Macro Trigger Point Thresholds for Longer Forecast Lead Times on the Green River

As various agencies and jurisdictions developed their Green River flood emergency operation plans it became very clear that, among other elements, complicated evacuations, staging of resources and equipment, and addressing vulnerable population needs were going to require considerable time to execute prior to the onset of flooding; well beyond the established NWS goal of approximately 72 hours for a Flood Potential Outlook. In addition to the long forecast lead time requirements there was a clear need to get the various emergency responders to agree to a few key levels of concern that would ensure they were elevating actions in a coordinated fashion.

FEMA Region X hosted a meeting in early November to address these key decision thresholds. Participants included: the Green River cities, King County, U.S. Army Corps of Engineers, State of Washington, FEMA Region X, and the National Weather Service – both WFO Seattle and the NW River Forecast Center. Several actions came out of that first meeting. Most critically, the USACE and the NWRFC agreed to identify at least two key flow levels and the associated mean areal precipitation thresholds required to reach those flows. The outcome of their analyses resulted in rather simple, “macro level” thresholds that could serve as the basis for the NWS to issue focused flood potential statements through 5 days (120 hours); thus addressing two critical concerns.

Once the thresholds were established the NWS Hydrometeorological Prediction Center and the Environmental Modeling Center developed several key objective guidance products to be used by the WFO Seattle forecasters in extending forecast lead time. Special product formatters were also developed at WFO Seattle to minimize the workload of the twice-daily forecast.

This multi-agency collaboration brought considerable clarity to the response planning and actions. The NWS was able to deliver a very specific decision support service to a spectrum of partners and emergency planners that aided in the coordination and planning of all those groups involved with the Green River flood threat. This presentation will give an overview of the background information, development process, and execution.