New rating system for Pacific storms

Meteorologists nationwide — the National Weather Service, California Department of Water Resources, U.S. Geological Survey and U.S. Army Corps of Engineers — have decided to begin rating atmospheric rivers (AR), storms that have the potential for millions of dollars in losses and severe flooding. But what exactly is an atmospheric river? It’s a long, narrow river of condensed water vapor in the atmosphere that moves with the weather.

A strong river can carry between 7.5 to 25 times the flow of water that passes through the mouth of the Mississippi River. These rivers in the sky can be 300 miles wide, a mile deep and more than 1,000 miles long. Most are small, but they can be massive and fast-moving, and when the storms make landfall, they often release rain or snow, sometimes in large amounts. They account for 50% of the annual precipitation in California and are the West Coast’s big storms.

The storms develop in the tropics and are also known as the “Pineapple Express.” They’re a default feature of the entire global water cycle, and are present somewhere on the planet at any given time.

Although most atmospheric rivers are small and beneficial, providing rain and snow to the western states, the massive atmospheric rivers can cause significant flooding.

Category definitions

As identification and prediction of atmospheric rivers has improved, a scale was developed to let people know what to expect and how to prepare if the storm is going to be a strong one. Similar to hurricanes atmospheric rivers are ranked 1–5. The ranking is based on Integrated Water Vapor Transport (IVT), which is the amount of water vapor in the system and the wind moving it around, and the duration of the storm.

• AR Category 1 is considered weak and is seen as primarily beneficial, lasting a short period of time, and producing modest amounts of rain. These storms last up to 48 hours and have an IVT of 250–500.
• AR Category 2 is considered moderate and mostly beneficial. These storms last longer and help replenish rivers, reservoirs and streams. These storms last up to 72 hours with an IVT of 250–1000.
• AR Category 3 is considered strong, beneficial and hazardous. Such storms may push rivers to near flood stage, and last up to 72 hours and beyond with an IVT between 500–1250.
• AR Category 4 is extreme, mostly hazardous, but also beneficial. These are larger storms dropping more rain, which pushes rivers even closer to flooding. They last up to 72 hours and beyond with an IVT of 750–1250 or greater.
• AR Category 5 is exceptional and hazardous. An AR 5 storm can bring as much rain as the strongest Atlantic storms and cause mudslides, loss of power and flooding.

A longer duration or stronger intensity can cause a storm to change categories. For example, a storm that lasts less than 24 hours with an IVT between 500–750 is a category 1 storm. That same storm becomes a category 3 storm when it lasts more than 48 hours, however.

The storm that stalls and stays for days is dangerous. Some experts say that Category 4 and Category 5 ARs can be compared to tornadoes ranked as EF 4 and EF 5 and hurricanes ranked as Category 3, Category 4 or Category 5. Tornadoes ranked EF 4 and EF 5 have winds of 166 miles per hour and upwards. Hurricanes ranked Category 3 have winds between 11–130 mph, and storm surge 9–12 feet. Category 4 hurricane winds are 131–155 mph, surge 13–18 feet and Category 5 are above 155 mph with surge above 18 feet.

Damage levels

Although no damage levels have been assigned to AR ratings, and the storms are different in nature, the following chart shows the damage associated with stronger tornadoes and hurricanes for reference. Insurance agents and claims adjusters may be able to use the chart as a starting point when preparing for or assessing damage from an AR storm. The rating scale may change over time, but this is a start on identifying and predicting another type of storm that may bring damage and claims with it.