2 , Example 



As an example of the steps necesc iry in preparing a prognostic wave 

 chart, the prognostic wave chart, verifying time 1230Z 12 January, figure 



10 is used. The basic principles described in C-l will be carried through. 

 The prognostic wave chart is based on the 30-hour weather prognostic chart 

 verifying time 123CZ, 12 January 195h and the synoptic wave chart of 1230Z, 



11 January 19!?U° It should be noted that the prognostic weather chart pro- 

 vides the forecaster only wit! the expected conditions for a specific time 

 2\x hours into the future. This is insufficient information with which to 

 establish the wave field for this time,-, The forecaster must establish the 

 complete wind history for the 2h-hour period. In the present example this 

 is done in six -hourly intervals for the forecast period. The particular 

 interval of time chosen will depend upon the expected variability of the 

 wind field* 



The following steps form a basic procedure for the construction of a 

 prognostic -wave charts (I) Selection of the points for which wave informa- 

 tion should be calculated in order to fix the predominant features of the 

 prognostic wave chart. (2) Evaluation of the vrave field as related to the 

 synoptic weather systems and as indicated by the latest synoptic wave chart. 

 (.3) Integration of (l) and (2). 



A general analysis of the prognostic weather chart will indicate the 

 predominant fetch areas and those areas that show a minimum of wave activity. 

 The centers of these areas will fix the predominant features of the prognostic 

 wave chart. A sufficient number of points is now selected to provide the 

 detail necessary for drawing the prognostic wave chart,, Wave plots are 

 calculated for these points using the form of table 1 as a guide. The prog- 

 nosticated wave values are entered on the prognostic wave chart and the prog- 

 nosticated frontal positions are drawn in. Isopleths of wave height are 

 sketched in: a height interval of three feet is used at present. 



In the evaluation of the synoptic wave chart, the following are noted: 

 (l) the positions and orientation of the major wave trains, (2) the position- 

 ing of the wave trains with respect to the frontal systems, (3) distribution 

 of heights within the trains, and (h) the history of the development of the 

 trains as indicated by previous synoptic wave charts. Such an evaluation of 

 the synoptic wave charts indicates the trend in the development and decay of 

 the predominant wave trains* The established trend forms the basis for 

 carrying the continuity into the forecast period. 



'The final form of the prognostic wave chart is determined by the inte- 

 gration of the calculated wave values and the established trends. This pro- 

 cedure becomes especially important in areas where wind direction and velocity 

 show considerable variability. Under such conditions calculations are subject 

 to large inaccuracies. The isopleths of wave height which have been sketched 

 in are now modified to conform to the past and prognosticated trends. The 

 prognostic wave chart is completed with graphic symbols as indicated on figure 

 2. 



Table 1 represents a convenient working form and tabulation sheet for 

 data necessary to carry through the individual wave forecasts for the selected 

 points. The step of selection of points can be eliminated by establishing a 



