NATIONAL WEATHER SERVICE PORTLAND SUPPORT FOR POST-FIRE FLOOD CONCERNS IN NORTHWEST OREGON FOLLOWING THE 2020 LABOR DAY FIRES (Invited Presentation)

The 2020 Labor Day fires devastated western Oregon on a scale not seen for generations. Along with the immediate concerns about loss of life, air quality, and property destruction, the potential for floods and debris flows garnered attention by geologists and hydrologists. Assessments of burn severity and debris flow threat were conducted in fall 2020, and the expected transition from dry and hot weather in September to the winter rainy season in November added urgency to the post-fire efforts.

The mission of the National Weather Service (NWS) is to “provide...forecasts and warnings...for the protection of life and property and the enhancement of the national economy.” For the NWS Weather Forecast Office (WFO) in Portland, Oregon, this mission took on a new dimension, with the potential for post-fire floods and debris flows and the threat to lives and infrastructure.

NWS Portland has a long history of providing flood services. Unlike much of the western U.S., flash flooding due to brief, heavy rain is a rare occurrence in NWS Portland’s forecast area. Historically, almost all floods are caused by stratiform precipitation on a time scale of 1 to 3 days. Following the Eagle Creek fire east of Portland in August 2017, NWS Portland began to incorporate services for post-fire flash flood and debris flow threats, and these services were greatly expanded following the multiple fires of September 2020.

Representatives of the NWS were active in BAER teams led by various federal and state agencies to evaluate the potential impacts of the 2020 fires in western Oregon. NWS staff focused on identifying locations and communities most vulnerable to flash floods and debris flows and then identifying potential data gaps and locations where new weather stations or rain gauges were needed. NWS Portland has also engaged with state, county, and local emergency managers to develop forecast and outreach services to enhance flood preparedness, mitigation, and response. In addition to the normal suite of forecasts, watches and warnings, NWS Portland developed an email briefing with distribution to appropriate officials with jurisdiction over the areas affected by the 2020 fires. Based on feedback from these officials, NWS Portland initiated a two-pronged approach to keep them informed: 1) a weekly overview assessing the daily risk for all of northwest Oregon, and 2) a burned-area update when there is a threat of heavy rain, flash floods, and debris flows caused by storms forecasted over the next 24 to 48 hours.