Previous Chapter 7 Deriving Product Parameters from Weather Elements

7 Deriving Product Parameters from Weather Elements

Once the forecast has been edited and stored in the database, product generators will be invoked to create text products for dissemination. Many of these products cannot directly use the information in the database. Some require information for specific points or over areas. Some require parameters that are not part of the weather elements.

Deriving product parameters from weather elements requires one or two basic steps:

summarizing the data for areas and points, and/or
computing product parameters from the (summarized) weather elements.

Product parameters needed for forecast verification will be stored in the database. This information can be viewed and edited if necessary. Since the format of much of the data is not gridded, the interactive tools will be different from those described in the previous chapter.

7.1 Extracting Point and Areal Data from Grids

Techniques for extracting areal data from grids have been developed by TDL as part of FEFS (see Section 1.4.4.1).

Summarizing weather and cloud information requires a meteorological assessment of the gridpoint data. The grids of weather and cloud information can contain many overlapping areas. These weather and cloud areas will normally not be aligned with forecast boundaries. The algorithms must determine

what fraction of an area must be covered before a weather or cloud area applies to that area,
which phases (liquid or solid) of precipitation are present,
the predominant character (steady or showery) of the precipitation,
the predominant intensity of the precipitation,
the proper handling of any probability qualifiers, and
an average sky cover for the area.

For public zone forecasts, the ICWF applies a special technique to reduce the amount of text produced. During the process of extracting a zone average from the grids, adjacent zones are combined when possible, so that the effective number of zones is reduced. Preferred and prohibited zone combinations may be specified by each local office.

This technique may be of even greater utility in the restructured NWS, since forecast zones will be as small as a single county. Until and unless some more specific dissemination system is devised, it will be necessary to create such zone combinations, lest the recipient be overwhelmed with repetitive text.

Zone combinations will be determined automatically by algorithms within the product generators. Forecasters may override the chosen combinations.

7.2 Computing Product Parameters

Specific product parameters are required for some forecasts. For example, public forecasts require wind chill, which is easily computed from temperature and wind fields.

More difficult problems are encountered in other programs. For example, marine forecasts include statements about heavy surf and beach erosion, when necessary. These can be derived from wind forecasts, plus information about coastal topography.

As an illustration, Table 11 lists the weather elements currently used to create routine zone forecasts. This list will be modified and expanded as necessary to reflect the requirements for public forecasts in the future. Similar lists have been developed for marine, hydrology, fire weather, agriculture, and aviation product generators.

Table 11 - List of Weather Elements Required by the ICWF Area Forecast 
		Generator
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Element                               Projection  Unit                   Values                                
                                       Interval                                                                
                                      (hours)                                                                  
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Maximum/minimum temperature           12          Degrees F              -80 - 150                             
3-h temperature                       3           Degrees F              -80 - 150                             
3-h dew point                         3           Degrees F              -80 - 150                             
Cloud                                 3           Tenths of sky cover    0 - 10                                
12-h probability of precipitation     12          Percent                0 - 100                               
Thunderstorm                          3           Parameters             slight chance, occasional, likely,    
                                                                         unqualified, null                     
Thunderstorm intensity                3           Parameters             frequent lightning, heavy down        
                                                                         pours, gusty winds, hail, slight      
                                                                         chance of severe, chance of           
                                                                         severe, damaging wind, large hail     
QPF                                   12          Hundredths of an inch  0 - 999                               
Wind direction                        3           Tens of degrees        0 - 36                                
Wind speed                            3           Miles per hour         0 - 200                               
Obstructions to vision                3           Parameters             fog, heavy fog, haze, blowing         
                                                                         snow, smoke, blowing dust, null       
Precipitation types 1, 2, and 3       3           Parameters             rain, rain showers, drizzle, snow,    
                                                                         snow showers, ice pellets, freez      
                                                                         ing rain, freezing drizzle, null      
Probability for types 1, 2, and 3     3           Parameters             slight chance, occasional, likely,    
                                                                         unqualified, null                     
Intensity for types 1, 2, and 3       3           Parameters             very light, light, moderate, heavy,   
                                                                         null                                  
Snow amount                           12          Inches                 0 - 99 where 99 indicates a trace     
Mixture of precipitation 1-2 and 2-3  3           Parameters             and, or, null                         
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