Toward Improved Consistency of Flash Flood Product Issuance in the Southwestern United States

In an effort to achieve an increased level of consistency and collaboration both within the National Weather Service (NWS) Flagstaff Weather Forecast Office (WFO) and between NWS WFOs in the southwestern United States regarding the issuance of flash flood watches and warnings, the forecast quality team at NWS WFO Flagstaff has been working on a multi-faceted project. The research subgroup examined upper air, satellite and surface data from past flash flood events in the WFO Flagstaff hydrologic service area (HSA) to find some common attributes. The research subgroup found that optimal conditions for an event associated with multiple flash flood reports occurred when northern Arizona was positioned on the western edge of a ridge aloft, with southerly to southeasterly flow below 500 hPa and dry southwesterly flow above 500 hPa. The information gathering subgroup consulted our own staff, key partners and the public to obtain their opinions and suggestions regarding our office flash flood program. Key partners were contacted via telephone, while the public was queried both via social media and in discussion groups during an open house at the Flagstaff WFO. The information gathering subgroup also consulted neighboring WFOs to obtain their current local flash flood watch criteria and flash flood warning guidance. Another subgroup locally clarified the working definition for the term “flash flood”.

In the coming weeks and months, several more subgroups will continue to move the flash flood watch/warning consistency project forward. One subgroup has obtained code which generates a gridded flash flood index across our entire HSA based on the office's gridded forecasts and model data. This subgroup will make changes to the flash flood index inputs based on local research. Other subgroups will develop and execute plans for a flash flood product verification scheme, partner/public education, and staff training. As the project continues, we plan to work closely with our neighboring WFOs to both share the results that we have obtained and coordinate any changes to our own criteria and guidance. Data collected from examination of past events and interactions with external users will support any suggested changes to our local flash flood watch criteria and flash flood warning guidance. The forecast quality team hopes to complete all aspects of the project and finalize a comprehensive solution by the start of the 2016 convective season. The status of the WFO Flagstaff flash flood product consistency project to date will be shared at the meeting, along with available results.