A massive rain event in southern Thailand

The severe flooding that hit southern Thailand in the middle of what was supposed to be the dry season was a “once-in-a-lifetime” event for Krabi Province.

Extremely heavy rain and flooding are quite common at the east side of the peninsula. They occur almost every year. But they happen in the rainy season of the east coast, mostly in November and December.

As for the west coast, torrential rainfall is most common in Ranong and Takua Pa in their rainy season, from May to the middle of November. In Phuket and Krabi the rain may be abundant in the rainy season but the amounts are not excessive. The rainiest month in Ao Nang since 2005 had 462 mm. The land has no problem in absorbing this rain.

This year something very extraordinary happened. March 2011 had 978 mm of rain.

How heavy was the rain?

The rain was heaviest in Surat Thani Province (with Ko Samui) and in Nakhon Si Thammarat Province. Ko Lanta at the west coast had hardly any heavy rain.

This was the rainfall (in mm) as measured at 12 places in one week, from 24 to 30 March 2011.

Krabi Province	24	25	26	27	28	29	30	1 Week
Ao Nang Moo 2	6	0	89	30	195	301	14	635
Ao Nang Moo 4	1	0	98	30	148	321	12	610
Nong Thale	4	0	90	32	135	305	24	590
Krabi Airport	6	0	79	21	131	161	9	407
Ko Lanta	6	0	17	6	46	12	0	87
Phuket Province
Phuket Town	0	0	70	22	46	102	13	262
Phuket Airport	0	0	25	36	46	185	70	362
East Coast
Surat Thani	15	69	138	6	148	242	86	704
Surat Thani agromet	30	275	267	8	250	247	224	1301
Nakhon Si Thammarat	230	290	112	7	249	91	190	1169
Nakhon Si Thammarat agromet	204	215	72	17	199	83	91	881
Ko Samui	55	87	145	95	415	196	96	1089

As the soil was already saturated after three weeks of bad weather, it was impossible to absorb any more and all the water had to flow to the sea. Imagine a vast countryside covered with about one meter of water that has to run off to sea. Flooding is then inevitable.

What caused the heavy rain?

La Niña

The weather pattern in the Pacific Ocean oscillates between two extremes: El Niño and La Niña. It has worldwide impacts. During a El Niño event the weather around Indonesia is drier than normal and during a La Niña it is rainier than average. We are in a La Niña event that started in June 2010. Since that month southern Thailand with Krabi had above average rainfall. The dry season in Ao Nang had some disappointing weather.

March had also a very wet start and it looked like it would tie with the very wet March 2009 during the last La Niña. The last week of the month brought us something quite unusual however.

Tropical Depression

On 24 March a disturbance reached the east coast from the Southeast.

The disturbance was forecast to move further into the Andaman Sea. It would become a Tropical Depression and even a Tropical Storm between Phuket and the Andaman Islands. This would be truly amazing as Tropical Storms and Hurricanes prefer more northern parts of the Bay of Bengal.

On 26 March the disturbance passed Krabi on its way out to sea with some heavy rain showers in Ao Nang. This was all forecast well in advance at the aonangweather.com website. The disturbance became a depression as expected. After the depression had passed 27 March became a quiet and mostly dry day in much of the south.

The Tropical Depression returns

In the night to 28 March the unexpected happened. The depression returned to the west coast. It did not become a Tropical Storm because it was not over sea any more.

After returning, the depression stalled and made a slow loop from Ko Lanta to Phang Nga to western Phuket and back to the northern tip of Ko Lanta.

The event could clearly be seen at Phuket radar. The depression had a dry center (that is why Ko Lanta got little rain) and a few spiral arms wrapped around the center. Rainfall was heavy under the spiral arms.

Ao Nang stayed under a spiral arm most of the time on 28 and 29 March and received half a meter of rain in 48 hours. This coincidence of events is a very rare happening. It can happen again of course but it is quite unlikely to happen any time soon.

La Niña is expected to end in May or June 2011.