The Hong Kong Observatory will start issuing seasonal forecasts of average temperature and total rainfall, which will be expressed in terms of 'above normal', 'near normal' and 'below normal'.

The Observatory's Assistant Director K H Yeung said in a media briefing today the forecasts will be issued in March, June, September and December through this website.

Mr Yeung said seasonal forecasts referred to forecasts of the average weather in the coming month, year, or season. For instance, a forecast could be for a warmer than average summer, or a colder than average winter. 

"These forecasts are useful for planning activities which are influenced by the weather or the climate.

"They can be used to assess the demand for soft drinks in the summer or for warm clothing in the winter, and to adjust the production or inventory levels accordingly. The Chicago Mercantile exchange in the United States even offers weather futures," Mr Yeung said.

The primary tool for producing the forecasts is a regional climate model adapted from the Experimental Climate Prediction Centre of the University of California at San Diego. Model-generated forecast maps will also be displayed on the website.

Outputs of climate models from other climate centres will be taken into consideration in formulating the seasonal forecasts.

Mr Yeung explained climate models are not too different from the numerical weather-prediction models. In both cases, computers simulate future weather changes. 

However, he said with the present state of technology, the accuracy of climate models is not yet as good as that of numerical weather-prediction models.

"That is why the Observatory is only issuing the seasonal forecasts on an experimental basis. This will be included as part of the regular forecasting service later when the technology has gained more maturity," Mr Yeung said, adding this is the practice other climate centres adopt.

He pointed out as proper use of seasonally forecasts required experience, those in weather-sensitive businesses as well as the community at large should try to make use of the seasonal forecasts and gain experience in the process so that they could use them more effectively in the future.

Also speaking at the briefing, the centre's director Dr John Roads said most centres use global climate models to produce seasonal forecasts, while his centre is one of the few to have developed regional climate models.

He explained while global models could cover a larger area, their resolution is relatively low. On the other hand, regional models cover smaller areas, but have higher resolution. 

Dr Roads said Hong Kong is located in an area of complex terrain, and it is appropriate to use a regional model for seasonal forecasts.

He expects more centres will use this model or this type of model to produce seasonal forecasts.