National Weather Service United States Department of Commerce

New Short Term Probabilistic River Forecast Guidance

A new service is available on the NWS River Forecast web page for several of the existing river forecast points.
Additional locations will be available over the next several months. The new guidance is a tool for assessing the
range of possible river levels that may occur. The goal is to better quantify the uncertainty in our river forecasts
for planning purposes.

Current Official River Forecast Example New Short Term Probabilistic River Guidance Example


How To Interpret the Graphic:

The shaded areas show the predicted range of river levels over the next 10 days. There is also a chance the
observed crest could exceed the highest crest shown. The forecast range can change over time and can be
much larger for high rainfall events than for lower rainfall events.


Differences Between the Official River Forecasts and the New Probabilistic Guidance:

Current, Official Forecast New, Probabilistic Guidance

Includes 24 or 48 hours of precipitation:

48 hours from Oct 1 to Mar 31 and

24 hours from Apr 1 to Sep 30

Includes 10 day of precipitation
Forecast created by a forecaster Automated, No quality control by a forecaster - Model could be off
Updated at least twice a day, sometimes 3 or 4 times a day during flooding Updated 2 times per day
Forecast may be too precise, and it does not show ranges of possible river crest Range of river crests can help with planning but still may not show the highest crest


Why Are There A Range of Forecast Crests?

The amount of rain and the time span it falls in affects the river level. More rain typically results in a higher river crest, while
less rain typically results in a lower crest. The amount of melting snow also plays a role.


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