The Excessive Rainfall Outlook (ERO) is a Weather Prediction Center (WPC) forecast map showing the probability that rainfall will exceed flash flood guidance within 25 miles (40 km) of a point. It covers the contiguous United States and is issued as separate daily outlooks for Day 1 through Day 5.
How the risk categories work
The outlook currently uses four categories. Marginal means at least a 5 percent chance of rainfall exceeding flash flood guidance near a point, Slight at least 15 percent, Moderate at least 40 percent and High at least 70 percent.
Flash flood guidance is the amount of rain needed in a set period to cause flash flooding over a given area, and River Forecast Centers supply those values. Each daily outlook covers a 24-hour period running from 12Z to 12Z, though later updates to the Day 1 outlook cover less than 24 hours.
A scale under review
In August 2026 the National Weather Service (NWS) asked for public comment, through 09/05/26, on a proposal to expand the ERO from four levels to five. The new Enhanced category would sit between Slight and Moderate at 25 percent.
Under that proposal the levels would run Marginal 5 percent, Slight 15 percent, Enhanced 25 percent, Moderate 40 percent and High 70 percent. As of early October 2026 no notice implementing the change had been posted and the WPC page still showed four categories, so read the legend on the current WPC graphic.
Where it fits among flood products
The ERO looks days ahead over broad areas. As an event gets closer, WPC issues Mesoscale Precipitation Discussions for the next few hours, and local NWS offices handle the alerts.
A Flash Flood Watch means conditions are favorable for flash flooding. A Flash Flood Warning means flash flooding is in progress, imminent or highly likely.
What it does not tell you
An ERO is not a forecast that a flash flood will happen at any particular spot. It accounts for uncertainty in where and when intense rain will fall, which is why it is stated as a probability over an area.
Even at 15 percent, most points in a Slight area would not see rain exceed flash flood guidance, while one spot under repeated heavy cells could still flood. Treat the categories as guides to attention, not as switches.