How the Recently Developed Atmospheric River Scale Can Improve NWS Forecasts and Services

Atmospheric rivers (ARs) and related impacts have been the topic of increasing study over the past several years. However, most studies establish one minimal threshold, above which every feature is an AR, with no differentiation between ARs of widely varying intensity. The recently developed AR Scale (Ralph et al., in review; online publication expected prior to conference) characterizes AR strength and impacts based on the intensity of vertically-integrated water vapor transport (IVT) and the duration of AR conditions (IVT ≥ 250). The AR category is calculated by first determining the maximum IVT intensity during the event:

IVT < 250 kg m^-1 s^-1: Not an AR
250 ≥ IVT < 500 kg m^-1 s^-1: AR Cat 1
500 ≥ IVT < 750 kg m^-1 s^-1: AR Cat 2
750 ≥ IVT < 1000 kg m^-1 s^-1: AR Cat 3
1000 ≥ IVT < 1250 kg m^-1 s^-1: AR Cat 4
IVT ≥ 1250 kg m^-1 s^-1: AR Cat 5

Second, event duration is considered. If duration is < 24 h, the AR is “demoted” by one category; if duration is ≥ 48 h, the AR is “promoted” by one category. For example, and AR with maximum IVT of 575 kg m-1 s-1 and event duration of 54 h is classified as an AR Cat 3 event.

Information regarding the use and interpretation of the AR Scale has been disseminated throughout western U.S. NWS forecast offices, as this scale will likely be a focal point for media and public interest during the 2018/2019 cool season. In this study, a selection of previous AR events are used to demonstrate that impacts vary as a function of AR category, and to highlight the potential that forecasts of AR category have to benefit the decision support services and messaging provided by NWS offices. Future work includes outreach to stakeholders, partners, and other users to more objectively determine how they respond to forecast messaging based on the AR Scale.