“Atmospheric River” – those two words have been thrown around a lot the past week or so in the West, and with good reason. The drought-ridden region just got it’s best soaking in years via one of these weather phenomena.
Parts of California – mostly Northern California – saw as much as a foot of rain over the course of the storm. Sacramento saw the most rain ever recorded within a 24-hour period at 5.44 inches, and San Francisco received 4.02 inches within 24 hours, the most rainfall in a single day during October and the fourth wettest 24 hours on record for the city. More than 16 inches fell on Mt Tamalpais in Marin County during the stormy 48 hours. In the Sierra, parts of Tahoe received as much as two feet of snow, prompting some resorts to announce earlier opening dates and bringing the water level of Lake Tahoe back above its natural rim after drought conditions sent the water level plummeting dramatically.
So that’s what an atmospheric river does – dump a ton of water on the places it lands, especially places like California and the Western U.S. But what is an atmospheric river, exactly? Well, according the National Oceanic and Atmospheric Administration, an atmospheric river is pretty much what the name says – a relatively long and narrow “river” of water vapor in the sky above the ocean. Outside of the tropics, this is how the majority of water vapor moves around the world. A strong atmospheric river can carry an amount of water vapor comparable to 7.5 to 15 times the flow at the mouth of the Mississippi River. When an atmospheric river makes landfall, it releases this water as rain or snow.
When an Atmospheric River stretches from Hawaii to the U.S. West Coast, it’s aptly called a “Pineapple Express.” Photo: NOAA
These types of storm systems are actually pretty common both around the world and more specifically in the Western U.S. We even have names for specific atmospheric river patterns. When an atmospheric river stretches from the Hawaiian Islands to the U.S. West Coast as this one did (see the above video) it’s aptly called a “Pineapple Express.” Weaker atmospheric rivers are always welcome weather patterns, bringing much-needed precipitation from more tropical and wetter climates to drier parts of the world. Thirty to fifty percent of the West Coast’s annual precipitation comes in just a few atmospheric river events, and these atmospheric rivers are a critical part of both the global water cycle and the water supply in drier parts of the world, such as the Western U.S. But stronger atmospheric rivers like the one that just hit Northern California can dump so much rain and snow that they cause flooding and landslides, wreck property and damage roads. The definition of too much of a good thing.
The strength of this most recent atmospheric river was in part due to the fact that it hit the West Coast at the same time as a “bomb cyclone”, a phenomenon that occurs when the pressure of a storm drops drastically, significantly increasing the storm’s strength. This bomb cyclone produced the lowest pressure ever recorded off of the Washington coast, at 942.5 millibars (mb) or 27.83 inches. “Try to find that reading on your barometer,” says NWS Seattle.
The 2010 drought, busted by an atmospheric river. Photo: Climate.gov
However, the long term gains of the precipitation we just received will probably over time outweigh the short-term issues of flooding and storm-damage. Another name for atmospheric rivers is “drought busters,” as they have effectively ended around three-quarters of all droughts in the Pacific Northwest from 1950 to 2010. However, for the true drought-busting effect, we need the right kind of atmospheric river, a cold one. A critical part of ending lasting droughts is building up snowpack. So if we receive a warmer atmospheric river that fails to produce snow, or even worse rains on pre-existing snow, despite the large amount of precipitation the loss of snowpack can be a net loss for the water supply as we rely on snowpack to keep reservoirs full during the dry summer months.
Luckily, this most recent atmospheric river was certainly a cold one as evidenced by the amount of snow it dropped on the Sierra. With any luck, we’ll get a few more storms like this one in the next few months, and be able to consider the scary drought we are living through “busted.”
