How strong is the atmospheric river about to hit SF?

San Francisco and the rest of the Bay Area are in for an unseasonal soaking starting Wednesday evening, raising the possibility of rain as late in the week as Sunday during Bay to Breakers frivolity.

This kind of storm is known as an “atmospheric river,” but that terminology refers both to storms powerful enough to cause major flooding and damage or mild enough to be considered part of the normal yearly precipitation cycle.

Until recently, researchers who study weather had no easy way of classifying these distinctions.

According to the National Oceanic and Atmospheric Administration, the term “atmospheric river” is pretty close to literal, referring to huge streams of water ferried through the sky in volumes comparable to waterways:

These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. [...] Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks—particularly in the western United States.

While atmospheric rivers are responsible for great quantities of rain that can produce flooding, they also contribute to beneficial increases in snowpack.

According to the forecast from the National Weather Service, this incoming storm—part of a series which will most likely yield significant rain every day this week and weekend except for Friday—does not bring any particular danger of flooding. But the absence of a potential emergency doesn’t tell us much about how potent it may be.

In February, researchers at Scripps Institution of Oceanography at the University of California San Diego published a new scale to measure the intensity of storms just like this one, similar to the widely cited scale used to measure the strength of hurricanes.

“The presence or absence of a few AR events can ‘make or break’ precipitation over the course of the water year,” meteorologist F. Martin Ralph wrote in the American Meteorological Society journal, noting that it’s unhelpful for states like California to have to rely on this key weather phenomenon without an easy way to measure severity.

Ralph’s scale designates atmospheric rivers as categories 1 through 5. An AR Category 1 is deemed “primarily beneficial,” bringing with it the smallest risk of flood danger and mostly consisting of just helpful precipitation.

An AR Category 5 is “primarily hazardous,” the kind of storm event that can last more than 140 hours and cause flooding up to 50 feet in the examples provided in the journal.

So how bad is this storm? According to the “atmospheric river outlook” produced by the Scripps Institution this week, “The ARs forecast to make landfall are predicted to produce AR Category 1 conditions for much of coastal Northern California and Oregon” starting today.

“The additional AR activity forecast to potentially make landfall between 18 and 22 May 2019 could bring AR Category 2 condition,” the forecast adds, meaning “moderate” severity that’s still mostly beneficial, but this is more likely further south.

Note that the outlook also anticipates as much as 3.5 inches of rain in higher elevations and as much as two inches in lower areas like the Bay Area, so while the hazards are not pronounced residents should still probably batten down the hatches. Just in case.