Verifying, Calibrating, and Redefining the Excessive Rainfall Outlook at the Weather Prediction Center

The Excessive Rainfall Outlook (ERO) issued by the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed one, three, and six hourly flash flood guidance (FFG). FFG is generated by River Forecast Centers (RFCs) and represents the amount of rainfall that is expected to bring rivers and streams quickly to bankfull conditions. WPC uses a variety of atmospheric models, situational awareness, and other tools to generate EROs valid for days one (current day), two (tomorrow) and three (two days into the future). The ERO consists of four categories: marginal, slight, moderate and high, each representing different probabilities of precipitation exceeding FFG. However, ERO forecasts have not been extensively verified against FFG or any flood observations in the past two years. Furthermore, WPC seeks to redefine the ERO definition from the probability of flooding at a point to the probability of flooding within 40 km of a point. This talk will detail the most recent verification of the ERO, the development of a new ERO verification website, and some new proposed changes to the ERO operational product.

The ERO is verified against one, three, and six hour FFG within 40 km of a point for 2015 and 2016 using stage IV precipitation data. When possible, the Model Evaluation Tools (METv6.0) software package is used in the verification procedure. This talk details the climatology of ERO issuance frequency. Also presented is the average fractional coverage of Stage IV exceeding FFG within 40 km of a point for each ERO category. Additional datasets to compliment FFG are considered; such as the annual recurrence interval (ARI) thresholds, local storm reports (LSRs), and United States Geological Survey (USGS) gauge observations.

Based on the results of this verification, WPC ERO categories of marginal, slight, moderate, and high are proposed to change from exceeding 2%, 5%, 10% and 15%, respectively, at a point to exceeding 5%, 10%, 20%, and 50% within 40 km of a point, respectively. In order to provide feedback to WPC forecasters on their recent ERO performance, an operational website is created to display the most recent ERO verification scores. A few recent verification metrics from the website are presented in this talk.