Making Sense of Uncertainty

How high will the river get during the storm? Will it reach my house?  Will it inundate the roads and cause emergency evacuations? For years, the National Weather Service has issued highly accurate hydrologic forecasts used by residents here in Easton and across the country, letting us know what level rivers and streams are expected to reach at a specific time.  If you visit, you can search for the river gage nearest you (for locals, search for the gage on the Easton-Phillipsburg Free Bridge, located on the map here:  National Weather Service Advanced Hydrologic Prediction Service) and determine where the river will be in the days ahead.

But as I noted in my last blog, the answers aren’t always as clear as they seem.  Every forecast has uncertainty, and new forecast capability is allowing NWS to share that range of uncertainty with users so they can know not only the official, expected river level, but the possibilities that the level could be lower, or higher – for practical purposes, what is the worst-case scenario?  The best?

This information is important – and, also, complex and not easy to show.  In 2018, NNC completed its first study of the use of these hydrologic probabilistic forecasts, which suggested that they have important value for users, and need further study and refinement to be most useful.  That study focused on users in the eastern part of the country, using a National Weather Service forecast model called Hydrologic Ensemble Forecast System (HEFS), which issues probabilistic water forecasts for rivers and streams across the country.

Now in 2021, Nurture Nature Center’s research team, including East Carolina University, has issued a new report from a social science study conducted 2018-2020, looking at the use of HEFS nationally.  Our study included focus groups and surveys with residents and professionals in Colorado, New York and California, and produced a series of recommendations to NWS about how to present these forecasts so they are easier to understand and likely to motivate people to identify their risk and take protective actions when needed.  We made recommendations for a national prototype of the forecast product, as well as regional variations that meet the needs of users in different geographies.  We also made a series of recommendations for forecasters for best practices for communicating probabilistic information.  NWS is beginning to transition recommendations from this study into operations now, adapting what we learned to their technical systems for displaying information.

Our report is online at Making-Sense-of-Uncertainty-2020-1.pdf (  There’s a lot of detail in the report, but some readers may enjoy learning, as we did, about the ways that people in different parts of the country use and respond to weather forecasts.

Next up for NNC is a study of the Weather Prediction Center’s Winter Storm Severity Index, an impact-based forecast tool that helps users understand the range of possible impacts from a winter storm, and the level of expected severity of those impacts.  I’ll tell you more about that in the next blog!