Extending atmospheric river research to the Pacific Northwest and 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. However, following a record high level of water behind HHD in January 2009, the U.S. Army Corps of Engineers (ACE) became concerned about the Dam's safety. Flood damage prevented by HHD from the January 2009 event is estimated at about $4 billion. 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. 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 Weather Forecast Office), there were no threatening floods observed along the Green River, primarily because the overall synoptic patter was progressive, 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. This paper will report on initial scientific findings resulting from the ARO deployments including orographic precipitation impacts and will document use of the ARO observations in daily forecast operations.