Northwest Drowned in Unusual Rainfall

The Pacific Northwest is experiencing an unusual amount of rainfall. This is due to a combination of warm water and air, and unusual weather patterns. These patterns can be traced back to tropical cyclones in Indonesia. The rain has been persistent and heavy, with almost 5 trillion gallons falling in the past week. The rain is expected to continue until the week of Christmas. Atmospheric rivers, which are long, narrow bands of water vapor, are responsible for the heavy rainfall. These rivers transport moisture from the tropics to northern latitudes. The Pacific Northwest typically experiences a few dozen of these rivers each year, but they are not usually this intense. The rain has led to forecasts of record-setting flooding, particularly on the Skagit River. This river flows through northern Washington and empties into the Puget Sound. The atmospheric rivers are continually reloading, with the amount of rainfall in the three-week period potentially reaching 20 to 30 inches. This is quite extreme. The moisture for these atmospheric rivers originated a few hundred miles north of Hawaii. The Pacific is a couple of degrees warmer than normal in this area, which fuels the atmospheric rivers. Warmer air also adds to this effect, causing more moisture to fall as rain than snow. These storms have been supercharged by a chain of events that began two weeks ago. This includes a natural seasonal weather pattern that moves around every 30 days, known as the Madden Julian Oscillation. This pattern was the strongest it has been this time of year in decades. It sent out waves that helped carry an "unbroken line of moisture" and energy from the Indonesia event toward the Americas. In a world made warmer by the burning of fossil fuels, atmospheric river storms will likely become bigger and wetter. Studies and computer models suggest that this may already be happening. Since 1980, these storms have increased in the area they soak by 6% to 9%, in frequency by 2% to 6%, and are slightly wetter than before.