After the storms that passed through the Hawaiian Islands in the weeks before Christmas, most people have now heard of the "Pineapple Express" and the heavy rain and snowfall it caused in California.

The Pineapple Express is one of several atmospheric rivers, relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. These rivers result from winds associated with mid-latitude 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 immense size of the Pacific Ocean.

In the eastern Pacific there is a semi-permanent 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 Hawaiian weather.

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

On the western side of the Pacific, mid-latitude 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 atmospheric rivers 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. 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 atmospheric rivers, use them in forecast models and monitor polar orbiter satellite imagery to provide warning of their presence and movement. This allows for warning of the Pineapple Express and its potential heavy rain five to seven days in advance along the Pacific coast.

The Pineapple Express is one of the more significant weather phenomena that are just beginning to be understood. But don’t blame it on Hawaii. We just happen to lie at the convergence of the great weather systems of the Pacific Ocean.