Backward Trajectory Map in September 2013

A transition from southerly to easterly flow over the coast of Guangdong in early September was clearly depicted by the backward trajectory map. Convergence between the southerlies and easterlies brought heavy rain to Hong Kong on 4 and 5 September, dumping more than 280 millimetres of rainfall over the 2-day period.

The trajectory map also showed that for several days in the latter part of the month, the air masses reaching Hong Kong originated from the north in the presence of tropical cyclones over the Luzon Strait and the northern part of the South China Sea. In particular, a subsiding continental airstream ahead of Usagi brought very hot weather to Hong Kong on 21 September, with temperature at the Observatory climbing to a maximum of 34.7 degrees, the highest for September since 1969.

Monthly backward trajectory map showing origins of air masses affecting Hong Kong can be found at the following webpage:

http://www.weather.gov.hk/en/wxinfo/trajectory/trajectoryMonthly.shtml

Backward Trajectory of Air Mass reaching Hong Kong in September 2013