Rapid Response to the Howard Hanson Dam Crisis

Dedicated in 1962, the Howard A. Hanson Dam (HHD) brought necessary flood relief to the Green River Valley in the Metropolitan Area of Seattle, Washington, and opened the way for increased valley development. For example, the flood damage prevented by HHD during the extreme precipitation event in early January 2009 is estimated to be about $4 billion. However, following the record high level of water behind HHD caused by this event, the U.S. Army Corps of Engineers (ACE) became concerned about the Dam’s safety. Despite short-term measures to improve HHD during 2009, the chance for a significant Green River Valley flood event was estimated by the ACE to be 1 in 25 for the 2009/10 winter season. In response to this elevated risk, NOAA organized a coordinated effort across research and forecast operations to implement new observations, modeling and dissemination tools, and knowledge of the role of atmospheric rivers (ARs) in producing extreme precipitation, prototyped in California within NOAA’s Hydrometeorology Testbed (HMT; hmt.noaa.gov). Fortunately, although seasonal observed precipitation (October 2009 through March 2010) in the Seattle area was slightly greater than normal (e.g., 112% of normal at the Seattle National Weather Service Forecast Office), there were no threatening floods observed along the Green River. This outcome was influenced by a synoptic pattern that was progressive, i.e., storms did not stall over this vulnerable watershed, which reduced the chance for an extensive (time and space) extreme event. More than a decade of West Coast winter storm research conducted primarily in California by NOAA’s Earth System Research Laboratory/Physical Sciences Division (ESRL/PSD) has identified atmospheric rivers (ARs), narrow regions of enhanced water vapor transport, as the culprits that cause extreme precipitation events, such as the January 2009 event that stressed HHD. ESRL/PSD extended this AR research to the coast of Washington by deploying a mobile AR observatory (ARO) at Westport, Washington, in October 2009. Development of the mobile ARO is based on two decades of instrument and technology development at ESRL/PSD. ESRL/PSD also responded to the HHD crisis by rapidly deploying a fixed ARO couplet closer to HHD in order to detect and monitor the AR conditions that potentially could lead to flooding along the Green River. These deployments complemented a set of newly telemetered rain gauges surrounding the Green River basin provided by the NWS Western Region Headquarters. This paper will report on initial scientific findings resulting from the ARO deployments including recent AR results for Washington and will document use of the ARO observations in daily forecast operations.