Differentiating Between Impactful and Non-Impactful Atmospheric River Events in Southeast Alaska

In Southeast Alaska, extreme precipitation triggered by atmospheric rivers (ARs) can result in impacts such as floods and landslides. Recent research has demonstrated a strong relationship between AR strength and extreme precipitation in Southeast Alaska. However, the National Weather Service (NWS) Juneau office has found that while forecasted AR magnitude and duration communicates the likelihood of a hazardous event, it does not sufficiently explain impacts to the general public and emergency management personnel. Therefore, ongoing collaborative efforts with NWS Juneau are pursuing the development of AR-based forecasting tools that leverage the strong relationship between ARs and extreme precipitation, but also consider other important factors such as antecedent soil/river conditions, freezing level, AR direction, and the occurrence of multiple ARs within a short period of time. Using National Oceanic and Atmospheric Administration’s (NOAA) Global Ensemble Forecast System, version 12 (GEFSv12) re-forecast data, Automated Surface/Weather Observing Systems (ASOS/AWOS) and Cooperative Observer Program (COOP) station precipitation data, NOAA Atlas 14 precipitation frequency estimates, and a catalog of impactful floods and landslides provided by NWS, this study analyzes AR events in Southeast Alaska to better differentiate between impactful and non-impactful ARs with otherwise similar characteristics. We generate a probabilistic range for low, medium, and high impact of all past AR events between January 2000 and December 2019 that aligns well with known previous impacts. This methodology can be applied to future forecasts to improve NWS Juneau forecasters' situational awareness and Impact Decision Support Services (IDSS) messaging before and during high impact weather events. This will allow NWS Juneau to communicate impact information caused by ARs to deep core partners (e.g., emergency managers, fire and/or police chief, village police safety officers, etc.) and the public, so they can take appropriate actions to prepare their community for impactful weather events.