The equivalent of 160,000 Olympic-sized swimming pools (400 million cubic metres of water) is set to run through Thailand's capital, which can only drain a small fraction daily, according to the government's flood relief operation centre on 26 October.
"Floods will hit every area of Bangkok, but each area will see different levels of water," said the director of the centre, Pracha Promnok, as quoted in local media.
Run-off from flooding in the north and a seasonal high tide are expected to push water levels in Bangkok's largest river above the city's 2.5m-high embankment.
The size of the population - more than eight million residents - coupled with the run-off, has made for an unprecedented and atypical emergency, said Kirsten Mildren, information officer for Southeast Asia at the UN Office for the Coordination of Humanitarian Affairs (OCHA), who has worked in disasters for almost a decade.
"I cannot think of another emergency where I have seen it like this, where you have got the authorities and emergency services really battling to get the water to move around a city of this size. It is really incredible."
The government's irrigation department has been trying to spare the city by pumping the deluge around the city's perimeter through canals and selectively opening flood gates.
While the Bangkok Metropolitan Authority (BMA) in a 23 October flood update requested residents not to panic, it did little to assuage fears: "Upon assessing the situation with all indicators, BMA would like to inform that a rather serious upcoming [disaster] is very imminent and inevitable."
These types of warnings have only amplified public uncertainty, said Bhichit Rattakul, a former governor of Bangkok and now executive director of the Bangkok-based NGO Asian Disaster Preparedness Center (ADPC).
Nationwide, 28 of 76 provinces have been flooded in this year's monsoon that started in late July; six of the country's major dams are at 99 percent capacity or higher, according to the national relief centre.
The airport where the centre operates has been closed, with two terminals under 80cm of water and all flights grounded.
As of 26 October, there have been 821 flood-related deaths in Thailand, Cambodia, Vietnam, Laos and the Philippines, where more than eight million people continue to be affected by severe flooding, according to the governments.
This article was produced by IRIN News while it was part of the United Nations Office for the Coordination of Humanitarian Affairs. Please send queries on copyright or liability to the UN. For more information: https://shop.un.org/rights-permissions