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Seasonal Flood Situation Report for the Lower Mekong River Basin, Covering period from 1st June to 31st December 2012

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Thailand
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Mekong River Commission
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1. Flood season 2012

1.1 Rainfall situation

The critical rainfall situation in the Lower Mekong Basin, as previous year, was concentrated in July and August, a period of Southwest Monsoon activity, low pressure troughs, storm and typhoon’s appearances in the South China Sea. In terms of total seasonal rainfall, the general picture was one of the average years (see Figure 1‐1). It can be seen that the total rainfall in the flood season 2012 at all stations in LMB (except Luang Prabang, Chiang Khan, Kratie, Kompong Cham and Phnom Penh Bassac) is lower than the long‐term average and all is lower than those of the previous flood season 2011.

The spatial variation of rainfall is high indicating that the intensity of heavy rain situations along the Lower Mekong Basin from upstream to downstream took place as a function of time (Annex A: 1. Graphs and Tables for monthly observed rainfall distribution during flood season 2012):

• The wet season started in early June; the heavy rain mostly occurred in upper reach of the LMB.

• During July ‐ August, the intensive and continued rain covered the most of upper and middle reaches of LMB and appeared more frequently in August.

• From September to October was the time of intensive rainfall in lower reach, especially from Kratie to Kompong Cham.

In 2012, three main weather patterns caused heavy rains, which are presented below:

• Southwest monsoon: influenced the Mekong River basin from early June to mid‐ October; the critical activity mostly occurred from the end of July to the end of August and from the mid‐September to mid‐October. As a common phenomenon, there was after mid‐October that a moderate to weak SW monsoon prevailed over lower parts of Thailand, Myanmar, Cambodia and Viet Nam. Figure 1-2 shows an illustration of weather map for Southwest Monsoon in the end of July.

• Tropical Low Pressure (TLP) and Inter Tropical Convergence Zone (ITCZ): these periodically appeared from early June to the mid of October with on average 3 to 7 days duration. In the flood season 2012, the frequent appearances of TLP and ITCZ during almost the entire flood season were one of the main phenomena which caused continuous heavy rain and rising water along the Mekong River. In August, TLP and ITCZ were observed and had significant influence on the upper and middle reaches of the LMB while the influence on the lower reach took place mostly in October. Figure 1‐3 shows an illustration of the appearances and influences of TLP and ITCZ to the LMB in August and October.

• Tropical depressions (TD), tropical storms (TS) or typhoons (TY): Same as previous year, there were about 8 tropical depressions, storms and typhoons which came to South China Sea and affected the Mekong River basin with different levels. Of these, the five storms DOKSURI, VICENTE, TEMBIN, BOLAVEN, and GAEMI were the most noticeable.

  1. DOKSURI was formed in the Philippines on 27th June, located in the South China Sea at latitude 22o N and longitude 112.8o E with maximum sustained wind about 65 kilometres per hour and moved West North‐ west ward and landed over China territory on 30th June.

  2. VICENTE was formed in the South China Sea on the 22nd July, upgraded to the Typhoon on 23rd July afternoon when moving north‐west ward close to South China coastline and made landfall on 24th July. It was downgraded into a tropical depression on 25th July and became a low pressure after moving deep into the mainland of China. Figure 1‐4 presents the recorded track of Typhoon VICENTE and weather maps before and after landing were shown in Figure 1‐5 and Figure 1‐6, respectively.

  3. TY‐TEMBIN and TY‐BOLAVEN were formed in the East of the Philippines on 19th and 20th August 2012, respectively. TEMBIN moved westward to the South of Taiwan while BOLAVEN moved northward to China, Korea and Japan. Figure 1‐7 and 8 show a Storm Track and weather chart of TAMBIN and BOLAVEN Typhoons, respectively.

  4. GAEMI was formed on 30th September 2012 on the South China Sea, upgraded to severe Tropical Storm named GAEMI on 4th October and made landfall over Binh Dinh and Phu Yen provinces in the Central of Viet Nam in the afternoon 6th October. After moving deep into Viet Nam territory, the tropical storm GAEMI downgraded into tropical depression in the evening 06th October and became a low pressure later. Figure 1‐9 presents the recording storm track of GAEMI and weather maps of the GAEMI before and after landing over the northern part of Central Viet Nam are showed in Figure 1‐10 and Figure 1‐11, respectively.