Weather forecasts

Press, 12 October 1985, Page 18

Weather forecasts

Sir,-A cheerful television newsreader informed me last night that the weather would be fine today, through until Sunday. So did the radio and your newspaper this morning. And so it was, until an unforecast south-west change arrived in mid-afternoon bringing clouds and light rain. How can this be? Comments lately from friends and workmates indicate that noone takes the weather forecast seriously because it is so often wrong. Was the purchase of sophisticated equipment by the meteorological service unnecessary? Could they have saved the taxpayer thousands of dollars by simply looking out the window each morning and tossing a coin to see if it would rain or not? Their successful prediction rate would probably remain unaltered using this method. It might even improve.—Yours, etc., L. TOBECK. October 3, 1985.

[Mr lan D. Miller, of the Christchurch Weather Centre of the Meteorological Service, replies: “Despite our best efforts, forecasting south-west chaoses and the OTpanyingtiweathff in Canterbury is often difficult Because of the orientation of the main ranges

in the South Island small changes in the direction of the airflow following a cold front can mean the difference between sunny weather and a dull, wet day. In addition, local effects caused by Banks Peninsula can be important, as was the case on October 3. On that day, a cold front moved north-east over the South Island, but as expected, it was very weak east of the Alps. It was not until it approached Banks Peninsula that it produced rain and this was largely caused by local convergence between north-east winds from Pegasus Bay and southeasterlies south of the Peninsula. This convergence was reinforced by a shallow trough of low pressure which followed the front about 12 hours later. The computer-produced weather maps we use normally give a reliable guide to the general weather patterns to be expected up to four or five days ahead, but often cannot cope with the details which, as you can see, are important for local weather. Nonetheless, I believe they still allow us to provide useful forecasts. This fall of 2.2 mm at the airport and 7.6 mm in the Botanic Gardens was in fact the only rain in Christchurch during the four-day period from Thursday, October 3, and sunshine averaged over five hours a day in the city and almost seven hours at the airport. I appreciate the concern presented about the accuracy of weather forecasts and would be pleased to show your correspondent around the Weather Office at the airport and explain in more detail the methods used to produce forecasts.”]