SEATTLE -- Hurricanes have the Saffir-Simpson Scale ("Category 5"), tornadoes have the Enhanced Fujita Scale ("EF-3") but now the West Coast has a storm scale of our own with a recent introduction of a new rating system for "atmospheric rivers" -- the causes of a vast majority of our annual autumn and winter flooding damage.
An "atmospheric river" is a term when a large storm system drives a narrow but intense band of moisture into a region. They occur worldwide but are particularly noteworthy along the West Coast where they are responsible for nearly all of the major flooding events, especially in Washington and Oregon. You might have heard the term "Pineapple Express" which is a type of atmospheric river -- just named for a specific one that has its tropical moisture origins near Hawaii.
"Over a decade of research and demonstration has led to the identification of AR type storms as key to both water supply and flooding in the western US," says Marty Ralph, Director for the Center for Western Weather and Water Extremes (C3WE) at UC San Diego's Scripps Institution of Oceanography, which developed the new rating system. "The AR scale was developed to fill the need expressed by the public and decision makers to be able to distinguish the largely hazardous ARs (the intense and long-lasting ARs) from those that primarily provide beneficial rain and snowpack."
MORE | Atmospheric River FAQ
The new scale rates the intensity of an AR event from 1-5 by measuring both its amount of water vapor embedded in the storm ("IVT" - Integrated Vapor Transport) and the expected storm duration.
The storm gets an initial classification based on the amount of water vapor measured in the storm. An IVT rating of 250 is considered the minimum amount for AR conditions, with each additional 250 considered another rung up the scale. A strong event (AR 3) is considered at 750-1000 with "exceptional" AR 5 values at 1250 or more.
Overall, AR 1s and AR 2s are considered "beneficial" events -- keeping the rivers full and water levels good without overloading the drainage system. These are quite common over our fall and winter and a lifeline to California to restock their reservoirs.
An AR 3 usually means enough rain that we start to lose the balance -- it's good in that it brings needed rain but now were starting to see potential flooding issues. The researchers term this level "both beneficial and hazardous".
When we get into AR 4 storm, it's getting into typical overload status with mostly hazardous impacts (though maybe some fringe benefits), and a top-rung AR 5 event is almost always hazardous frequently leading to major flooding events with past events leaving damage that has reached into the hundreds of millions if not more, Ralph said.
But unlike hurricanes and tornado scales which are given a blanket rating based on wind speed alone, the new AR scale will be adjusted based on its duration, as an intense rainstorm that lasts an hour might not have as much impact as storm that rains less heavily but lasts all day or more.
WEATHER GEEK STUFF: Atmospheric River Forecast Page
A typical atmospheric river lasts 24-48 hours. Any AR event that is expected to last more than 48 hours gets pushed up one level, while any storm that will be done in less than 24 hours gets downgraded a level.
Ralph says this is a challenge of other scales, such as the Saffir-Simpson scale. He noted that recent slow-moving Hurricanes Harvey and Florence caused catastrophic flooding as feet of rain fell but their categories were solely based on their wind speeds.
Also unlike hurricane and tornado ratings, where the entire storm gets a single rating like "Category 3", the AR scale varies geographically depending on the storm's expected impact at that location. So areas under the brunt of the storm can get a higher-rated AR than areas more on the fringe. A storm that hits the Olympic Mountains might see AR 4 or AR 5 conditions but areas along the central Oregon Coast may only see AR 2 conditions from the same storm.
Almost all flooding damage in the Northwest is from atmospheric rivers
The research into better understanding ARs and their potential impact is important because Ralph's team found 90-99% of all flood-related damages and insurance claims in Western Washington and Western Oregon are from atmospheric rivers.
In the Puget Sound region, Pierce and King County both report 97% of all flood insurance losses were from atmospheric river events while it's just over 90% in Snohomish County. In Lewis County it's nearly 99 percent!
Clackamas County in Oregon was also quite high at 97 percent.
Ralph found that expected flooding damage increased exponentially with each level on the AR Scale. Damage was minimal to zero in AR 1 an AR 2 events, while averaging around $3 million in AR 3 events. But AR 4 events averaged $20 million while AR 5 events average $260 million in damage with some individual AR 5 events in California costing over $1 billion in damage.
But current conditions can affect how much of an impact an atmospheric river can have. Very early season major AR events, such as the AR 4/5 event of Sept. 23, 2020, were not expected to have much impact despite 7-10 inches of predicted rain in the mountains because the rivers were low and the soils were dry coming off the very dry summer season, allowing the region to take in that much water without flooding.
Ralph says his research team found the region typically gets about a 5-6 week reprieve at the start of autumn where rivers are low and soils are dry before these storms begin to have major impact.
But if you were to copy-and-paste a similar storm into December or January where we've had prior heavy rain events, the rivers are fuller, the mountains are snowier and the soils are more saturated, this could be a major event