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Coldest day in 59 years unexpected: Hong Kong meteorologists describe forecast challenges as imperfect science

Observatory and other weather experts respond to criticism for underestimating the severity of the weekend cold snap

PUBLISHED : Monday, 25 January, 2016, 7:55pm
UPDATED : Tuesday, 26 January, 2016, 10:11am

Forecasts for extreme weather systems are never 100 per cent accurate and not simply the work of looking at statistics, local meteorologists emphasised.

The point came as the Hong Kong Observatory was criticised on social media for underestimating the severity of the weekend cold snap.

Minimum temperatures Sunday dropped to 3.1 degrees on Sunday afternoon – more than five degrees lower than what was originally predicted and the city’s lowest reading since 1957.

READ MORE: Hong Kong shivers through its coldest day since 1957: kindergartens, primary schools closed today

Only last week, the Observatory stated that its grasp of Hong Kong weather was much better than overseas bodies or unofficial forecasters on the internet, as US and European-based systems indicated temperatures would be far lower than the Observatory’s “six to eight degree” estimate.

Clarence Fong Chi-kong, a meteorologist at the Macau-based UN ESCAP World Meteorological Organisation typhoon committee, said the discrepancy was probably caused by an unforeseen rainband moving over southern China on Sunday morning.

READ MORE: Nowhere else to go: Hong Kong working parents take children to school despite cold weather suspension

“They were not able to forecast this, which eventually brought wetter and colder weather to the region,” said Fong, in charge of a website called Weather Underground of Hong Kong, an unofficial alternative weather forecaster operating online.

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Dr Alexis Lau Kai-hon, a meteorologist with the University of Science and Technology, said making forecasts for such extreme weather events was as “difficult as forecasting movements in the stock market”.

Lau, who studies numerical weather prediction, said that “the rate of false alarm and the probability of detection” had to be taken into account.

READ MORE: Hong Kong chills on social media: foolhardy frost chasers, ice selfies, videos and pictures of our coldest day for 60 years

He added that deriving a forecast from computer models was easy but that it was not surprising the Observatory drew upon further human interpretation or adjustment, which he said was prudent, as looking only at quantitative data would make the forecaster prone to “systemic bias”.

The Hong Kong Meteorological Society’s Leung Wing-mo, formerly an assistant director at the Observatory, said weather was essentially a “chaos system” involving huge uncertainties and unpredictable factors that could swing forecasts another direction entirely.

READ MORE: Freeze frame! Images of Hong Kong’s chilling return to work on Monday, inside the polar vortex

He said the recent inaccurate forecast was a minor “slip” and an isolated incident. “This is a one-off case and judging their performance based on this one incident is just not scientific,” Leung said. “Forecasts are never 100 per cent accurate”.

Regarding the rise of alternative forecasting platforms, Leung expressed reservations about unofficial online forecasters as no one could tell whether their projections resulted from sheer luck or superior skill. Fong, operating such a site, said there was no harm in looking at forecasts from different sources.

The Observatory said it made reference to the “best computer numerical weather prediction models from around the world”.

The models include the European Centre for Medium-Range Weather Forecast, Japan Meteorological Agency and National Oceanic and Atmospheric Administration.

“After adjusting these model forecasts based on such factors as their past performance, forecast position and actual observations, a consolidated forecast will be generated for the public,” the observatory said in a statement last night.

Last year, the average forecast error in temperature based was about one to two degrees.