Ice-Jam Flooding and NWS Decision Support Services in Northern New York and Vermont During January 2018

Historic ice jam flooding occurred along the Lamoille and Missisquoi Rivers in Northern Vermont, and along the Ausable and Saranac Rivers in Northern New York during January 2018. Rapid river rises and ice breakup were induced by record breaking temperatures in the 50s to lower 60s °F (13-16 °C). These temperatures were more than 30 °F above normal for January yielding rapid snowmelt from the antecedent snowpack which was 8 to 16 inches (203 to 406 mm) above normal. Basin averaged rainfall totals of 0.6 to 2.1 inches (15 to 53 mm) onto frozen ground also contributed to excess runoff and contributed to the rapid rises.

Operational forecasters at the National Weather Service (NWS) forecast office in Burlington, Vermont utilized calculations of thawing degree hours (TDH) based on forecast temperatures to assess the potential for river ice breakup. With forecast values more than double the benchmark of 300 TDH required for river ice breakup, forecasters had increased confidence that ice jams would occur and issued a flood watch with 24 hours of lead time prior to the first flood reports. Significant ice jams remained in place after their initial occurrence, causing sustained concerns for several weeks in a few towns and communities.

Severe flooding in Swanton, Vermont and Plattsburgh, New York resulted in home evacuations. In Johnson, Vermont, high water on the Lamoille River backing up from an ice jam along State Route 15 flooded the post office and grocery store. The ice jams in Swanton and Johnson remained in place and resulted in a prolonged period of time that NWS Burlington provided Decision Support Services (DSS) - in the form of on-site visits, phone calls and email briefings - to the town Emergency Managers. DSS provided the needed lead time for Emergency Managers to order evacuations, preventing any casualties and minimizing property loses.

This presentation will detail effective messaging strategies unique to ice jams learned and utilized during the January 2018 event.