Sat, Aug 29, 2015 - Page 3 News List

CWB redefining rainfall terminology for clearer cautions

By Shelley Shan  /  Staff reporter

The Central Weather Bureau (CWB) yesterday said that starting on Tuesday, it would use new standards to define different categories of rainfall to give government agencies and the public greater warning of potential disasters caused by extreme weather.

At present, “heavy rain” means cumulative rainfall reaching 50mm within 24 hours, including at least one hour in which the accumulation reaches more than 15mm.

Under the new standards, “heavy rain” will be defined as cumulative rainfall reaching 80mm within 24 hours, or hourly rainfall of 40mm.

“Extremely heavy rain,” which is now defined as cumulative rainfall of 130mm, would mean cumulative rainfall of more than 200mm within a 24-hour period or 100mm within three hours under the new standards.

“Torrential rain” was used to refer to cumulative rainfall of 200mm, but it will now require 350mm to reach that term, while the level to qualify for “extreme torrential rain” has been raised from 350mm to 500mm.

The bureau said that the old definitions can no longer meet the weather forecasting challenges brought about by extreme weather.

Many previous heavy-rain alerts, which were issued when forecasted rainfall was expected to reach 50mm, did not lead to obvious disasters, the bureau said, while strong precipitation over a very short period of time could sometimes lead to disasters.

CWB First Division head Hsieh Ming-chang (謝明昌) said the new standards take into considerations the different standards used by other agencies, based on which they act to prevent disasters and human casualties.

The new standards would also inform the public about the possible damage brought by rainfall in any one of the four categories.

For example, during a heavy-rain alert, flooding could occur in low-lying areas as well as areas with poor drainage systems. With the heavy rainfall, landslides, mudflows or fallen rocks could occur in mountainous areas, while the public should be alert to dangers of gusting wind and thunderstorms.

The new standards should help reduced the number of days the bureau is obligated to issue heavy-rain alerts, from an average of 187.8 days per year to 166.2 days per year, it said.

Fewer heavy-rain alerts would hopefully mean the public would pay more attention to the ones that are issued, thereby reducing casualties, it said.

However, the public should continue to pay attention to the bureau’s round-the-clock weather updates, as there may be rainstorms that do not meet the standards for heavy rain, but could lead to substantial damages, the bureau said.

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