More than a few inquisitive readers have sent in questions about meteorological phrases and abbreviations that weather forecasters often use. To be honest, these terms can make a forecast resemble a coded U.S. Navy antisubmarine warfare tactics manual.

Awkwardly, with the increased popularity of social media like Facebook and especially Twitter, even a greater number of shortenings will have to be deciphered. Some of these terms have been in use for many decades, while others are just a few years old.

A question I’m often asked: What is Z?

“Z” stands for Zulu Time, also known as Greenwich Mean Time (GMT). GMT refers to the mean solar time at the Royal Observatory in Greenwich, London. GMT is the same as “Universal Time Coordinated” (UTC) and allows meteorologist, and for that matter, everyone else, to synchronize to one standard time throughout the world.

Along the West Coast, Z time is eight hours ahead of Pacific Standard Time (PST) which occurs in late fall and winter, and seven hours ahead of Pacific Daylight Time (PDT) for the rest of the year.

Another question: What is GFS?

GFS stands for Global Forecasting System, a model that’s run every six hours (00 Z, 06 Z, 12 Z and 18 Z) by the National Oceanographic and Atmospheric Administration. So for example, the 12 Z (noon in Greenwich) run for the GFS weather model occurred at 4 a.m. Pacific time.

In addition to GFS, some other model abbreviations are: European Center for Medium-Range Weather Forecasts (ECMWF) and the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) brought to us by the U.S. Navy ‘s Fleet Numerical Meteorology and Oceanography Center in Monterey.

Another weather term that’s being thrown around quite a bit lately is Atmospheric Rivers (AR).

The term Atmospheric River hasn’t been around very long. None of my oceanographic and atmospheric textbooks show any reference to it. Turns out, the phrase was coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s.

These rivers in the sky can stretch for thousands of miles across the world’s oceans but are only a few hundred miles wide. They can draw huge amounts of water vapor into narrow bands ahead of cold fronts and transport fantastic amounts of water across vast expanses. In fact, they can carry more fresh water than the Amazon River.

Along the West Coast, they are informally called the “Pineapple Express.” The Pineapple Express is a subset of an atmospheric river event that originates in the tropical waters near Hawaii; hence the pineapples. In the past, meteorologist simply referred to these as “the hose.”

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative.

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