The Pineapple Express, a local name for the atmospheric disturbance that drenches the U.S. West Coast, is one of several atmospheric rivers, a relatively narrow region in the atmosphere that is responsible for most of the horizontal transport of water vapor outside the tropics.

ARs result from winds associated with midlatitude storms that concentrate moisture into a narrow region just ahead of a cold front.

The conditions that produce the Pineapple Express are due in part to the size of the Pacific Ocean, which covers nearly half of the planet.

In the eastern Pacific there is a semipermanent high-pressure region (the subtropical high) around which air circulates in a clockwise gyre. It moves north and south with the changing seasons and is also influenced by the unpredictable jet stream.

This is the source of the tradewinds that characterize Hawaii weather.

The Pacific is so large that the subtropical high cannot sustain circulation throughout the ocean basin.

In the eastern Pacific, midlatitude weather is controlled by low-pressure systems that form around 60 degrees latitude and move southeastward.

Air circulation around low-pressure systems is counterclockwise. Their movement is likewise influenced by the jet stream. They form at more southerly latitudes during the winter.

As the low-pressure center moves southeastward, it eventually encounters warmer air circulating through the tropics from the clockwise gyre.

A front or shear line forms when warm, moist tropical air meets cold, dry polar air. The cold air tunnels under the warm air, lifting it and causing clouds to form.

The intensity of frontal storms depends on the difference of temperature and humidity between the two air masses.

The Pineapple Express is one of the most persistent of all ARs with a connection to the tropics near Hawaii.

Even during the summer months, when the polar and subtropical air masses have similar temperature and humidity, the convergence of clockwise and counterclockwise gyres causes a persistent northeasterly flow of air along a shear line that centers on the Hawaiian Islands.

Hurricane and tropical storm tracks reflect this, as they nearly always turn to the north near the islands. In September 1992 Hurricane Iniki got caught in that northeasterly flow and made a drastic northerly turn to hit Kauai head-on.

National Weather Service forecasters can now identify ARs, use them in numerical forecast models and monitor polar orbiter satellite imagery to provide advance warning of the presence and movement of ARs in the Pacific. This allows for advance warning of the Pineapple Express and its potential heavy rain five to seven days early along the Pacific coast from California to Washington.

The Pineapple Express is not just a catchphrase. It is one of the more significant weather phenomena that are just beginning to be understood.

Don’t blame it on Hawaii. We just happen to lie at the convergence of the great weather systems of the Pacific Ocean.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.