NWS Seeks Input on Long-Range Probabilistic Outlook Improvements

To users of the National Weather Service's Long-Range Hydrologic Probabilistic Outlooks...

As a regular user of the NWS Probabilistic Hydrologic Outlook, we would like your opinion on the examples below. 

We’ve been issuing these types of outlooks for several years now, and would like to incorporate some of the requests for changes we’ve received. As you are probably aware, we currently provide a range of probabilities from 10 to 90% which show the chance for a river to exceed a particular stage. There have been some concerns voiced that there are too many numbers, so we are looking at other options with fewer numbers. We have also found that many river forecast users like the percentages displayed for the different flood category levels (e.g., minor, moderate and major) as well as including the associated departures from normal to give an idea of how the current outlook compares with historical data.
 
If you would be so kind as to take a look at the attached three examples and let us know which you prefer and why by responding to the short survey at http://www.surveymonkey.com/s/23939ZZ it would be greatly appreciated. It’s possible you may like part of one example and part of another, or you may have a different suggestion. Whatever your opinion, we welcome any feedback you are willing to give.
 
We thank you very much for your input!!

 


Examples of Long-Range Probabilistic Hydrologic Outlooks
 
Long-range hydrologic outlooks contain probability values that are calculated using current conditions of the river with multiple season scenarios from 30 or more years of historical data. Current conditions include height or flow of the river, estimated water content of any existing snow cover and soil moisture. Additionally, 30 to 90 day long-range outlooks of temperature and precipitation may also be considered. By providing the complete range of probabilities, the level of risk associated with long-range planning decisions can be determined. These probabilistic outlooks are part of the National Weather Service's Advanced Hydrologic Prediction Service (AHPS).
 
The flood categories used by the National Weather Service are as follows:
 
Minor flooding - when the river reaches flood stage and causes minimal or no property damage, but possibly some public threat.
 
Moderate flooding – when river levels cause some inundation of structures and roads near stream with some evacuations of people and/or transfer of property to higher elevations.
 
Major flooding – when river levels cause extensive inundation of structures and roads with significant evacuations of people and/or transfer of property to higher elevations.
 
 

Three examples follow that use various customer input received over the last several years. Graphical representations follow the text examples.


In Example 1, the probabilistic hydrologic outlook contains two tables, the first table provides information on the probability of reaching minor, moderate and major flooding, and the second table provides information on the probability (at 10 % intervals) of exceeding a given stage at a given location over the next 90 days.
 
Example 1

THIS OUTLOOK IS VALID FROM FEB 1 2011 – MAY 2 2011

 

THE FOLLOWING TABLE GIVES THE CHANCE OF REACHING DIFFERENT
FLOOD LEVELS AT THE GAGES DURING THE VALID PERIOD.
 
...TABLE 1--PROBABILITIES FOR MINOR...MODERATE AND MAJOR FLOODING...
                                          APPROXIMATE CHANCE OF
                  FLOOD LEVELS (FT)       REACHING FLOOD LEVELS

 

LOCATION          MINOR  MOD  MAJOR       MINOR   MOD   MAJOR

--------          ----- ----- -----       -----  -----  -----
MISSISSIPPI RIVER
 DUBUQUE LD11 IA   16.0  17.0  20.5        98%    93%    67%
 DUBUQUE IA        17.0  18.0  21.5        98%    98%    72%
 BELLEVUE LD12 IA 17.0  18.0  20.0        88%    86%    65%
 
IN THE TABLE BELOW...THE 90 THROUGH 10 PERCENT COLUMNS INDICATE THE
CHANCE THE RIVER COULD RISE ABOVE THE LISTED STAGE LEVELS IN THE
NEXT 90 DAYS.
 
...TABLE 2--EXCEEDANCE PROBABILITIES...
 
CHANCE OF EXCEEDING STAGES AT SPECIFIC LOCATIONS
 
LOCATION        FS(FT) 90%  80%  70%  60%  50%  40%  30%  20% 10%
-----------     ----   ---  ---  ---  --- ---  ---  ---  ---  ---
MISSISSIPPI RIVER
DUBUQUE LD11    16.0  17.4 19.1 20.3 21.5 22.2 23.2 24.0 24.7 26.2
DUBUQUE         17.0  19.0 20.7 21.9 22.9 23.6 24.7 25.4 26.3 29.3
BELLEVUE LD12   17.0  16.9 18.6 19.8 20.7 21.3 22.0 22.6 22.8 23.8
 


In Example 2, the probabilistic hydrologic outlook contains three tables, the first table (1A) provides information on the probability of reaching minor, moderate and major flooding in a slightly different format than used in Example 1; the second table (1B) provides additional information on the departure from normal of reaching minor, moderate, and major flooding; and the third table (2) provides the probability (at 10 % intervals) of exceeding a given stage at a given location, similar to Example 1. 

 The main difference between this example and the previous example is the information in the second table (1B), which shows how the flood threat deviates from normal for minor, moderate, and major flooding.

 

Example 2
 
...TABLE 1A--PROBABILITIES FOR MINOR...MODERATE AND MAJOR FLOODING...
 
VALID 2/1/2011 - 5/2/2011
 
                       -----------FLOOD STAGE-----------    
LOCATION             MINOR          MODERATE        MAJOR      
                    STG   PCT      STG   PCT      STG   PCT   
                                                        
MISSISSIPPI RIVER
 DUBUQUE LD11       16.0 98%       17.0 93%       20.5 67%   
 DUBUQUE            17.0 98%       18.0 98%       21.5 72%   
 BELLEVUE LD12      17.0 88%       18.0 86%       20.0 65%   
 
 
...TABLE 1B--DEPARTURES FROM NORMAL OF REACHING MINOR...MODERATE
AND MAJOR FLOOD STAGE...
 
VALID 2/1/2011 - 5/2/2011
                          -----------FLOOD STAGE-----------
LOCATION               MINOR            MODERATE         MAJOR
                    STG   DEPARTURE  STG   DEPARTURE  STG   DEPARTURE                                          
 
MISSISSIPPI RIVER
 DUBUQUE LD11       16.0  53% ABOVE  17.0  56% ABOVE  20.5  60% ABOVE
 DUBUQUE            17.0  51% ABOVE  18.0  58% ABOVE  21.5  65% ABOVE
 BELLEVUE LD12      17.0  58% ABOVE  18.0  67% ABOVE  20.0  59% ABOVE
 
NR NORMAL = WITHIN 5% OF NORMAL
 
 
...TABLE 2--EXCEEDANCE PROBABILITIES... (same as Ex. 1 Table 2)
 
CHANCE OF EXCEEDING STAGES AT SPECIFIC LOCATIONS
VALID 2/1/2011 - 5/2/2011
 
LOCATION          FS(FT) 90%  80%  70%  60%  50%  40%  30%  20%  10%
 
MISSISSIPPI RIVER
 DUBUQUE LD11     16.0  17.4 19.1 20.3 21.5 22.2 23.2 24.0 24.7 26.2
 DUBUQUE          17.0  19.0 20.7 21.9 22.9 23.6 24.7 25.4 26.3 29.3

 BELLEVUE LD12    17.0  16.9 18.6 19.8 20.7 21.3 22.0 22.6 22.8 23.8

 


In Example 3 the probabilistic hydrologic outlook contains only two tables, the first table (1) combines information on the probability of reaching minor, moderate and major flooding in a similar format to what was used in example 2(1A) above, but with only information on the departure from normal of reaching flood stage; and the second table (2) provides the probability of exceeding a given stage at a given location but uses different and fewer percentage intervals than what was used in examples 1 and 2. 

 The main difference in this example is that it only gives you departure from normal for reaching flood stage (not for moderate or major flooding), and this example uses different, and fewer, percentage intervals for exceedance probabilities.

 

Example 3
 
          CHANCE OF EXCEEDING MINOR TO MAJOR FLOOD STAGE
                 DEVIATION FROM NORMAL FLOOD RISK
             VALID PERIOD: 2/1/2011 - 5/2/2011
 

...TABLE 1--PROBABILITIES FOR MINOR...MODERATE AND MAJOR FLOODING...

 

                                                          DEP FROM
                        ---------FLOOD STAGES---------      NORMAL
                         MINOR     MODERATE    MAJOR     OF REACHING
 LOCATION               STG PCT   STG PCT    STG PCT   FLOOD STAGE
----------------------------------------------------------------------
MISSISSIPPI RIVER
 DUBUQUE LD11           16   98%   17   93%   21   67%   ABOVE NORMAL
 DUBUQUE                17   98%   18   98%   22   72%   ABOVE NORMAL
 BELLEVUE LD12          17   88%   18   86%   20   65%   ABOVE NORMAL
 
 
...TABLE 2--EXCEEDANCE PROBABILITIES...
 
          CHANCE OF EXCEEDING STAGES AT SPECIFIC LOCATIONS
               VALID PERIOD: 2/1/2011 - 5/2/2011
 
                  95%    90%    75%    50%    25%    10%     5%
                  ---    ---    ---    ---    ---    ---    ---
MISSISSIPPI RIVER
 DUBUQUE LD11    16.5   17.4   19.1   22.2   24.5   26.2   27.3
 DUBUQUE         18.1   19.0   21.1   23.6   26.0   29.3   31.5
 BELLEVUE LD12   16.3   16.9   18.7   21.3   22.8   23.8   24.7
 
 LEGEND
STG = STAGE (FEET)
PCT = PERCENT
--- = DISTRIBUTION COMPLETELY BELOW THIS STAGE
DEP = DEPARTURE
 
NEAR NORMAL = 0.5 - 1.5 times normal
BELOW NORMAL = 0.25 - 0.50 times normal
MUCH BELOW NORMAL = <0.25 times
ABOVE NORMAL = >1.5 - 3.0 times normal

MUCH ABOVE NORMAL = > 3.0 times normal

 


If you have any questions or need additional information, feel free to contact:

Mike Welvaert, Service Hydrologist



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