Integrating Research with Community Outreach to Improve Messaging and Forecast Methods for Mendenhall Glacier Dammed Lake Outburst Floods and Their Impacts to Downstream Residents in Juneau, Alaska

Glacier lake outbursts floods (GLOFs) from Mendenhall Glacier’s Suicide Basin glacier dammed lake near Juneau, Alaska began in 2011. The resulting GLOFs have produced record water levels and river discharges in Mendenhall Lake and River, and inundated the highly populated valley. Glacial dammed lakes pose unique forecasting and communication challenges, and the National Weather Service (NWS) Juneau Forecast Office and the Alaska-Pacific River Forecast Center are tasked with forecasting these dynamic annual outburst flood events. The forecast method relied upon using previous event hydrographs as analogs to estimate river discharge during an event. The 5 August 2023 record GLOF exceeded all previous events by a wide margin causing widespread inundation and erosion. The National Weather Service responded to this event by updating its forecast and messaging strategy.

Immediately following the first Mendenhall GLOF in 2011, research efforts to quantify glacier runoff dynamics has been a joint partnership among the NWS, the University of Alaska Southeast, and the U.S. Geological Survey. A reevaluation of historical Mendenhall outburst floods found that the event hydrographs had similar shape and duration. A hydrograph scaling method based on the range of potential release volumes was developed to estimate a maximum release hydrograph using the current volume impounded in the glacial dammed lake. This method requires an understanding of the glacier dammed lake bathymetry, which changes over time, and the current lake water surface elevation to calculate lake volume in real time.

The Mendenhall GLOFs are unique among Alaska’s glacier lake outbursts. Prior to 2023, each outburst flood was only a partial release of the total glacier dammed lake volume capacity. Immediately after the record August 2023 flood, conversations between the National Weather Service and its core partners recognized the need to improve its Impact Decision Support Services (IDSS) during the next large Mendenhall glacier dammed lake release.

For 2024, the NWS coordinated with research partners at the University of Alaska Southeast, U.S. Geological Survey, core Federal, State of Alaska, Tribal, and Municipal partners to prioritize data needs and a timeline for monitoring and messaging. We determined that communicating the uncertainty of the GLOF forecast was critical to support local decisions. Therefore, NWS developed probabilistic event hydrographs from historical release events and included these within a daily “what if the glacier dammed lake released now” forecast scenario that included flood impacts, which were then shared for situational awareness. This preparation led to improved community IDSS during yet another unprecedented 6 August 2024 record Mendenhall GLOF. Despite catastrophic flood inundation, the pre-outburst flood forecast scenarios and improved messaging prepared Juneau to respond and take action, saving lives and property. As the Mendenhall Glacier rapidly changes, Juneau’s impacts from future GLOFs remain uncertain. NWS continues to collaborate with research partners and local community leaders to improve its understanding of these dynamic events as it relates to future GLOF forecasting, and effectively communicate the forecast uncertainty of these very impactful events.