Cyclone Harold - a category five, the highest possible - has sat just to the west of Vanuatu's central islands for much of the past day, gathering significant strength in the past 12 hours.
On Monday afternoon, just after 1pm, local time, it made lanfall on Santo's southwestern coast, and is forecast to continue on a track that takes it very close to Luganville, the country's second-largest town with a population of more than 16,000 people.
Fred Jockley, a managing forecaster at the Vanuatu Meteorological Service, said this storm was the most serious since Cyclone Pam, which destroyed much of the country in 2015, killing few people, but setting livelihoods, infrastructure and the economy back years.
Mr Jockley said Harold was displaying the signs no one wanted to see: it had effectively stalled, moving as slow as two knots, which allows it to suck up moisture from the warm ocean and gain ferocity; it was growing in size, and its force would likely envelop much of Santo and Malekula, Vanuatu's two largest islands; and its current track had it skirting very close to Luganville, the country's second-largest city with more than 16,000 people.
"It's very slow now. It's been very slow the past six hours, but now it's beginning to pick up speed," Mr Jockley told RNZ Pacific. "It's going to go through Santo and Malekula."
"The winds range is covering the whole of Santo and part of northern Malekula."
Hundreds of people across Santo had already sought shelter in evacuation centres, and flooding has been reported in many areas. On Monday morning, authorities evacuated people from remote areas where rivers had burst their banks into villages. Communication has since been lost with areas outside Luganville.
The official number of people in evacuation centres is so far unknown.
"We're expecting more rainfall and flooding to continue over Luganville and Malekula, even extending to Penama and Torba," Mr Jockley said.
Cyclone Harold comes at the worst possible time for Vanuatu. The country was under a state of emergency because of the Covid-19 pandemic, with the country's borders sealed, and mass gatherings of more than a few people banned.
Yesterday, the government relaxed its conditions as the country contends with dual emergencies, allowing people to travel to seek shelter, and to congregate in evacuation centres. Vanuatu is yet to confirm any cases of the coronavirus.