Flash flooding hit the Nang Rong waterfall at Khao Yai National Park, stranding 30 tourists and a film crew. (Photo via Post Today)

Tourists at Khao Yai National Park in Nakhon Nayok were warned yesterday to steer clear of Nang Rong and Wang Takhrai waterfalls and stay on high ground because of mountain runoff caused by heavy rain.

The water level at the two waterfalls rose rapidly Monday afternoon, prompting authorities and resort operators to issue stern warnings.

About 30 tourists including a film crew have been stranded at a riverside resort but all the crew are safe.

Worapan Suwannut, deputy governor of Nakhon Nayok, said operators of riverside resorts had been asked to monitor official announcements closely over the next two days and suspend services for safety reasons if necessary.

Thai PBS broadcast an alert Monday night that floods would soon hit the downtown area.

Dramatic footage captured by the network show the fury of Monday's flash floods.

He said authorities are on standby around the clock and were mobilising resources to help the stranded tourists and escort them out of the resort.

Akkharadet Yodjumpa, a singer better known as Kong Huayrai, live-streamed that he was stuck at the resort in Wang Takhrai following heavy rain and runoff.

The singer, who was with a film crew, said he and the others were safe and at about 6pm he posted that the water was receding.

Sirinan Maneechote, chief of Nakhon Nayok's disaster prevention and mitigation office, said provincial authorities had issued a warning to local residents about a possible surge in the water level.

Wichai Pornleesaengsuwan, a national park official, said the runoff, the worst in 10 years, was triggered by heavy rain on the mountain. On Sept 25, 1994, a flash flood killed 21 people at Wang Takhrai.

Flash floods hit this resort near Nang Rong waterfall, flooding the hallways and inundating a car. (FB/Chumnan Maunghong)

In Phuket, in the South, tourists were warned Monday not to swim at several beaches due to rough seas and strong winds.

Red flags were put up at popular beaches where trees have been toppled by gusting winds brought on by the monsoon surge. Many beach roads were also covered with thick sand.

In Chumphon, Monday rainstorms caused damage in Lang Suan district in the early afternoon.

Motorists were left stranded when a section of the southbound Asian Road at the 70th-kilometre marker was blocked by fallen trees.

On the Bangkok-bound route, a tree fell on a car, shattering its windscreen and bonnet. No one was injured.

Motorists on Phetkasem Road have been warned to be wary of loose soil on the road between Ranong and Chumphon.

The Office of National Water Resources has warned residents about possible flooding along the Phetchaburi River in Phetchaburi, the Kra Buri River in Ranong, and Takua Pa River in Phangnga.

In Phetchaburi, high-risk areas include the low-lying and riverside communities in districts of Kaeng Krachan, Tha Yang, Ban Lat, Muang and Ban Laem.

Meanwhile, residents in the North and Northeast have been urged to brace themselves for rain tomorrow and Thursday as a tropical storm moves across the upper part of Vietnam.

People near the Andaman Sea and the Gulf can also expect rough seas and strong winds, with larger vessels told to proceed with caution and small boats advised to stay ashore.

In Kanchanaburi, officials at Srinakarind dam and Vajiralongkorn dam Monday gave assurances that the reservoirs can receive more water and their structures are not compromised.