There are phrases that have been around the meteorological lexicon for years, and then get adopted into the media, and become part of what many find to be a sensationalized parlance. The polar vortex stands out to me, as does the bomb cyclone. These are real, definable things that suddenly take on a more ominous tone because they are uttered on the news regularly.
Right now, we are hearing about the “Atmospheric River”. Allow me to show you what that result of the atmospheric river is on satellite. It’s unusual, I think, to look at satellite over the north Pacific, because it isn’t somewhere we usually look at, and the perspective is a tad wonky, but here it is.
And that, boxed in red, is an area of low pressure, showing off the typical comma shaped cold front with a little warm front evidenced as angling towards San Francisco Bay. It’s not particularly unusual, but the discussion surrounding this feature is probably amplified by the persistent drought, which has rendered rainmakers rare over the last several years, and ongoing concerns over climate change flaring anxiety over every weather phenomenon.
The jet, in response to oceanic circulations, tends to rise and form a ridge at the California Coast, leading to the semi permanent Gulf of Alaska low (which feeds moisture into British Columbia and the Pacific Northwest with regularity) and a semi permanent area of surface high pressure west of the Baja Peninsula. In the winter, the jet is more apt to dig a little bit further south, because the cold California coastal waters don’t cause the atmosphere to differ as extensively as happens in the summer, and that coastal ridge can break down.
So the jet sinks south, and a regular old area of low pressure moves into California. It is coming off the ocean, so it will be laden with moisture, and it is interacting with the terrain, so it will dump a lot of precipitation, especially in the Sierras. This will do a lot of work in refilling dried reservoirs as the snow melts in the spring, and is how California usually maintains a sustainable quality of life. The southern source of the jet has led, in the past, to some “atmospheric rivers” to be dubbed “the Pineapple Express”.
All they are, really, is a more southerly jet that brings consequential weather to the West Coast. he phenomenon isn’t strange, but the location is, made a little bit more unusual by the recent climatology.