Washington ? Moisture-laden storms from the north, west and south are likely to converge on much of America over the next several days in what could be a once-in-a-generation onslaught, meteorologists forecast Tuesday.

If the gloomy computer models at the U.S. Climate Prediction Center are right, we’ll see this terrible trio:

• The “Pineapple Express,” a series of warm wet storms heading east from Hawaii, drenching Southern California and the far Southwest, which already are beset with heavy rain and snow. It could cause flooding, avalanches and mudslides.
• An “Arctic Express,” a mass of cold air chugging south from Alaska and Canada, bringing frigid air and potentially heavy snow and ice to the usually mild-wintered Pacific Northwest.
• An unnamed warm, moist storm system from the Gulf of Mexico drenching the already saturated Ohio, Tennessee and Mississippi valleys. Expect heavy river flooding and springlike tornadoes.

All three are likely to meet somewhere in the nation’s midsection and cause even more problems, sparing only areas east of the Appalachian Mountains.

“You’re talking a two- or three-times-a-century type of thing,” said prediction center senior meteorologist James Wagner, who’s been forecasting storms since 1965. “It’s a pattern that has a little bit of everything.”

A city snowplow in Flagstaff, Ariz., prepares to knock down one of several large tree limbs that broke under the weight of a 17-inch snowfall. The western United States has seen much precipitation recently in the form of both rain and snow. Meterologists say that a convergence of three moisture-laden storm systems could spread the wicked weather over much of the country in the next week.

While the predicted onslaught is nothing compared with the tsunami that ravaged South Asia last week, the combo storms could damage property and cause a few deaths.

The exact time and place of the predicted one-two-three punch changes slightly with every new forecast. But in its weekly “hazards assessment,” the National Weather Service alerted meteorologists and disaster specialists Tuesday that flooding and frigid weather could start as early as Friday and stretch into early next week, if not longer.

“It’s a situation that looks pretty potent,” Ed O’Lenic, the Climate Prediction Center’s operations chief, told Knight Ridder. “A large part of North America looks like it’s going to be affected.”

Kelly Redmond, the deputy director of the Western Regional Climate Center at the Desert Research Institute in Reno, Nev., where an unusual 18 inches of snow is on the ground already, said the expected heavy Western rains could cause avalanches. Since Oct. 1, Southern California and western Arizona have had three to four times the normal precipitation for the area.

“Somebody is in for something pretty darn interesting,” Redmond said.

The last time a similar situation seemed to be brewing — especially in the West — was in January 1950, O’Lenic said. That month, 21 inches of snow hit Seattle, killing 13 people in an extended freeze, and Sunnyvale, Calif., got an unusual tornado.

The same scenario played out in 1937, when there was record flooding in the Ohio River Valley, said Wagner, of the prediction center.

Meteorologists caution that their predictions are only as good as their computer models. And forecasts get less accurate the farther into the future they attempt to predict.

Wagner was worried about the Ohio and Tennessee River valleys as the places where the three nasty storm systems could meet, probably with snow, thunderstorms, severe ice storms and flooding. Some of those areas already are flooded.

The converging storms are being steered by high-pressure ridges off Alaska and Florida and are part of a temporary change in world climate conditions, O’Lenic said.

Over equatorial Indonesia, east of where the tsunami hit, meteorologists have identified a weather-making phenomenon called the Madden-Julian Oscillation. It’s producing extra-stormy weather to its east. Similar oscillations in the north Atlantic and north Pacific are changing global weather patterns. Add to the strange mix this year’s mild El Niño — a warming of the equatorial Pacific — which is unusually far west, Redmond said.