Like much of Asia, Thailand is accustomed to serious precipitation during the rainy season. But as sheets of rain lashed Bangkok on Wednesday and loud thunderclaps reverberated across the city, Thais—from government leaders to business owners to ordinary citizens—saw it as an ominous sign that the flooding that has hobbled the country for months and drowned a number of provincial areas could well devastate the capital, toward which the water has been making inexorable and dangerous progress.

The floods have hit other countries in the region, notably Cambodia and the Philippines, killing hundreds, uprooting tens of thousands, and causing hundreds of millions of dollars in damage. But the threat may be the gravest in Thailand, where 2.5 million people have been displaced, more than 3.5 million acres inundated, and some $1.5 billion in damage reported. The surging waters have ravaged Thailand’s all-important rice crop, apparently knocking the country from its No. 1 perch among nations that export the commodity. And the flooding constitutes a huge test for the government of Yingluck Shinawatra, in office a scant three months.

Yingluck, the country’s first female prime minister, has seemed overwhelmed at the scale of the disaster, acknowledging in a televised address this week a fear that the local and national governments would not be able to contain the water that’s making its way south to Bangkok. “The widespread flood problem is reaching crisis level, the worst in decades,” declared Yingluck, the sister of controversial former prime minister Thaksin Shinawatra, who is now an overseas fugitive after being ousted in a bloodless coup in 2006 and subsequently convicted in absentia on corruption charges.

Authorities are bracing for the mighty Chao Phraya River, which flows through Bangkok, to break its banks and overwhelm the teeming city, jammed with both ultramodern high-rises and squatters’ hovels. The river has already spilled over some dams upstream.

"If we cannot divert the flood water into the sea, it will cause extensive damage to the provinces in its path," Yingluck told Thais. She and others are awaiting with growing alarm the arrival of peak tide on Oct. 15 and 16—which coincides with the water flow hitting Bangkok. More than 250 people have died in the flooding, which has damaged about 60 provinces, half of those badly, with provinces such as Ang Thong and Ayutthaya hardest hit.

The latter province—center of the Ayutthaya Kingdom that prevailed from the mid-1300s until 1767, and the location of an ancient national heritage site—remains under water, with the river having risen approximately 15 feet. Authorities use jet skis to navigate some areas, while some residents get around in small boats. Many are stranded in camps along the Asian Highway linking Bangkok and Chiang Mai in the north. Ayuttthaya Vejthani Hospital evacuated patients after the rising waters took over its ground floor; occupants moved out of an art center for national artists.

“Business has ground to a halt. All the factories are closed. Nobody’s working, and people are stuck at places of business like banks,” artist and businesswoman Arisara Punyanitya told The Daily Beast. “Everybody is wearing shorts and flip-flops. You can’t tell who’s who. The [provincial] governor looked like a tuk-tuk driver. You can’t go anywhere; the water is too strong.”

Agriculture officials had hoped to increase the kingdom’s industry-leading rice crop this year, but those ambitions have been submerged. Instead of seeing production rise to 25 million metric tons, the government has had to settle for a drop of 3 million metric tons, to 21 million, over last year. In a way, the decline has created a shortage that benefits Thailand, pushing up prices around the region—but it also has caused a spike in prices at home.

Ironically, Yingluck and her Pheu Thai party came to power in part on a promise to restore the so-called paddy-pledging program designed to increase the minimum guaranteed price for rice farmers—and supposedly result in greater wealth distribution. That and other “populist” strategies such as increasing the minimum wage have brought harsh criticism from some economists, and political opponents such as the last prime minister, Abhisit Vejjajiva. Analysts also had been discussing whether the powerful Thai army would move against Yingluck, especially if she were to make any move to bring back her exiled brother—regarded by many as running the country in any event, by proxy.

Those issues are now taking a back seat to the question of how well the new premier, a political novice, handles the crisis created by nature. She has been politically savvy enough to reach out to Abhisit, consulting him on flood-prevention plans in a show of national unity. In the meantime, the government pursues efforts to divert floodwater to the sea and dredge several city canals to take in more water, as the military builds floodwalls to protect inner-city Bangkok, as well as the eastern part of the city where the international airport is located.

Authorities are making food drops by helicopter to stranded people, and Thais around the kingdom have mobilized to raise donations—food, water, clothes, and supplies—to send to the most stricken areas. At Bangkok’s massive Mah Boon Krung shopping mall last weekend, rock bands played at a fundraiser as volunteers set up numerous stalls to collect donations and sell recycled clothes to raise money. Those efforts, however, will soon have to be directed at people who live in the capital itself. Prime Minister Yingluck is concerned about whether the Chao Phraya Dam will be able to do its job sufficiently to protect nearby areas. Bangkok’s governor told the Bangkok Post that speeding up draining is difficult, since the city’s waterways are already at very high levels.

Many city residents are heeding warnings that the floods will hit big this week. In districts such as Sathorn, merchants are taking protective measures. A branch of Siam Commercial Bank has set up a ring of sandbags in front of its building, as has the owner of a bakery called Pear’s, who explained that her shop is on the lower side of the street. Manutsupa Kung, owner of Only You Boutique, watched as a workman built a three-foot wall of red bricks and cinderblock in front of her store. “This is supposed to be the worst flooding in our lives,” she said, laughing nervously. “We’re not taking any chances.”