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Atmospheric River systems ranked? A new scale intends to do just that

Posted at 8:41 PM, Mar 04, 2019
and last updated 2019-03-05 16:51:43-05

The Central Coast is no stranger to “atmospheric rivers.” One is expected to impact the area Tuesday PM into Wednesday. (It’ll be just another in a series of these events the Central Coast has seen this year). The term essentially replaces “pineapple express” and refers to a narrow band of subtropical air driven by strong winds. The moisture heads toward the Western U.S., sometimes aimed right at the Central Coast.

The term isn’t exactly new. “Atmospheric river” was coined in 1998 by climate researchers, but the physical process has been there just as long as modern weather has happened. In the early 2000s, the term became more popular. Since atmospheric rivers (AR) contain as much water vapor as some hurricanes, they can deliver months worth of average rain in just a single storm. Understanding where AR events will hit is critical because they often produce flooding (80 percent of levee breaches in California’s Central Valley are associated with landfalling atmospheric rivers).

A team of researchers led by Scripps Institution of Oceanography at the University of California San Diego has created a scale to characterize the strength and impacts of “atmospheric rivers”.

The scale ranks ARs as follows:

  • AR Cat 1 (Weak): Primarily beneficial.
  • AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous.
  • AR Cat 3 (Strong): Balance of beneficial and hazardous.
  • AR Cat 4 (Extreme): Mostly hazardous, but also beneficial.
  • AR Cat 5 (Exceptional): Primarily hazardous.

The classification system is currently in a “beta” phase. It is not expected to be in widespread use for several years. For example, the current storm headed to California is not classified based on its forecast. Most AR storms to hit California annually are AR Cat 2 systems. The classification system is meant to simplify forecasts. It’s something emergency managers and county officials can use to prepare the public for potential storm impacts.

The classification system also has its critics who argue the system doesn’t actually change the forecast, rather, it may over-simplify it since AR events can be narrow. Just 50 miles north or south can make the difference between heavy rain or almost nothing at all. A simple classification system may not make those differences clear. Cite specific forecasts are already being made for storm details like rain, rain rates, wind, and potential impacts. A classification system doesn’t change what we already forecast, but the hope is an AR classification system would work much like the Saffir-Simpson scale for hurricanes which assigns simple numbers for potential storm impacts.

Here is a deeper dive into what an “atmospheric river” is, done by University of California TV: