Atmospheric rivers (ARs) play vital roles in the western United States and related regions globally, not only producing heavy precipitation and flooding, but also providing beneficial water supply. This paper introduces a scale for the intensity and impacts of ARs. Its utility may be greatest where ARs are the most impactful storm type and hurricanes, nor’easters, and tornadoes are nearly nonexistent. Two parameters dominate the hydrologic outcomes and impacts of ARs: vertically integrated water vapor transport (IVT) and AR duration [i.e., the duration of at least minimal AR conditions (IVT ≥ 250 kg m–1 s–1)]. The scale uses an observed or predicted time series of IVT at a given geographic location and is based on the maximum IVT and AR duration at that point during an AR event. AR categories 1–5 are defined by thresholds for maximum IVT (3-h average) of 250, 500, 750, 1,000, and 1,250 kg m–1 s–1, and by IVT exceeding 250 kg m–1 s–1 continuously for 24–48 h. If the AR event duration is less than 24 h, it is downgraded by one category. If it is longer than 48 h, it is upgraded one category. The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snowpack, while stronger ARs can become mostly hazardous, for example, if they strike an area with antecedent conditions that enhance vulnerability, such as burn scars or wet conditions. Extended durations can enhance impacts. Short durations can mitigate impacts.

An anomalous lightning event occurred on 20-21 June 2008 in central and northern California. Most of the thunderstorms produced frequent lightning with little precipitation, resulting in over 1500 new fires. Many of these fires became large, and massive firefighting efforts continued through August in order to contain all the fires. This event was not well forecast, with only isolated lightning strikes expected. Several computer model forecast parameters pointed toward the impending lightning outbreak, including satellite and lightning observations, as well as model data from 19 June. An existing procedure to forecast dry thunderstorms showed the strong possibility of a thunderstorm outbreak over California. This procedure proved to be helpful, but has been modified to better handle moisture advection and low-level forcing, which provides a more comprehensive framework for forecasting potential dry thunderstorm outbreaks.