Trio of storm systems could have devastating impact on U.S.By SETH BORENSTEINKnight Ridder Newspapers 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....