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

Atmospheric rivers (ARs) driven by strong vertically-integrated water vapor transport (IVT) trigger floods, landslides, and avalanches that threaten lives and livelihoods across Southeast Alaska. This study investigates this relationship by focusing on extreme precipitation days (precipitation >95th percentile) that are associated with ARs. ARs reach Southeast Alaska 8–15 days per month, yet only six AR days per year account for up to 93% of extreme precipitation days. Furthermore, 80–96% of extreme precipitation days have >75th percentile IVT, demonstrating the strong relationship between IVT and extreme precipitation. However, the National Weather Service (NWS) Juneau has found that forecasted AR intensity alone does not sufficiently explain impacts to the public and emergency managers. Therefore, collaborative efforts with NWS Juneau are pursuing the development of AR-based forecasting tools that leverage the strong relationship between AR intensity and extreme precipitation, but also consider freezing level, low-level winds, and AR direction.
For example, in Yakutat, 80–90% of extreme precipitation days have south-southwesterly or south-southeasterly IVT, while extreme precipitation days in Klukwan and Skagway are most common with southwesterly IVT. Additionally, we demonstrate the utility of a Model Climate (M-Climate) based on NOAA’s Global Ensemble Forecast System reforecast data. This places other important forecast elements in the context of reforecasts with the same lead time and seasonal window. For example, large M-Climate IVT anomalies indicate ensemble-mean IVT values are large for a given lead time. This has an advantage over comparisons to reanalysis climatologies, which feature smaller forecast anomalies at longer lead times due to averaging of ensemble members. This methodology can be applied to future forecasts to improve forecasters' situational awareness and Impact Decision Support Services messaging before and during high impact weather events.