Rivers of rain

Now they’re called atmospheric rivers, a term that gives more respect to these thousands-of-miles long, hundreds-of-miles wide cascades of flooding rains that routinely make landfall in the fall and winter months. Typically, dozens of atmospheric rivers make landfall each year, usually in California, but Oregon and Washington aren’t exempt. In the 2023 season, there were 46 atmospheric river events in California. And when they arrive, they last for as long as three to four days, soaking the terrain for hundreds of miles inland. Two to four feet of total rainfall is routine.

As you might expect, the result is widespread flooding and mudslides. On the other hand, atmospheric rivers do have benefits. They bring half of California’s total annual rainfall, fill reservoirs, and put an end to drought and wildfires. As for aviation, it takes little imagination to think of the impacts: widespread instrument meteorological conditions in cloud and rain, plus icing conditions as the atmospheric rivers’ flows move up and over high terrain.

Atmospheric rivers tend to form when low pressure occurs in the central areas of the Pacific Ocean. (It’s those that originate near the Hawaiian Islands that prompted the Pineapple Express handle.) Convergent winds then lift the plentiful, warm, moisture-soaked air up into the westerly trade winds and then this soggy plume is on its way east, picking up more and more water vapor as it goes.

Atmospheric rivers are quantified by measurements of integrated vapor transport (IVT)—in other words, the weight of the water vapor flowing through the air, in a column from the surface to the flight levels. Depending on the amount of integrated vapor and the duration of the event, atmospheric rivers are further categorized from levels 1 (weak) to 5 (exceptional). In this sense, atmospheric river rankings are somewhat reminiscent of those used for categorizing hurricanes.

How much integrated vapor transport makes for a “weak” atmospheric river? A column of water vapor weighing from 500 to 750 kilograms (about 1,100 to 1,650 pounds). A Category 5 atmospheric river? More than 1,250 kilograms, or about 2,750 pounds.

Let us now give thanks, for the satellite sensors and computer models that make it possible for us to locate these atmospheric river plumes—and contour their IVT levels. Perhaps the best website for seeing current and forecast atmospheric river dimensions and movements is run by the Center for Western Weather and Water Extremes (CW3E). It shows not just Pacific Ocean plots but extends the notion of atmospheric rivers to other areas of the United States—like the notoriously rainy Mississippi and Ohio valleys—as well as the North Atlantic Ocean.

Any pilot thinking of a cool-season flight around the West Coast would do well to consult the posted IVT plots on the CW3E website. They make use of the Global Forecast System (GFS) model, so you can click on the future forecast Zulu times to see what the model’s predictions show. Just make sure the initialization times are up to date. Atmospheric river information is covered in forecast discussions and on public weather sources, but the IVT website gives more granular detail on the situations at hand.

Climatology is frequently considered too vague and general, and atmospheric rivers are certainly climatological features. The IVT plots give you a better idea of what’s in the offing. Remember: Climate is what to expect, weather is what you actually get.

Thomas A. Horne is a former editor at large for AOPA media and the author of Flying America’s Weather.