Atmospheric rivers are a good source of water for the western United States, but can also lead to flooding if the timing and strength is right. According to NOAA, atmospheric rivers provides the west coast with 30-50 percent of their water supply for the year on average. Our last big storm was an atmospheric river and dumped a lot of snow in the mountains. The snow was rich in water too. Not cold and fluffy snow, but dense snow that is good for building snowmen. Not every atmospheric river is the same, but there are similarities.
An atmospheric river is a river of flowing moisture or condensed water vapor that starts in the ocean and is aimed at the coastline. Atmospheric rivers can produce significant amounts of rain and snow as they move onshore. According to NOAA, they can be pretty big too, but on average they are between 250 and 375 miles wide. While we don’t always have one impacting the west coast, there is always an atmospheric river somewhere on the earth at any given time. Not every atmospheric is strong, but those that are can move enough water vapor similar to 7.5 to 15 times the average flow of the mouth of the Mississippi River.
North America from space. Elements of this image furnished by NASA
Atmospheric rivers can also produce a lot of wind, but some have less wind than others. The wind also plays a big role in the weather forecast itself. For example, with our last storm, as winds picked up temperatures rose too. What happened was the warm air from the atmospheric river road over top of the cold air in the valley to begin with, producing a lot of snow pretty much all day long. Once our winds picked up the cold air that was stuck in the valley was able to break up and temperatures began to rise. Usually atmospheric rivers goes from warm weather and heavy rain to snow, but it was backwards this time. It was a tricky forecast, especially for areas below 5000’.
There are no big signals of an atmospheric river hitting northern California and Nevada anytime soon. The earliest would be February 18th, but that is still a couple weeks away and a lot can change by then.
