Korea Typhoon Season (Peak Months, Jeju, Typhoon Warning)
Korea is the country in this part of the world that people do not associate with typhoons, and for most of the year that instinct is correct. Then a fortnight arrives in late summer when it is very much not, and the surprise does more damage than the storm.
The useful facts here are the boring ones. When the window actually is, how many storms actually arrive, which part of the country gets them, and what the two Korean alerts mean when you see them on a phone in Korean.
Photographs here are stock images for illustration and are not pictures of the events described.
The Peak Months are August and September
The Korean season runs from June to October, and the storms that matter arrive in August and September. That is the shortest honest version of it.
The reason is steering rather than heat. Storms form far to the south and spend most of their lives moving west. Whether one reaches Korea depends on when it turns north, and that turn is governed by the position of the subtropical high sitting over the Pacific. Through midsummer that ridge sits far enough north and west that storms are pushed into the Philippines, Taiwan and southern China. By late August it has begun to retreat, and the door to the Korean Peninsula opens.
So the peak is not the peak of storm formation. More typhoons form in July than in September. It is the peak of storms being aimed at Korea, which is a different thing and explains why the Korean season is later than the Philippine one despite the same ocean producing both.
October is not empty either. Late-season storms do reach Korea, and because the surrounding air is cooler they often arrive already changing character, which tends to spread the rain over a wider area rather than concentrating it.
How many actually hit, and why the count is argued
You will find two very different numbers for this, and both are printed by serious sources.
The Korea Meteorological Administration frames it this way. Around 25 to 28 tropical cyclones form in the northwest Pacific in a typical year, and two or three of them influence the Korean Peninsula between June and October. Other references put the figure at about seven a year.
Neither is wrong. They are counting different things. The low number counts storms that meaningfully affect the country. The high number counts every system that enters Korea's area of influence, including ones that pass well offshore and produce a windy afternoon in Jeju and nothing anywhere else.
This matters more than it sounds, because it is the reason travel advice about Korea is so inconsistent. One guide tells you typhoons are rare and one tells you to expect several, and they are both quoting a real statistic. If you are planning a trip, the number you want is the low one. If you are planning a fishing business, you want the high one.
The other thing worth holding onto is that a two-or-three-a-year country has no muscle memory. Korea handles storms well institutionally and badly personally, in the sense that most people go years between having to think about one.
Jeju gets it first, and usually gets it worst
Jeju sits about 90 kilometres off the southern coast, directly in the path of anything recurving out of the East China Sea. It is the first Korean land a storm meets and it takes the storm at close to full strength, before the shallow water and the peninsula have had a chance to take anything out of it.
Alongside Jeju, the regions most affected are South Jeolla in the southwest and, less intuitively, Gangwon in the northeast. Gangwon is on that list because of terrain rather than track. The Taebaek range forces moist air upward on the eastern slopes, and the rainfall totals that produces are frequently the worst numbers of the whole event, from a storm whose centre never came close.
That is the general pattern in Korea. The wind story is a southern coast story. The rain story is a mountain story, and it can happen a long way from where the storm actually went.
Seoul is a partial exception in the other direction. It is far enough north and inland that a fully intact typhoon reaching it is unusual, and what the capital typically gets is a weakening system delivering a great deal of rain over a day.
A Typhoon Warning is not about wind alone
Korea uses a two-step system, an advisory and then a warning, and the thing to understand is that the warning has a rainfall route into it as well as a wind one.
On the wind side, the land threshold for a strong wind warning is a sustained 21 metres per second, or gusts of 26 metres per second. That is about 76 and 94 kilometres per hour respectively, and it is the level at which the KMA considers ordinary outdoor activity to have become a problem.
On the rain side, a typhoon warning can be issued when total expected rainfall from the system reaches 200 millimetres. No wind threshold has to be crossed for that.
This is why you can see a typhoon warning in Korea on a day that does not feel especially windy. The alert is describing the storm's whole package, and in Korea the water half of that package causes more of the damage than the wind half does. Landslides and river flooding, not roofs.
The current warning map is published in English at https://www.weather.go.kr/neng/forecast/warning.do and the full threshold table for every advisory and warning type is at https://www.kma.go.kr/eng/weather/forecast/standard_warning_info.jsp
What it means if you have a trip booked
The practical shape of a Korean typhoon is short. One bad day, occasionally two, rather than the multi-day siege that the Philippines or Okinawa can get.
Flights to and from Jeju go first and go in volume, because Jeju is both the most exposed airport and the busiest domestic route in the country. If your itinerary has a Jeju leg in late August, that is the single booking most worth making flexible.
Rail is the reassuring part. The Korean network usually keeps running through a storm with reduced speeds and some cancellations on exposed coastal sections rather than a full stop, which means a domestic plan can often be rescued by moving it onto the ground.
The part people underestimate is the day after. Mountain roads and national park trails close on their own timetable, and in a country where a large share of the tourism is hiking, a cancelled trail is the thing most likely to ruin a specific plan long after the sky has cleared.
If there is one thing to take from the numbers, it is that Korea is genuinely a low-frequency country for this, and that low frequency is exactly what makes the individual events go badly. Places that get hit constantly build the habit into daily life. Places that get hit twice a year send everybody out to work as usual, and then find out which hillside was not going to hold.
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