The Weather of October 2011 - A cooler and wetter October

     October 2011 started off gloomy and wet with mostly fine and dry weather prevailing in the latter half of the month.  On the whole, the month was cooler and wetter than usual.  The mean temperature of the month of 24.8 degree was 0.5 degrees below the normal figure of 25.3 degrees.  The total rainfall recorded in the month was 172.4 millimetres, about 13 percent above normal.  However, the accumulated rainfall since 1 January of 1398.3 millimetres still suffered a deficit of 40 percent compared to the normal figure of 2313.1 millimetres for the same period.
 
     Under the influence of a strong northeast monsoon, local weather was windy with a few showers on the first day of the month.  Meanwhile, Severe Typhoon Nalgae had moved across Luzon and entered the South China Sea.  While edging closer to the south China coast on 2 October, Nalgae weakened and became a severe tropical storm.  Affected by its associated rainbands, the weather in Hong Kong was cloudy with some rain.  Under the combined effect of the northeast monsoon and Nalgae, local winds strengthened on 3 October and became strong at times offshore, reaching gale force occasionally on high ground.  As Nalgae moved away, Hong Kong came increasingly under the influence of the northeast monsoon and remained windy on 4 October.  With the monsoon relaxing and further weakening of Nalgae over Beibu Wan on 5 October, local winds moderated gradually while the weather remained generally cloudy with a few light rain patches from 5 to 7 October.

     A dry northeast monsoon brought generally fine and dry weather with some haze to Hong Kong on 8 and 9 October.  A broad trough of low pressure began to dominate over the northern part of the South China Sea, enhancing the moist easterly air flow over the coastal area of Guangdong.  Local weather became windy with rain for the ensuing three days.  The rain was particularly heavy on 12 October when more than 70 millimetres of rainfall were recorded over parts of the territory.  A weak cold front over Guangdong edged southwards gradually on 13 October and crossed the coast the next morning, bringing some rain to the territory during the two days.  Dominated by a dry northeast monsoon over southern China, local weather was generally fine and dry for the following ten days.  

     Another cold front formed over inland Guangdong crossed the coastal areas of Guangdong on the morning of 25 October.  Locally visibility deteriorated to below 5000m during the passage of the cold front.  The northeast monsoon behind the cold front maintained slightly cooler weather and brought a few rain patches to the territory from 25 to 28 October.  Affected by a dry continental airstream, local weather became mainly fine and dry for the rest of the month.

     Three tropical cyclones occurred over the South China Sea and the western North Pacific in the month.

     Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.4.  Monthly meteorological figures and departures from normal for October are tabulated in Table 2.