Editor's note: This is a news release made by the Pacific Northwest Region of the Forest Service after the large floods in Washington over Thanksgiving weekend, 1990.

Thanksgiving weekend brought the "Pineapple Express" to a heavy snowpack.

In a period of two weeks, two significant flood events hit a large section of western Washington causing loss of life and significant loss of property and forest resource values.

The kind of weather phenomenon responsible for both of these events is referred to as the "pineapple express," a strong, persistent flow of warm, moist air from the vicinity of the Hawaiian islands. As this air moves onshore and over the Olympic and cascade mountains, it "drops its load." This moist, warm system follows the same track as the jet stream. This is opposed to the other weather systems that are colder and flow out of the gulf of Alaska during the late fall and winter in the northwest. One difference existed between the most recent Thanksgiving weekend event and the one that occurred two weeks earlier: Snow existed at lower elevations prior to the Thanksgiving weekend flood. In the earlier event, snowmelt did occur at higher elevations, but there was little snow below 3200 feet. However, prior to the Thanksgiving weekend flood, snow did accumulate as low as 2,000 feet. The melting of this snow along with the increased rainfall and previously saturated ground in many watersheds intensified the flooding.

Studies of major flood events in the past indicate that 25-30% of the runoff during a major rain-on-snow event can be attributed to snowmelt. The snowpack at Stampede Pass was 20 inches on November 21, but two days later, after more than 7" of rain, the snowpack had diminished to only 3 inches. On the average, this amount of snowmelt can be equivalent to another 2-3 inches of rainfall. Those watersheds where accumulations of snow were greater would be expected to have a more severe storm runoff.

Using the Skagit watershed as an example, rainfall measurements for the period Nov. 22-25 can be approximated at 14", which includes an estimate of that contributed by snowmelt. Using an estimate of the acreage of the entire watershed, about 75 billion cubic feet ran off the Skagit watershed. That equates to about a cubic mile of water or enough to cover all of downtown Seattle about 900' deep.

Another factor that can have an effect on amount and timing of runoff in a particular watershed is the amount of acres recently harvested or where replanted stands have not yet reached the development necessary for them to respond hydrologically as mature forest would. The Forest Plan recognized this situation across the Mt. Baker-Snoqualmie N.F. and adjusted harvest rates over the next few decades to provide for a higher level of hydrologic recovery. When looking at the effects of timber harvest on runoff and downstream impacts the issue of heat transfer needs to be considered also.

During small to moderate rain-on-snow events (which is characterized by the Nov. 9-10 event), wind aided transfer of heat to snow is the major source of heat and can be affected by forest cover. As rainfall increases, as it did during the Thanksgiving event, rain heat becomes the dominant source of heat and is much less affected by forest cover. The larger the rainfall, the more difficult it will become to detect differences in runoff between clearcut forests and those with cover.

The flat, alluvial valleys characteristic of most of the major rivers draining the cascades of northwestern Washington indicate a natural history of flooding as part of their formation over time. These alluvial valleys are floodplains, they formed by flooding, and they will always be prone to flooding in major runoff events as we've experienced this November.

It is extremely difficult to separate the effects of human activities on the severity of runoff and downstream flooding under the conditions that existed during both of the recent storm events in northwestern Washington. The amount of rainfall and the added effect of snowmelt from the rain so overwhelmed what might be attributed to human disturbance that it is nearly impossible to separate causes and define any meaningful cause and effect relationships.

Over the next few months the Pacific Northwest Research Station, the Mt. Baker-Snoqualmie, Wenatchee, Olympic, and Gifford Pinchot National Forests, in cooperation with other agencies, will be examining rainfall, snowmelt, and runoff data in order to place the Thanksgiving weekend flood in perspective.