Coming changes to the Weather Prediction Center Excessive Rainfall Outlooks

As the broad view component of a forecast funnel approach, the National Weather Service (NWS) outlines areas at risk from flash floods out to three days using the Excessive Rainfall Outlooks issued from the Weather Prediction Center (WPC). The Outlooks often capture flash floods associated with larger weather systems, but are not as efficient at capturing flash floods whose causative rainfall is organized on smaller scales. The product specifications also make it difficult for forecasters to convey a consistent message and to gain lead time on high impact events. Valid periods continually “slide” forward by 12 hours so that successive issuances under the same product title cover different times. A graphical label of “See Text,” used to identify events of lesser coverage or lesser confidence, offers little spatial information, and is only permitted on Day 1. Given that confidence is typically lower on Days 2 and 3, very little information about potential threats is communicated with much lead time aside from the best defined, more easily modeled large scale weather systems.

WPC proposes to improve the performance and utility of the Excessive Rainfall Outlooks through several changes. The sliding valid periods could be fixed by binding the end points to 12z. Thus, all initial issuances and most product updates would cover a 12z to 12z period. The valid period for updates to the Day 1 Outlook would decrease in length but would always keep the same end point of 12z the following morning. Issuance times could also be simplified, with initial Outlooks for Days 1-3 sent simultaneously at 08z and afternoon updates synced at 20z. The evening update to the Day 1 Outlook could be moved to 01z. In addition to improving internal consistency, these changes would increase consistency between National Centers, following the 12z to 12z “convective day” concept and Day 1 update times employed at the Storm Prediction Center (SPC). The proposed product schedule would likely be easier for users to remember.

While Slight, Moderate, and High risk categories would remain, the “See Text” function could be replaced by a low-probability contour separating areas of near zero risk from areas in which convective rain rates capable of contributing to flash floods are possible - even if organized on small scales. WPC also plans to design new web graphics, with potential enhancements including color fill and addition of optional map layers. In the longer term WPC is working with the hydrometeorological community to design an Outlook product whose definition would circumvent the weaknesses of Flash Flood Guidance and emphasize prediction of the observed impacts of flash flooding.