Applying the WPC Excessive Rainfall Outlooks in Decision Support

The Weather Prediction Center (WPC) is committed to crafting a scientifically-robust Excessive Rainfall Outlook (ERO) – one that relates easily to users, is well calibrated, and solidifies WPC’s role within the National Weather Service information cascade ahead of flash floods. In October 2017, the ERO moved from a point-based to neighborhood approach that yields more relatable probabilities (e.g., High Risk >= 50% probability of rainfall exceeding Flash Flood Guidance within 40 km (25 miles) of a point; legacy point-based threshold was 15%) and is consistent with Storm Prediction Center Outlooks. The text discussion component, where forecasters add value by synthesizing their thought process, high-resolution meteorological and hydrologic guidance (e.g., full suite of National Centers for Environmental Prediction hi-res windows; National Water Model; High-Resolution Rapid Refresh hourly precipitation on saturated soil, etc.), and plausible worst-case scenarios was then added to the Days 2 and 3 projection - a critical planning period for users.

In recent years, developers have provided WPC forecasters a number of real-time verification tools focused on forecast goodness and accuracy so that EROs remain calibrated. Increasingly, resources applied to the Excessive Rainfall Outlooks are commensurate with their importance. WPC has worked with local NWS offices to establish more aggressive criteria for initiating real-time coordination so that all have a stake in the message carried by this powerful forecast vehicle. User feedback speaks to a palpable hunger for national scale, probabilistic flash flood outlooks, and WPC is poised to continue leading this effort. This presentation will summarize the science inputs to this manually crafted outlook product, examine ERO performance (particularly High Risks), and, using observed example cases, demonstrate how the ERO can ideally play a role in end-to-end forecasts of both large scale and small scale flash floods.