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  • Is it really very hot?

  • Thursday, 21st July 2016

Have you come across a Very Hot Weather Warning icon on the TV screen early in the morning with a temperature of just 28 degrees displayed next to it? How come such temperature is considered as "very hot"?

In fact, forecast is a main component of the Very Hot Weather Warning. Even if the temperature at the Observatory is only 27 or 28 degrees in the morning or the weather is relatively cloudy, forecasters may still issue or maintain the Very Hot Weather Warning when they forecast more sunshine leading to a significant rise in temperature. This is to remind the public to get well-prepared before they depart home, such as wearing suitable clothing, bringing sufficient drinks and sunscreen.

Figure 1

Sometimes the weather is very hot during the day with relevant warning in effect but the temperature at night only drops slightly (say 28 degrees or above, which is known as a "hot night"), the forecaster usually will not cancel the warning at night if very hot weather is forecast to recur the next day. This is to avoid confusion caused by the frequent changes in warning status.

The Hong Kong Observatory takes into consideration several factors when issuing the Very Hot Weather Warning. These include the atmospheric conditions, weather situations over different parts of Hong Kong, and the Hong Kong Heat Index which reflects the combined effect of temperature, humidity, wind speed and solar radiation. When the warning is in force, we should take appropriate actions to prevent heat strokes.

T.S. Tsoi & L.S. Lee

  • Forecasting Nepartak

  • Thursday, 14th July 2016

While Super Typhoon Nepartak (Figure 1) brought havoc to Taiwan last week, it was fortunate that Hong Kong remained safe from its direct impact. Although no Tropical Cyclone Signal No. 1 was necessary, our forecasters had been keeping a vigilant eye on the evolution of Nepartak.

Figure 1

Figure 1      Satellite imagery at 1 p.m. on 7 July, showing the distinct eye of Nepartak (Source: Japan Meteorological Agency)

As early as 4 July, the Hong Kong Observatory (HKO) began to issue the forecast track of Nepartak and predicted that it would land in the middle part of Taiwan on 8 July. At that time, other meteorological centres and some computer forecasts (including the European Centre (EC) and the National Weather Service (NWS) of USA) forecast that Nepartak would hit the northern part of Taiwan or even just skirt across the offshore waters. Thereafter, the Observatory revised the forecast track in the morning of 6 July, predicting that Nepartak would land over the southern part of Taiwan. This turned out matching quite closely the actual situation (Figure 2).

Figure 2

Figure 2      Forecast tracks of Nepartak by different meteorological centres

Why did the forecasts of various meteorological centres and computer forecasts differ?

The relatively westerly track forecast by the Observatory was mainly based on the consideration that the subtropical ridge of high pressure over the western Pacific would extend westwards from 4 July to 6 July, and thereafter should remain over there for some time. The flow at its periphery would steer Nepartak to keep its west-northwest track. As such, forecasters chose a relatively westerly track among the computer forecasts. As it turned out, even though the main body of the subtropical ridge weakened on 8 July (i.e. the day when Nepartak landed over Taiwan), a weak ridge still persisted over Taiwan and caused Nepartak to maintain its west-northwest movement (Figure 3).

Figure 3

Figure 3      Background field of geopotential heights at 500 hectopascals level (after removing the circulation of Nepartak), depicting the extension of subtropical ridge over Taiwan

It was likely that the eastward bias of the forecast tracks of the other centres was due to the eastward-biased tracks of different computer forecasts (Figure 2). As it turned out, forecast tracks of computers (e.g. European Centre) gradually shifted westward day by day from 4 July to 7 July. Only until 7 July did the computer forecast track become steady and resemble the actual conditions (Figure 4).

Figure 4

Figure 4      Forecast tracks of Nepartak from the European Centre from 4 July to 7 July

Some weather enthusiasts may recall a number of occasions where computer forecasts displayed such systematic bias. Instead of relying too much on computer forecasts, we should take into account all available data and forecast products, and integrate them with our forecasting experience plus analysis of historical cases. Nonetheless, many complicated weather phenomena still remain unsolved by scientists. Forecasters still need to endeavour to deal with each and every challenge of such "unpredictable weather".

L.S. Lee & C.M. Shun

  • Heavy rain at our doorstep

  • Tuesday, 12th July 2016

It is nearly my habit to glance at the radar imagery every morning before leaving home. The situation I saw this morning (12 July 2016) is worth sharing. There was rain, some rather heavy, in many places around Hong Kong, yet Hong Kong was almost rain-free (Figure 1). Why did this happen?

Figure 1

Figure 1      Radar imagery at 8.36 a.m. on 12 July.

Actually, an active southwesterly airstream affected the coast of Guangdong today. The airstream came from the ocean and brought with it abundant moisture. Together with the effect of an upper-air disturbance, the atmosphere was relatively unstable as a whole. These conditions favoured precipitation, and according to past experience, rain could be heavier in the morning under such active southwesterlies. However, even though the large-scale weather pattern favoured the occurrence of heavy rain, the weather was not bad everywhere. Along the coast of Guangdong, rain was rather heavy in some areas, but there was almost no rain in places such as Hong Kong. Rainfall carries a degree of randomness (please note that luck may not visit the same place every time). Moreover, the upper-air disturbance which affected Hong Kong today was moving relatively slow. This caused not much change to the overall distribution of rain around Hong Kong. Those areas of heavy rain in our vicinity did not come and affect us. As shown in Figure 2, the rainfall (as estimated using radar data) over Hong Kong from midday yesterday to midday today was significantly less (areas in blue and green) when compared with our adjacent areas.

Figure 2

Figure 2      The total rainfall over a 24-hour period ending noon 12 July, as estimated using radar data.

Given the above situation, the state-of-the-art "numerical weather prediction models" on computers are still unable to forecast perfectly where and when heavy rain will occur. One of the reasons is that observation data are relatively sparse over the seas south of Hong Kong. In view of this, weather forecasters will adopt nowcasting techniques to analyse the changes in precipitation in our vicinity and the movement of rain areas on the radar, in order to assess the change of weather in Hong Kong in the next few hours and to update the weather forecast accordingly. As such, on a day with changeable weather, it is advisable to keep monitoring the latest weather forecasts, warnings, Special Weather Tips and radar imageries provided by the Observatory.

LEE Lap-shun