Original 2019
Meteorologists nationwide have decided to begin rating atmospheric rivers (AR), storms that have the potential for millions of dollars in losses and severe flooding. However, 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 -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 percent 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 are a default feature of the entire global water cycle, and are present somewhere on the planet at any given time.
While most atmospheric rivers are small and beneficial, providing rain and snow to the western states, the massive atmospheric rivers can cause significant flooding. As identification and prediction of atmospheric rivers has improved, the scale was developed to let people know what to expect and how to prepare if the storm is going to be strong. Similar to hurricanes atmospheric rivers are ranked 1-5. The ranking uses 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. They 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 last less than 24 hours with an IVT between 500 – 750 is a category 1 storm. However, that same storm becomes a category 3 storm if it lasts more than 48 hours. The storm that stalls and stays for days is dangerous. Some experts say that Category 4 and 5 ARs can be compared to tornadoes ranked as EF 4 and 5 and hurricanes ranked as Category 3, 4, 5. Tornadoes ranked EF 4 and 5 have winds of 166 miles per hour and upwards. Hurricanes ranked 3 have winds between 11-130 mph, and storm surge of 9-12 feet. Category 4 hurricane winds are 131-155 mph, surge 13-18 feet and category 5 are above 155 mph and surge above 18 feet.
While no damage levels have been assigned to AR ratings, and the storms are different in nature, the following shows the damage associated with stronger tornadoes and hurricanes for reference. This may be usable as a loose guide when preparing for or assessing damage from an AR storm. The rating scale may change overtime, but this is a start on identifying and predicting another type of storm that may bring damage and claims with it.
| Category | Tornadoes | Hurricanes |
| 3 | Extensive damage-Large trees, some signs down. Structural damage to small buildings, homes; mobile homes destroyed. Serious flooding at coast, waves/debris damage large, destroys small structures. Roads cut off 3-5 hours before storm, flat terrain <5 feet above sea level flooded inland 8 miles or more. Local evacuations occur. | |
| 4 | Devastating damage=houses destroyed, some structures lifted off foundations and blown some distance, cars blown some distance, large debris airborne. | Extreme damage – Trees, all signs down. Structural damage severe. Complete failure of some roofs. Flat terrain < 10 feet above sea level flooded 6 miles inland. Major damage to lower levels of structures near shore. Roads cut off 3-5 hours before storm. Major beach erosion, all residences within 500 yards, single-story homes on low ground within 2 miles of shore evacuated. |
| 5 | Incredible damage, trees debarked, automobile-sized debris airborne, reinforced concrete structures damaged, strong frame houses lifted from foundations. | Catastrophic damage- Roof/window/door damage/ complete failure. Small buildings fail, some overturned, mobile homes destroyed. Major damage to lower floors of all structures < 15 ft. above sea level and within 500 yds. Roads cut off 3-5 hours before storm. Major erosion. Evacuations within 5-10 miles of shore. |
Christine G. Barlow, CPCU
Christine G. Barlow, CPCU, is Executive Editor of FC&S Expert Coverage Interpretation, a division of National Underwriter Company and Arc Network. Christine has over thirty years’ experience in the insurance industry, beginning as a claims adjuster then working as an underwriter and underwriting supervisor handling personal lines. Christine regularly presents and moderates webinars on a variety of topics and is an experienced presenter.