Severe Storms/Atmospheric Rivers
Overview of Atmospheric Rivers
Atmospheric rivers are narrow bands of enhanced water vapor transport. They are often found in the warm sectors of mid-latitude cyclones that form over water. The warm sector can also entrain moisture from oceanic regions.
Characteristics of Atmospheric Rivers
Atmospheric rivers (ARs) are narrow, usually only about 100 kilometers wide, move quickly, about 85 kilometers per hour at the core, and are centered about one kilometer above the surface. ARs are responsible for almost all (>90%) of the global north-south transport of water vapor. Some ARs carry up to 50,000,000 liters of water per second. Atmospheric rivers can change locations, and often do with a change in weather pattern. ARs are especially relevant as they contribute vastly to flooding risk and potent water levels in the Western United States. This AR, known as the "Pineapple Express," draws its moisture from near Hawaii and flows to the West Coast. As much as 30-50% of the total rainfall and snowfall in any given year can be attributed to a relatively small number of atmospheric river events in this region. Scientists have attributed the especially extreme events of flooding to the orography, or the mountains and highlands, of the regions associated with the location of the atmospheric river. The total rainfall is almost the same as that of flat areas, but almost all the precipitation falls rapidly on the windward side of the mountain, increasing the likelihood of flash flooding and mudslides.
The evidence surrounding the impacts and effects ARs have on weather patterns is substantial, and will continue to increase as NOAA and other important weather organizations conduct research on how the rivers function. NOAA sent a ship on a mission called CalWater 2015 to "'Improv[e] our understanding of atmospheric rivers,'" according to Chris Fairall, the chief of Weather and Climate Physics at NOAA’s Office of Oceanic and Atmospheric Research Earth System Research Laboratory in Boulder, Colorado. This will enable meteorologists to better forecast where and when the storm will strike, and how much it will deliver.
Atmospheric rivers are largely responsible for incidences of extreme precipitation in the west coast regions of the middle latitudes (e.g., the West Coast of North America, western Europe, and western North Africa). These incidences of extreme precipitation can result in severe flooding and mudslides, both of which can be hazardous to human life and property.
However, atmospheric rivers can also provide significant and beneficial snowfall in late fall into early winter. This pads the snowpacks in the mountainous areas of the regions affected, which increases water supplies and water tables as it melts off in the following spring. For example, in the Sierra Nevadas, 75% of the snowfall came before December 22nd, 2010 as a result of this phenomenon.