Atmospheric Rivers

The average atmospheric river carries roughly as much water as flows out of the mouth of the Mississippi. The exceptional ones carry up to fifteen times that. None of it is visible as water — it is vapour, moving invisibly through the air above the ocean in a long, narrow filament, and it only becomes weather when it hits something.

What the comparison actually measures

Comparing a river in the sky to a river on the ground takes a conversion step, and it is worth understanding because it explains why the numbers sound implausible. Scientists calculate the mass of water vapour moving through the atmospheric river and divide by the density of liquid water. The result is the volume that vapour would occupy if you condensed all of it, expressed as a flow rate. That is what gets set beside the Mississippi.

The definition of the feature itself is straightforward: a relatively long, narrow region of the atmosphere that transports water vapour. What is striking is the share. Atmospheric rivers move most of the water vapour travelling outside the tropics. Not a meaningful fraction — most of it. The rest of the mid-latitude atmosphere is comparatively dry, and the moisture that matters is concentrated into these corridors.

They vary enormously. Most are weak, and a weak one delivers a useful soaking and nothing more. The ones that cause trouble are those combining the largest vapour loads with the strongest winds.

Why they matter on west coasts

An atmospheric river over open water is a transport system with nowhere to unload. Vapour stays vapour as long as the air holding it stays warm enough. Condensation is not about reaching some fixed temperature; it is about the gap between the air temperature and the dew point, the temperature at which air becomes saturated and can hold no more vapour. Close that gap and the vapour has to go somewhere.

Landfall closes it. When moist air runs into a coastline backed by terrain, it is forced up and over, and rising air cools. If the cooling is sufficient, vapour condenses into cloud. Cool it further and you get rain or snow. This is orographic lifting, and it is the reason precipitation totals climb steeply with elevation while a valley station a short distance away records something far more modest.

The best-known corridor is the Pineapple Express, a strong atmospheric river capable of pulling moisture from the tropics near Hawaii across to the United States West Coast. It is a specific case of the general pattern: a supply line anchored in warm tropical water, aimed at a coast with mountains behind it.

The same storm fills reservoirs and drowns people

A series of atmospheric rivers drove the storms that struck the West Coast from western Washington to southern California between 10 and 22 December 2010. Certain areas recorded 11 to 25 inches of rain. The same systems loaded the Sierra Nevada with snow: by 22 December, the first full day of winter, the range had received 75 percent of its annual snowfall.

That is the whole tension in one event. Atmospheric rivers are tied directly to water supply in the western United States, and snowpack is the most valuable form the delivery can take, because it releases slowly through spring rather than all at once. Rain arriving on the same day at lower elevation runs off immediately.

Flooding follows when the corridor stops moving. A stalled atmospheric river parked over a watershed vulnerable to flooding keeps the supply line open for hours, and the same principle applies at smaller scale with thunderstorms — repeated cells crossing the same ground, which forecasters call training. Either way the ground saturates and stops absorbing, and everything that falls afterwards becomes runoff. The result can include catastrophic debris flows and mudslides on steep terrain, and floods that roll boulders, tear out trees, and scour new channels.

What to look for, and what the gauges miss

The useful thing to notice is not the storm but the gradient. After a landfalling atmospheric river, compare rainfall totals from a station on high ground with one in a nearby valley. The difference is orographic enhancement made legible, and it is why forecasts of a storm's impact depend as much on terrain as on the storm.

Rainfall varies over remarkably short distances even without mountains. When Tropical Storm Allison sat over southeast Texas for five days in June 2001, totals across Harris County ranged from under 5 inches to nearly 37. Official gauge networks are sparse enough that this variation frequently goes unrecorded. The flood that hit Fort Collins, Colorado in 1997 produced the clearest demonstration: when researchers went back through the precipitation data afterwards, the heaviest rainfall had missed every official gauge. The volunteer CoCoRaHS network grew out of that failure and now has over 26,000 active observers. Joining it is the most direct thing an interested person can do about the problem.

Forecast skill for these events has improved. Research using satellite, radar and aircraft observations, together with numerical model improvements, now lets National Weather Service forecasters issue warnings for potential heavy rain and flooding as much as five to seven days in advance in areas prone to atmospheric river impacts. Where the science remains harder is in pinning down exactly where the corridor makes landfall and how strong it will be when it does — the difference between a beneficial soaking and a disaster is often a matter of a hundred miles and a few hours of stalling.

The hazard is almost always the water on the road

Flooding kills more people than any other weather hazard except heat, with a 30-year national average of 88 deaths per year, against 41 for lightning and 68 for tornadoes. Nearly half of flash flood fatalities are vehicle-related.

The reason is a consistent underestimate of what moving water does. Six inches of fast-moving flood water will knock an adult off their feet. Twelve inches will carry away a car. Twenty-four inches will move an SUV. A flooded road also hides whatever has happened to the road bed underneath, which may no longer exist. If you come to one, turn around.