Rivers in the Sky: Understanding atmospheric rivers

Atmospheric rivers can be both beneficial and hazardous to a region
The source of all the flooding in Haines, is attributed to an atmospheric river
The source of all the flooding in Haines, is attributed to an atmospheric river(KTUU)
Published: Dec. 4, 2020 at 6:57 AM AKST
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ANCHORAGE, Alaska (KTUU) - The world of weather is sometimes strange, fascinating and at times even scary. After all, it plays a huge role in transporting the most essential substance on earth around the globe.

Nearly three-quarters of the planet we call home is covered by water and one of the key features of transporting this water is atmospheric rivers. These rivers of moisture in the atmosphere can both provide the water supply to regions and inundate an area bringing catastrophic flooding.

The most well-known example, the Pineapple Express. No, it’s not the next great cinematic masterpiece or even a train ride, but it is a narrow region of moisture in the atmosphere that builds up in the tropical Pacific. As global prevailing winds cross over the ocean, they not only create surface currents but they pick up water vapor along the way. These rivers in the sky can transport nearly 15 times the average flow of water at the mouth of the Mississippi River but can be as high as 27 times. They are always forming and consistently moving until they slam into something like the mountains of Southeast Alaska. As these rivers of moisture crash into the mountains, the moisture is forced upwards into the atmosphere where it condenses and falls as rain. The rivers are often accompanied by fronts, which help dump the moisture from the atmosphere. It’s here where rainfall amounts can be extreme at times, often exceeding record amounts.

Atmospheric rivers come in all shapes and sizes and on average are roughly 250 to 375 miles wide in the atmosphere. While some are weak and provide essential water supply and snow to western communities, others can pack quite a punch and bring record-breaking rain and even damage communities.

Southeast Alaska is often the target for many of these powerful atmospheric rivers due to its location and topography. As storms move in from the Pacific Ocean, winds play a key role in just how much rain an area will see in the Southeast. The weaker the winds, the less moisture that is transported into the Southeast. The stronger the winds, the more powerful a storm will be, with southerly or southwesterly winds enhancing the rainfall amounts. This is due to the steep topography of the region, as the winds blow perpendicular to the mountains. Add in a system that stalls over the Gulf of Alaska and you have a recipe for disaster.

This was recently experienced in the area, as the Pineapple Express began to encounter the higher terrain of the Northern Inner Channels. It was here where nearly a foot of rain fell in just 72 hours, leading to widespread flooding, mudslides and washed-out roads.

Thanks to the satellite era, scientists are better able to study and determine just how significant an atmospheric river event will be. A team of researchers recently released a new scale to categorize the strength of atmospheric river events, to better help the public prepare. Much like hurricanes, these rivers in the sky come with their own categorical system from weak to exceptional.

They are as follows:

  • - AR 1 (weak) - Primarily beneficial for a region in replenishing the water supply or snowpack.
  • - AR 2 (moderate) - Mostly beneficial, but can produce some hazards.
  • - AR 3 (strong) - An equal chance of being both beneficial and hazardous to a region
  • - AR 4 (extreme) - Mostly hazardous
  • - AR 5 (hazardous) - Primarily hazardous

An added concern to atmospheric rivers, as detailed in the study, is that a changing climate is yielding the potential for stronger atmospheric river events. The warmer the atmosphere, the more moisture that will flow in these rivers in the sky, which ultimately could make them more intense in the future.

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