Weather Matters [here]

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 Oct. 1 and Feb. 8, our local area received 29.28 inches, which is almost 4 inches above the average for that period, 25.45 inches.

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

Ingredient three: Maybe. 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 to 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 snowdrifts and had assistance from the National Guard to dig them out. And that at a place that’s only 1,700 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 Feb. 1 of this year, total snow water was 22.1 inches (this represents a depth of about 80-100 inches). I looked back at Feb. 1 values for previous years, and found that this year’s total was the highest since 1950 — which was the snowiest winter Oregon has ever had!

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 percent of average for this time of year. But below 4,000 feet, the pack is about 250 to 300 percent 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.



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