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Science: What happened to the Pineapple Express?

Science: What happened to the Pineapple Express?

By Shannon Lee

Growing up in San Diego I learned to see rain as an uncommon event. Those rare times when we got heavy rain were extra exciting and thus seared in my memories. The phrase Pineapple Express was used to describe those storms which, as I came to learn later, referred to how the moisture arrived direct, on an almost straight line, from the Hawaiian Islands to our southwest. FYI, if you do an internet search for this term, the entire first page of sources will be covering the 2008 Seth Rogan movie and a variety of marijuana, which is entertaining to be sure, but if you’d like to learn more be sure to add the word ‘weather.’ In the past several years it seems I’m hearing several other words far more often, for example, Atmospheric River, Bomb Cyclone, Bombogenesis. So, what has happened?

Pineapple Express (PE) is a non-technical term for a central Pacific Ocean direct to the U.S. West Coast mass of warm moisture-laden air driven by a southern portion of the jet stream. These large-scale rain events can bring heavy rain to western states from California to Washington (and even into Alaska) and are also common during El Nino conditions. PE is an example of an Atmospheric River.

In the fall of 2021, the west coast experienced several strong storms that brought high rainfall rates and flooding in some areas. These were described as being the product of a Bomb Cyclone. Cyclones are large storms rotating around a center of very low pressure. They are then described by their location of origin, speed of the winds, direction of travel, and the rapidity and intensity of the drop in air pressure, etc…. Tropical cyclones develop in the tropics, where warm sea surface temperatures can assist in driving rotation and speed. Hurricanes (Atlantic and Eastern Pacific) and typhoons (Western Pacific) are both examples of tropical cyclones. The simple term cyclone is used for these large storms when they develop in the Indian and South Pacific Oceans.

A Bomb Cyclone is a mid-latitude, or extratropical (extra = outside of) cyclone. These are rotating large storms whose origin is temperate latitudes (such as the north central Pacific). Bombogenesis refers to the process of development of one of these Bomb Cyclones (genesis = origin). In addition to the mid-latitude origin, the other feature of these storms is that the air pressure must drop rapidly (at least 24 millibars in 24 hours). Significant differences between air pressure and temperature of side-by-side atmospheric features are what lead to winds and precipitation events.

All of these storms often bring not only heavy rain but significant winds, both of which can lead to widespread damage. As you can see from the included wind chart, the storms we have on this coast do produce conditions that rival both tornadoes and tropical cyclones, such as hurricanes. Predictions of these events should spur preparations for both the potential of flooding and of damaging winds. Perhaps that’s why we’ve started hearing more about Bomb Cyclones; Pineapple Express may sound a bit too ‘pleasant’.