CORVALLIS - A major winter storm expected to lash the Pacific Northwest in the next 24 hours with snow and freezing rain might be only the prelude to more serious weather problems to come, experts say, because an old-fashioned "pineapple express" is bearing down on Oregon and Washington, and a serious flood could be the result.

These warm, extremely wet weather events usually emerge from the sub-tropics two or three times each winter and often account for the single rainiest day of the year, according to George Taylor, the state climatologist at Oregon State University.

But when combined with the large amounts of low elevation snow that has occurred in the past couple weeks, a pineapple express can be the trigger that causes serious flooding, Taylor said. Exactly those conditions are now present in Oregon, and the remaining wild card is how much rain the storm brings. If it's very high, the region may experience a major flood.

"After a transition day on Tuesday, our forecasts clearly show that temperatures will rise, some snow melt will begin and it's going to rain for the next four or five days," Taylor said. "It's less certain exactly how much rain we will get."

What's pretty clear, however, is that the stage is set.

"The two largest flooding events in the Pacific Northwest during the past 100 years were in 1964 and more recently in February, 1996," Taylor said. "The '96 flood was accompanied by record-setting rains, such as the eight inches that Corvallis got during a four-day period. But in terms of low-elevation snow, our current conditions more closely resemble those of 1964."

The '64 flood is generally considered the worst in recorded Pacific Northwest history, Taylor said. It struck a huge area, causing 47 deaths in four states, inundating a region from Northern California through Oregon, Washington and Idaho. To this day there's a marker in place above the Rogue River near Agness in Southern Oregon, where the river crested about 100 feet above its normal level.

"Obviously we have no indication at this point that the flooding we may experience soon would resemble either of those two major events," Taylor said. "But we do have a great deal of snow on the ground and a big sub-tropical storm headed this way. It's a serious concern."

Just in the past couple weeks, he said, the snow pack in most of the Pacific Northwest has gone from below normal to well above normal, and at the moment it extends to the floor of the Willamette Valley. At mid-elevations of about 3,000 to 4,000 feet in the Cascade Range, there currently is about 10-15 inches of "snow water" waiting for warmer conditions to cause melting. The lower elevation snow will go first, Taylor said.

The combination of rapid snow melting and significant precipitation is the recipe for a flood, he said. In the next few days, at least some minor flooding along local streams might be expected, but it will take fairly significant rain to cause problems with the major river systems, including the Willamette River.

"That's what we don't know at the moment, exactly how much rain to expect," Taylor said.

"The name that has caught on with the media and public is pineapple express, although I think of these storms as a subtropical jet stream," Taylor said. "Regardless, they are warmer storms coming in from the southwest, and they are particularly unpredictable in the amounts of moisture they carry. It's usually a lot, but we can't say for sure how much."

In 1996, Taylor said, some areas of the Oregon coast range were just drenched. Laurel Mountain northwest of Corvallis got 28 inches of rain in four days, and one location near Forest Grove added 20 inches of rain to 15 inches of melting snow water to cause the Tualatin River to hit its highest level ever recorded.

The current five day forecast from the Flood Forecasting Center suggests that the Willamette Valley might get 2-3 inches of rain in the next few days, with far more in the Coast Range and some larger amounts in Washington, Taylor said.

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George Taylor, 541-737-5705