Reading a typhoon warning

The forecasters

The CWA’s Weather Forecast Center watches the weather around the clock. Its tasks include forecasts, close monitoring of typhoons and timely typhoon warnings, and public information channels such as the automatic weather hotlines 166 and 167.

The agency itself was founded in 1941 in Chongqing as the Central Weather Bureau. It came under the Ministry of Transportation and Communications in 1947 and moved to Taiwan in 1949. In 1957 its structure was simplified and most of its work passed to the Taiwan Provincial Weather Institution; in 1971 it was restructured and returned to the ministry, and on 15 September 2023 it was elevated to the Central Weather Administration.

How many typhoons

According to the CWA, roughly 21 to 25 typhoons typically form in the western North Pacific between June and December, and around 3 to 5 of them affect the vicinity of Taiwan. Each season is different, which is why the CWA also publishes seasonal outlooks.

A typhoon does not need to make landfall to matter. The CWA’s warnings and probability products, described below, are built around the area of strong winds that surrounds the centre, because that area can reach a coast even when the eye passes offshore.

Sea and land warnings

The CWA issues two kinds of typhoon warning, for the sea and for land, and explains when each is issued in its weather FAQ. The English note on its warning page ties the warning to the moment a typhoon’s radius of 30-knot winds is expected to reach land in Taiwan, Kinmen or Matsu, with 18 hours as the threshold. Thirty knots is about 56 kilometres per hour, force 7 on the Beaufort scale. Once a warning is in force, bulletins follow every hour, more often if needed.

The same FAQ explains whom to ask about whether work and school are suspended during a typhoon, and what it means when a typhoon “makes landfall”.

Why forecasts spread out

A typhoon forecast is a best estimate of where the centre will go, and that estimate gets less certain with every extra day. The CWA publishes its own average track errors for 2019 to 2023:

• 12 hours ahead: 56 km
• 24 hours ahead: 83 km
• 48 hours ahead: 143 km
• 72 hours ahead: 215 km
• 96 hours ahead: 275 km
• 120 hours ahead: 357 km

In other words, the further ahead a forecast looks, the wider the band of places the typhoon might reach, and the more careful one has to be with any single line on a map.

What a probability means

To show this uncertainty, the CWA publishes wind speed probabilities, based on its track errors from 2019 to 2023. A value of 80% means that in 80 out of 100 similar forecasts, the storm’s radius of 30-knot winds passed over that place.

The product covers up to 120 hours ahead and is updated every 6 hours until the typhoon dissipates. It includes a map of the area within which the centre has a 70% chance of passing, probabilities for major cities and islands, a chart showing how the probability has changed from one forecast to the next, and two arrival times for strong winds: the most likely one and the earliest reasonable one. The earlier time is meant for people who need longer to prepare, such as vulnerable groups.

In its guide, the CWA uses Typhoon Koinu in 2023 as an example. For Taitung City, the probability rose from 11% at 2 pm on 29 September to 72% at 8 pm on 1 October.

What a probability does not say

A probability is not a measure of strength: the CWA notes that it alone cannot tell how severe the wind and rain will be. It does not give the probability of landfall, of particular kinds of damage or of a warning being issued, and it is not suitable for slow-moving, stationary or looping typhoons, whose tracks are even less certain.

The CWA presents it as an early reference about three to five days before a typhoon approaches. Within 24 hours, and whenever a warning is in force, the warning bulletins, which are updated more often, remain the main source of information.

Making forecasts better

The CWA’s strategic plan includes extending how far ahead tropical depression tracks are forecast and improving the accuracy of its regional model’s typhoon track and intensity forecasts, with a stated target of 8%. Other items in the plan are a new-generation global forecast system, a regional model with a resolution of 1 kilometre, reception of data from new Japanese, Korean, European and American weather satellites, and the use of artificial intelligence to add value to model output.

Typhoons and the city’s tallest building

In Taipei, typhoon winds also shaped the city’s tallest building. Guinness World Records reports that during Typhoon Soudelor in August 2015, when Taipei saw winds of about 160 kilometres per hour, the damper of Taipei 101 swung by 1 metre, its largest recorded movement. Scientists at Academia Sinica’s Institute of Earth Sciences have studied the tower’s vibrations during Typhoon Fanapi in 2010. The story is on A pendulum in a skyscraper.