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Thursday, 15th August 2013

How to decide on changing tropical cyclone signal?

The Hong Kong Observatory issued a No. 8 signal early yesterday morning (14 August) while Severe Typhoon Utor approached us. When Utor was about to make landfall near Yangjiang in western Guangdong, local winds were weakening. The Observatory then changed the tropical cyclone signal to No. 3 at 1:40 pm. This subsequently drew considerable attention from the public. Some members of the public have opinion on the time of signal change, considering that we changed the signal too hurriedly. Some other public have opinion on the wordings in the tropical cyclone signal messages.

We understand that signal change has an impact to the work and living of the public, and may cause inconvenience to some of them. Our decision on signal change is based on science and the safety of the public is the prime factor. Then, what did we consider before the signal change? How did we inform the public of the timing for signal change in the tropical cyclone signal messages?

In yesterday morning, we expected that Utor would make landfall at Yangjiang in the afternoon and would gradually recede from Hong Kong (Figure 1). We also anticipated that local winds would weaken progressively in the afternoon. In view of this, we indicated for the first time in the tropical cyclone signal messages issued at 10:45 am that we would consider changing the signal to No. 3 around 2 pm. Why did we use the word "consider" but not indicated more definitively that we "would" change tropical cyclone signal? Actually, there was a rainband approaching Hong Kong from the south. Based on our past experience, such rainband of typhoon might bring high winds, and local winds might subsequently strengthen again. We thus could not rashly indicate that winds would definitely weaken three hours later to enable changing of tropical cyclone signal.

The rainband then reached Hong Kong later (Figure 2), but did not bring high winds. There was also no intense rainband upstream to the south of Hong Kong. We expected that local winds would then continue to weaken and there would not be heavy rain and squalls for a while. We then remove the word "consider" in the tropical cyclone signal message issued at 11:50 am. About two hours later, we issued the No. 3 signal at 1:40 pm.

Different people have different needs and arrangement in tropical cyclone situation. As such, when we change tropical cyclone signal, it will unavoidable cause inconvenience to some people. Issuance of tropical cyclone signal involves a degree of forecast and may not be 100% accurate. We hope the public would understand. If situation allows, we would strive to provide the latest tropical cyclone signal messages to the public in clearer wordings in the future.

CM Cheng

Figure 1

Figure 1      Utor about to make landfall near Yangjiang at 2 pm, 14 August 2013

Figure 2

Figure 2      Radar image* at 11:48 am, 14 August 2013, indicating a rainband affecting Hong Kong.

 *   Radar composite image made from radar data of Hong Kong and Guangdong.

Last revision date: <04 Oct 2013>