Sometimes, you hear odd terms, especially related to weather. “Bombogenesis” and “Polar Vortex” for example.
And when you hear it for the first time you may wonder if the idea is new. If the term was recently ‘invented’, or even if it is a real thing!
Usually, these are terms in use by Meteorologists for decades, yet they’ve only recently become relevant on the News side of the business.
“Pineapple Express” is such a term. Is it a real thing? Yes, it is.
Across the globe, there are jet streams. These are just concentrated rivers of wind, stronger than north or south of their central location. The basic jet streams are in the mean flow of where weather systems are born, or die. So these carry weather systems (high and low pressure), around the globe.
But, just as there are other circulations like El Nino or La Nina, or the North Atlantic Oscillation, there are “moisture rivers”, sometimes referred to as “Atmospheric Rivers”. These are often associated with widespread and major flooding events, like along the West Coast. (The presence of an atmospheric river doesn’t mean flooding will occur, but most West Coast flooding events do happen because an atmospheric river is plowing ashore.)
So, because this atmospheric river is flowing almost entirely over an ocean (the Pacific), it skims off an abundance of moisture, which makes it easy to spot, because you will see a long stretch of clouds, often emanating from Hawaii, to the West Coast. Because Hawaii is associated with all things tropical, this river is referred to as “The Pineapple Express”. It will often stretch several thousand miles, and will greatly enhance moisture along the shore.
Remember, in order to get widespread rain/snow, a storm has to saturate the entire layer of the Troposphere. But, with the presence of an atmospheric river, upper level moisture is well supplied, and often it can also fill low layers near shore, too.
So, if as a Meteorologist, you were seeing a storm push ashore at a time similar to a “river” setting up and pushing in to the same shore, you can almost bet, moderate to major flooding is coming.
So there you go. Atmospheric Rivers. A real thing.