NWS Probabilistic Flood Outlook Tutorial
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Probabilistic Flood Outlooks, or AHPS outlooks are based on what is called the Ensemble Streamflow Prediction or ESP. What this means is that the NWS River Forecast model takes the present conditions and then applies the weather conditions from a specific historical year (say 1995) for a 90-day period of interest to the model. In other words, the model takes the present-day numbers representing all of the scientific factors involved in a flood and applies to them the observed temperatures and precipitation from every day in that 90-day period from the year 1995. The result is what the river height would be if 1995’s weather occurred with the current conditions.

 
Wait! What does 1995 have to do with today? Well, just a little bit. The reason the AHPS outlooks are called an ensemble is because they create simulations by applying weather for 60 different years to the present conditions. This means that the extremes of weather in the region at this time of the year are captured…both very dry and very wet and very warm and very cold periods during the melt. The graphic below shows an example of what the river stages at Grand Forks would look like based on each of the possible weather scenarios from 1948 to 2008.
 

 

Grand Forks Trace j12/22/10

Figure 1. ESP traces at East Grand Forks for AHPS run on December 22, 2010.

The statistics go to work once graphs like the one above are created. The year with the maximum simulated crest is ranked as number one, the year with the second highest as second, and so forth.   The probability of reaching the stages is calculated by dividing its rank by the number of years in the ensemble (plus one, actually…which is a well-accepted statistical formula). This ends up meaning that the lowest simulated crest has the highest probability of occurring. So, in most cases this means that a 90% confidence is based on what would happen to the river even with very little additional precipitation. The low probabilities (like 2%) represent what could happen if a rapid melt or excessive precipitation occurred with the present basin conditions.

 

Luckily, all the hard work is already done for you. Click here to see the forecast points in the area, and after you select a point of interest, click on “Chance of Exceeding Levels During Entire Period” from the tabs above the hydrograph. A graphic like the one below will appear. The black triangles (labeled CS for “Conditional Simulation”) mark the probability of a certain height being reached. In the Grand Forks example, this means there is a 90% chance or risk of the Red River rising to around 40 feet. The blue dots (labled HS for Historical Simulation) represent what the chances of the river rising to a certain height would be using average snow cover, soil conditions, and spring river levels, not the present ones. It’s merely a judge of how unusual the pattern is (the farther apart the CS and HS markers are, the more atypical the present conditions are). 

 

90 day grand forks 12/22/10

Figure 2.  Chance of Exceedence Values at East Grand Forks for AHPS run on December 22, 2010.

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