A Typhoon Can Be "Large" Without Being "Strong". Japan Grades Those Two Things Separately, and a Storm This Week Showed Exactly Why That Matters.
Here is a thing that confused me for years, and I say that as somebody who spent seven years in a country where typhoons are a normal part of the calendar rather than an event.
You will see a Japanese bulletin describing a system as a large typhoon. You will look up the same storm on an American site and find it listed as a tropical storm, which is a rung below typhoon entirely. Neither of them is wrong. They are not measuring the same thing, and once you know which is which, a whole category of confusing storm coverage stops being confusing.
Two grades, not one
The Japan Meteorological Agency, which is the regional centre responsible for this entire basin, files a typhoon under two separate headings. One is size. The other is strength. They are independent of each other, and a storm can score on one and not the other.
Size is measured by the radius of what the agency calls the strong wind area, meaning the zone where sustained winds are 15 metres per second or more. If that radius is at least 500 kilometres but less than 800, the storm is graded large. At 800 kilometres and above it becomes very large. Below 500 there is no size grade at all, and the bulletin simply does not mention size.
Strength is measured by the maximum sustained wind near the centre, and the thresholds are these. From 33 metres per second up to 44, the storm is graded strong. From 44 to 54 it is very strong. At 54 and above it is violent, which is the top of the scale.
Notice what falls out of that. A storm whose peak winds are below 33 metres per second gets no strength grade whatsoever. The field is left blank. It is still officially a typhoon, and Japan will still name it and number it and issue bulletins on it, but the strength column is empty because the wind never reached the bottom of the scale.
So "large typhoon, no strength grade" is not a contradiction or a typo. It is a completely ordinary combination, and it describes a real and fairly common kind of storm: broad, sprawling, wet, and not especially windy at the core.
There is one more number underneath all this that is worth knowing, because it explains the mismatch with English-language coverage. For the JMA, a tropical cyclone becomes a typhoon when its maximum sustained wind reaches roughly 17 metres per second, which is 34 knots. That is the entry point. Everything above it is a typhoon as far as Japan is concerned, whether or not it ever earns a strength grade.
English-language sources, working from a different convention, will call that same system a tropical storm and reserve the word typhoon for something considerably more violent. So you get the situation where a Japanese bulletin says large typhoon and an American one says tropical storm about the identical system on the identical day, and readers assume somebody is exaggerating. Nobody is. The line is just drawn in a different place.
Where those numbers actually come from
The reason the two scales exist separately is that they answer different questions.
Wind speed at the centre tells you how violent the worst part of the storm is. It is the number that decides whether roofs come off. It is also the number almost every other system in the world is built around, including the Saffir-Simpson scale that produces the Category 1 through 5 figures you see quoted for hurricanes.
Radius tells you how much of the map is involved. That is a different problem. A compact, ferocious storm can destroy a town and leave the next province untouched. A sprawling, mediocre storm can put four prefectures under a wind advisory at once without doing serious damage to any single one of them.
Both of those situations are worth warning about. Only one of them shows up if your scale is built on wind speed alone.
I noticed this most in the Philippines, where PAGASA issues wind signal numbers province by province rather than for the country as a whole. On a big sprawling system the list of provinces under a signal goes on and on, and people who have never looked at the wind figure assume from the length of the list that something enormous is coming. Sometimes it is. Sometimes the list is long precisely because the storm is spread thin.
The reverse case is the one that actually catches people out. A small, tight, genuinely violent storm produces a short list. Two or three provinces, maybe. If you are reading the list as a proxy for severity, a short list reads as reassuring, and for the provinces on it that is the worst possible conclusion to draw.
And it is worth being clear that Japan's size scale has no grade below large. There is large, there is very large, and below 500 kilometres there is simply nothing printed. That silence is not the same as saying the storm is small and harmless. It only means it did not clear the bar for a label, which is why a compact monster and a nondescript system look identical in the size column. Both are blank.
Nangka this week
The storm that prompted me to write this is the one sitting out over the open Pacific right now.
Typhoon number 17 of the season, named Nangka, formed near the Ogasawara Islands on the morning of 12 August. It was the third earliest that a season's seventeenth typhoon has ever formed, which tells you something about the year 2026 that is worth a separate article.
On the morning of 13 August, the agency's own listing for it read like this. Size grade: large. Strength grade: blank. Central pressure 994 hectopascals, maximum sustained wind 18 metres per second near the centre, gusts to 25.
Eighteen metres per second is not close to the 33 needed for a strength grade. It is barely past the 17 needed to be called a typhoon in the first place. Sit those two facts next to each other and the storm comes into focus: it cleared the entry requirement by about one metre per second and then spread itself over five hundred kilometres of ocean.
The strong wind area was wide enough to qualify as large, though, and Chichijima and Hahajima, out in the Ogasawara chain, both went inside it and got winds of 15 metres per second or more. If you live on those islands, the strength grade being blank does not do you a great deal of good.
That is the entire phenomenon in one bulletin. Wide enough to be graded on size, too weak to be graded on strength, and still perfectly capable of making life unpleasant on any island that happened to be inside the radius.
Nangka never came near mainland Japan. It wandered around the waters near Minami-Torishima, weakened, and by the evening of 14 August it had dropped to a tropical depression. The forecast never had it making landfall. It was, in the plainest terms, a storm that did not do very much.
And forecasters were still telling people to be careful, which is the part of this that actually matters.
Why a big weak storm can still ruin your day
A storm graded large and ungraded on strength will not take your roof off. What it will do is take up space, and taking up space has consequences that have nothing to do with the wind at the centre.
The strong wind area is where advisories get issued, so a large storm triggers warnings across a much wider stretch of coast than its intensity would suggest. Ferries make their suspension decisions on sea state rather than on category, and sea state responds to the size of the wind field as much as to its peak. Anybody who has watched a schedule board go red on a day with blue sky overhead has met this already.
Then there is the water. A broad system has more area over which to pull moisture upward, and if it is also moving slowly, which sprawling weak systems very often are, it sits over the same stretch of ocean or coast for a long time. Rainfall totals are driven far more by how long a storm lingers than by how fast its winds spin.
That is worth saying twice, because the headline number is always the wind. The thing that historically kills the most people in these systems is water, and water does not care about the strength grade at all.
What to look at instead
If you are trying to work out whether a storm matters to a specific trip, the two grades are a starting point rather than an answer.
Look at the radius first if you want to know whether your location is even involved. A storm three hundred kilometres away with a six hundred kilometre wind radius is your problem. The same storm with a two hundred kilometre radius is somebody else's.
Look at the strength grade if you want to know how bad it gets at the centre, and then remember that unless the centre is coming to you, that number is describing weather you will never experience.
Look at forward speed if you want to know about rain, because a slow storm is a wet storm, and that is true regardless of what either grade says.
And look at the agency that actually covers your coast rather than whichever one an English-language article happened to quote. The Japan Meteorological Agency publishes its typhoon information in English at jma.go.jp, PAGASA issues the Philippine provincial signals at pagasa.dost.gov.ph, and the Hong Kong Observatory is the fastest place to see several agencies' tracks for the same storm side by side, which is a good habit when the forecasts disagree.
Anyway. The next time you see a storm described as large and you cannot find a strength rating anywhere, nothing has gone wrong with the bulletin. It just means somebody measured two different things and only one of them scored.
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