Under the influence of an intense cold surge, the Hong Kong Observatory recorded a minimum temperature of 3.1 degrees on 24 January 2016 (Sunday). This is the lowest temperature since 1957. Such an extreme event is a big challenge in weather forecasting. Currently, the Observatory made reference to the best computer numerical weather prediction models from around the world, including those from the European Centre for Medium-Range Weather Forecast, the Japan Meteorological Agency and the 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. In the past year, the average forecast error in temperature based on this method is about 1 to 2 degrees.
In forecasting the current cold surge, all these computer models adjusted their forecasts progressively based on more and more information collected. For instance, about a week ago on 17 January, all these computer models forecast a minimum temperature of about 10 degrees for Hong Kong on 24 January (Figure 1). The Observatory also made a similar forecast on the same day. Then on 23 January, the Observatory adjusted the forecast based on the prevailing situation, consistent with the forecast from computer models (Figure 2). It turned out that the actual minimum temperature on 24 January (black lines in Figures 1 and 2) was lower than these forecasts. This is related to the rain being more persistent than forecast on 24 January. The Observatory would like to stress that the forecast accuracy is different for different forecast period. In general, the longer the forecast period, the lower the forecast accuracy.
There were reports saying certain overseas computer model once forecast snow on 24 January. However, such forecast changed from time to time and was not dependable. The Observatory predicted that the minimum temperature near the ground on 24 January would be above freezing point. As other atmospheric conditions such as the upper air temperature variations and amount of rainwater were not favourable for snow, no snow was forecast.
In view of the intense cold surge, the Observatory issued a Special Weather Tip on 20 January pointing out that the weather would turn cold significantly and there would be icing and frost on high ground and the New Territories. The Observatory further issued a Cold Weather Warning and arranged a press conference on 21 January to alert the public of the various effects of the very cold weather. The Observatory subsequently issued a Frost Warning on 23 January to specifically alert the farmers and the people concerned of likely ground frost on high ground and the northern part of the New Territories.