Typhoon Matmo threatens Taiwan with flash floods

Taiwan is bracing itself for Typhoon Matmo to strike on Tuesday night (local time), giving damaging gusts of wind, torrential rain and a storm surge at the coast.

The combination of all these weather elements will bring a significant threat of damage and disruption as the storm moves across the island, before heading on to reach the coast of south east China 24 hours later.

In preparation for the storm, the Taiwanese military is collecting and distributing sandbags to guard against the expected flooding. 

Although sustained winds of 90mph, with 115mph gusts will bring down trees and power lines, the greatest danger will come from torrential rain, flash flooding and landslides.

Extremely torrential rainfall warning

Ahead of Typhoon Matmo’s arrival, the Taiwanese Central Weather Bureau issued a number of heavy rain warnings across the country.

Greatest concern lies for central and eastern parts of the island, where warnings of extremely torrential rainfall (purple on map below) have been issued.

In these areas, around 350-400mm of rain is expected to fall in just 24 hours.

The reason that rainfall is going to be so heavy is that the very moist, tropical air from the typhoon is going to be forced up over the mountain chain of eastern Taiwan, which peaks at 3,952 metres.

As the very moist air is forced to rise upwards over the mountains, the air cools and condenses, producing clouds laden with a huge amount of moisture that then falls to the ground as torrential rain.

Even for the other parts of the island rainfall warnings have been issued for extremely heavy rain (orange on map above) and torrential rain (red on map above), with 130-350mm of rain expected in one day.

Flash flooding and landslides

As torrential rain slides down the steep slopes of the mountains, torrents of water will have the potential to give dangerous flash flooding, as large amounts of water enter the river catchments very quickly.

Another concern will be landslides, as the ground quickly becomes saturated with water and subsequently destabilises, potentially collapsing without warning. Typhoons are common at this time of year across the north west Pacific Ocean, as they draw up energy from the tropical waters that are current as warm as 31C in places – five degrees warmer than the minimum sea temperature required for a tropical storm to form.

After hitting Taiwan, the typhoon will carry on to south east China, taking the risk of heavy rain and flooding there as well.

I’ll be keeping an eye on the developments of Typhoon Matmo and posting updates on Twitter – @liamdutton