https://www.sciencedaily.com/releases/2023/03/230309164716.htm

Atmospheric rivers typically form when warm temperatures create moist packets of air, which strong winds then transport across the ocean; some make landfall. The intensity scale ranks these atmospheric rivers from AR-1 to AR-5 (with AR-5 being the most intense) based on how long they last and how much moisture they transport.

In part because some West Coast weather outlets are using the intensity scale, "atmospheric river" is no longer an obscure meteorological term but brings sharply to mind unending rain and dangerous flooding, the authors said. The string of atmospheric rivers that hit California in December and January, for instance, at times reached AR-4. Earlier in 2022, the atmospheric river that contributed to disastrous flooding in Pakistan was an AR-5, the most damaging, most intense atmospheric river rating.