The Central Weather Administration (CWA) grades hazard-causing rainfall in four levels: heavy rain (大雨) is 80 mm or more accumulated in 24 hours or 40 mm or more in 1 hour; extremely heavy rain (豪雨) is 200 mm or more in 24 hours or 100 mm or more in 3 hours; torrential rain (大豪雨) is 350 mm or more in 24 hours or 200 mm or more in 3 hours; extremely torrential rain (超大豪雨) is 500 mm or more in 24 hours. The way the advisory name is written differs between two CWA documents: the advisory page issues it as the extremely heavy (heavy) rain advisory (豪（大）雨特報) and lists torrential and extremely torrential rain as levels within it, while question 93 of the CWA typhoon Q&A (颱風百問) calls them the torrential rain advisory and the extremely torrential rain advisory directly.
Reminder: rainfall levels have no direct link to work and school closures; this is the original wording of the CWA FAQ. When flooding occurs, do not wade through water to switch off the electrical panel and do not go down into a flooded basement; for the procedure of cutting power first and then dealing with the problem, call a plumber/electrician (水電師傅) or 119. Whether rainfall reaches an advisory threshold is decided by what the CWA issues at the time.
The official definitions of the four rainfall levels
The CWA's extremely heavy (heavy) rain advisory page and question 93 of its typhoon Q&A list the same set of definitions. Each of the four levels has a 24-hour threshold, and the first three also have a short-duration threshold; reaching either one counts.
| Level | Accumulated rainfall in 24 hours | Short-duration accumulated rainfall | Advisory name | Basis |
|---|
| Heavy rain (大雨) | 80 mm or more | 40 mm or more in 1 hour | Heavy rain advisory (大雨特報) | CWA extremely heavy (heavy) rain advisory page, typhoon Q&A question 93 |
| Extremely heavy rain (豪雨) | 200 mm or more | 100 mm or more in 3 hours | Extremely heavy rain advisory (豪雨特報) | Same as above |
| Torrential rain (大豪雨) | 350 mm or more | 200 mm or more in 3 hours | Torrential rain advisory (大豪雨特報) (the wording of typhoon Q&A question 93; the advisory page lists it as the torrential rain level of the extremely heavy (heavy) rain advisory) | Same as above |
| Extremely torrential rain (超大豪雨) | 500 mm or more | None | Extremely torrential rain advisory (超大豪雨特報) (the wording of typhoon Q&A question 93; the advisory page lists it as the extremely torrential rain level of the extremely heavy (heavy) rain advisory) | Same as above |
An advisory is issued when the threshold is "forecast or observed". Question 93 of the typhoon Q&A puts it this way: a heavy rain advisory is issued when accumulated rainfall over 24 hours is forecast or observed to reach 80 mm or more, or when at least one hour of it reaches 40 mm or more. That means the rain may not have fallen yet when an advisory goes out; it is a warning, not a tally.
The two thresholds only make sense read together. 80 mm in 24 hours works out to under 4 mm per hour on average, so the rain can be a steady all-day fall; 40 mm in 1 hour is rain concentrated into a short time. What follows is this article's experience, not an official statement: the former is more likely to bring out wall efflorescence and peeling (壁癌) and seepage around window frames, while the latter is more of a test of whether drain outlets are blocked.
Why there are both 24-hour and 3-hour thresholds
Question 2 of the CWA FAQ explains that the 2015 revision of the rainfall levels was made after collecting the warning and response trigger points of disaster prevention and relief agencies in Taiwan and abroad and analyzing debris flow and road damage, taking into account the hazard of both 24-hour accumulated rainfall and short-duration intense rainfall; it was confirmed by the central disaster prevention and response meeting (中央災害防救會報) and took effect on September 1, 2015. On March 1, 2020, a new standard of "accumulated rainfall of 200 mm or more in 3 hours" was added for torrential rain, to reflect the hazard of short-duration intense rainfall.
Question 4 gives a figure: based on rainfall data for all of Taiwan from 2009 to 2018, using the 2015 definitions, the number of days reaching the torrential rain standard averages about 3.8 days a year; with the 200 mm in 3 hours added in 2020, it is about 7.2 days, an increase of 3.4 days. In other words, short intense rainfall was already common; it just was not counted as torrential rain before.
Question 1 of the CWA FAQ also cautions that the causes of disasters are complex, rainfall levels cannot describe individual disaster events, and different places tolerate rainfall differently, so the degree of disaster differs. When sudden or multi-day rain does not reach an advisory level but is judged to pose a risk of disaster, the CWA issues a real-time message as a reminder, such as the severe thunderstorm real-time message; see what the severe thunderstorm real-time message is.
Rainfall levels and work and school closures are two separate standards
The first sentence of question 5 in the CWA FAQ is "stopping work and classes has no direct link to rainfall levels". Work and school closures (停班停課) follow the central personnel administration agency's (人事行政總處) natural disaster work and class suspension measures (天然災害停止上班及上課作業辦法): the head of a special municipality or county (city) decides, only when there is a concern about disaster, by making an overall judgment from the CWA's rainfall observations or forecasts against the "rainfall alert values set for each area" (各地區所定雨量警戒值).
The alert values in the schedule to Article 4 of those measures are in terms of "forecast accumulated rainfall for the next 24 hours"; for example, Taipei City and New Taipei City are 350 mm, Keelung City is 200 mm, and Taoyuan City is 200 mm in mountain areas and 350 mm on the plains. Compared with the table above, the rainfall reference for closures in Taipei and New Taipei equals the 24-hour value of the "torrential rain" level, and the schedule has no short-duration 3-hour threshold, so a torrential rain advisory alone does not mean a closure. However, item 2 of Article 4 reads "based on weather forecasts or actual observations", item 3 also provides that an announcement may still be issued when wind or rainfall has not reached the standards of the first two items but there is a risk of disaster, and Article 10 further allows the reporting authority to issue one at any time according to actual conditions; if short intense rain cuts off transport or poses a risk of disaster, the head may still decide on a closure. For the decision process and announcement times, see how typhoon day-off is decided.
What to do at home at each advisory level
The table below lists home actions compiled by this article; they are not official rules. The reminder on the CWA advisory page is that heavy rainfall often causes landslides, rockfalls and debris flows in mountain areas, that low-lying areas should guard against ponding and flooding, and that people near rivers should watch for rapidly rising water and flash floods.
| Advisory level | Home actions suggested by this article |
|---|
| Heavy rain advisory | Clear fallen leaves from the balcony downspout inlets and drain outlets; take in the balcony floor mat; open the hyperlocal real-time rain trend radar to see whether the rain band is staying over your home. |
| Extremely heavy rain advisory | Ground-floor and basement residents should get their drainage check valves and flood barriers ready; move cars parked in basements or at low-lying roadsides first; confirm the basement sump pump can start. |
| Torrential and extremely torrential rain | Do not go down into a basement that is already flooded; when ground-floor outlets may be soaked, do not operate the electrical panel yourself, put personal safety first, and have a qualified electrician or plumbing and electrical worker handle it, calling 119 if there is immediate danger; after the water recedes, have someone check in the order set out in drying outlets and insulation testing after a flood; use the real-time flood report board to see whether flooding has been reported nearby. |
| After the rain stops | For a house that has flooded, follow the order for cleaning up after a flood: confirm the power is off, photograph for evidence, then clean up, and watch the deadline for flood relief applications. |
For what "100 mm in 3 hours" looks like on a radar map and how the colors correspond to rainfall intensity, see what the colors on a radar echo map mean. Extremely heavy rain mostly comes with typhoons or plum rain fronts; for full preparation during a typhoon, see the typhoon and heavy rain self-rescue guide.
What not to do yourself
- If standing water has reached an outlet or extension cord, do not pull it out with bare hands, and do not step into the water to switch off the main breaker.
- While the basement water level is rising, do not go down to rescue a scooter or belongings.
- If drains are backing up sewage, do not dismantle floor drain pipes or the toilet yourself; wait until the water recedes and have a plumber/electrician (水電師傅) install a check valve.
- If the exterior wall or roof leaks in heavy rain, do not go up on the roof to patch it; deal with it after the rain stops.
FAQ
Does an extremely heavy rain advisory mean a day off?
Not directly. The CWA states plainly that work and school closures have no direct link to rainfall levels. Closures depend on the 24-hour rainfall forecast alert values in the schedule to the measures (350 mm for Taipei and New Taipei) plus whether there is a risk of disaster, and are decided by the head of the county or city. An extremely heavy rain advisory is a CWA warning, and having one issued does not necessarily mean a closure.
Which is more serious, a heavy rain advisory or a 90% chance of rain?
They are two different things. By the CWA's definition, the probability of rain is the chance of rainfall of 0.1 mm or more occurring within the forecast period, and it has no direct relation to rainfall intensity or accumulated rainfall; a heavy rain advisory answers the question of whether rainfall will reach 80 mm in 24 hours or 40 mm in 1 hour. For details see what a 30% chance of rain means.
The advisory says "torrential rain" but it is barely raining where I am. Is it wrong?
Not necessarily. Advisories are issued by county, city or region, and question 1 of the CWA FAQ also says different places tolerate rainfall differently. Rainfall in the mountains and on the plains of the same county or city can differ several times over, and the advisory is aimed at the places in the whole region that might reach the threshold.
Where can I find the actual figures for 80 mm of rain?
The observation data on the CWA website has hourly and accumulated rainfall for each rain gauge station, and the extremely heavy (heavy) rain advisory page also links to the Taipei disaster prevention information website (臺北市防災資訊網) and the New Taipei disaster information e-point (新北市災訊 E 點通). This site's rain trend radar shows the short-term trend for your location.
Last updated: 2026-10-30
This article was compiled by SOSfix editors from public information and is for general reference only. For individual cases, contact the Central Weather Administration, your county or city disaster response center, or a qualified plumbing and electrical worker.
Sources
- Central Weather Administration: extremely heavy (heavy) rain advisory page, definitions of heavy rain and extremely heavy rain (豪（大）雨特報頁面「大雨」與「豪雨」定義; verified 2026-10-30) (in Chinese): the 24-hour, 1-hour and 3-hour thresholds for heavy, extremely heavy, torrential and extremely torrential rain; reminder text on hazards from heavy rainfall.
- Central Weather Administration typhoon Q&A: question 93, what are a strong wind advisory and an extremely heavy rain advisory (何謂強風特報？豪雨特報？) (in Chinese): the wording that an advisory is issued when forecast or observed values reach a threshold, and the advisory names for the four levels (heavy rain advisory, extremely heavy rain advisory, torrential rain advisory, extremely torrential rain advisory).
- Central Weather Administration FAQ: question 1, why have rainfall levels, and how do they relate to disaster alerts (為什麼要有雨量分級？與災害警戒有什麼關係？) (in Chinese): levels cannot describe individual disasters, places tolerate rainfall differently, a real-time message is issued when rainfall falls short of an advisory.
- Central Weather Administration FAQ: question 2, how the rainfall levels were set (雨量分級是怎麼制定的？) (in Chinese): in effect from September 1, 2015, with 200 mm in 3 hours added for torrential rain on March 1, 2020.
- Central Weather Administration FAQ: question 4, how the 2020 revised rainfall levels differ from the 2015 version (109 年新修訂的雨量分級與 104 年版的雨量分級有什麼差異？) (in Chinese): days reaching the torrential rain standard rise from about 3.8 to 7.2 a year.
- Central Weather Administration FAQ: question 5, are rainfall levels related to stopping work and classes, and whom should you ask about it (雨量分級和停止上班上課有關嗎？停止上班及上課到底要問誰？) (in Chinese): closures have no direct link to rainfall levels and are judged overall by the county or city head.
- National laws and regulations database (全國法規資料庫): schedule to Article 4 of the natural disaster work and class suspension measures, rainfall reference table for suspension by each reporting authority (各通報權責機關停止上班上課雨量參考基準一覽表) (PDF) (in Chinese): Taipei City and New Taipei City 350 mm, Keelung City 200 mm, Taoyuan City 200 mm in mountain areas and 350 mm on the plains.
- National laws and regulations database: natural disaster work and class suspension measures (天然災害停止上班及上課作業辦法; amended April 24, 2026) (in Chinese): Article 4 item 2 "based on weather forecasts or actual observations", item 3 an announcement may be issued when the standard is not reached but there is a risk of disaster, Article 10 may be issued at any time according to actual conditions.
- Central Weather Bureau (中央氣象局): weather knowledge Q&A booklet (氣象常識問答集; compiled March 2020, PDF) (in Chinese): question 4, the probability-of-rain forecast is the probability of rainfall (0.1 mm or more) occurring and has no direct relation to rainfall intensity or accumulated rainfall.
- Central Weather Bureau: meteorological data standard (氣象領域資料標準（v1.0，110 年 10 月）; version 1.0, October 2021, PDF) (in Chinese): probability of rain defined as "the probability of a precipitation event of 0.1 mm or more within the forecast period".
