Hydrologic Ensemble Forecast System (HEFS)

The Hydrologic Ensemble Forecast System (HEFS) is run at the 13 River Forecast Centers (RFCs) to produce a set (ensemble) of members, each of which represents one possible outcome. Together, the ensemble members make up a probabilistic forecast.

This data is available from the experimental HEFS API, which is used to power the NWPS HEFS displays.

How do I interpret the options I've selected?

When you choose a duration (ex: 10 days) and a probability level (ex: 50%), you're setting the criteria for what kind of flood risk you're looking at on the map: Major Moderate, Minor, Action or No Flooding.

Here's how the settings work:

• If you select 10 days and 50%, the system will look at each gauge and determine if there's a 50% or higher chance of that level of flooding happening at any given point during those 10 days.
• If a gauge is green (No Flooding category), it means that on no day during those 10 days is there a 50% or higher chance of flooding.
• If the gauge is not green, it means that on at least one of those 10 days there is a 50% or higher chance of it exceeding the respective flood level (Major, Moderate, Minor or Action).

Depending on the duration and probability level you select, the display will show you which areas are at risk.