TYPHOON DUJUAN — THE WIND EASES AS IT TURNS ONTO TOKYO, AND THE RAIN IS WHAT ARRIVES

On the board · September 20, 2026 →

CRESTING · September 20, 2026 · NO. 0920

THE STORM

There is a way of reading a storm that follows only its wind, and it would get this morning wrong. The system turning up the Japanese coast is weaker in the wind column than it was — 970 hPa, maximum sustained winds of 35 m/s, about 126 km/h on a 10-minute averaging period, gusts near 50 m/s, about 180 km/h — and forecasters expect it to stop being a tropical cyclone at all by Tuesday. None of that is relief. A storm that transitions does not switch off; it spreads out, and what it spreads is rain. The forecast has it reaching the Izu Islands and the Kanto region from Monday afternoon into early Tuesday, passing offshore south-east of the Boso Peninsula on Monday morning. The wind figure is coming down on precisely the days the water figure goes up.

The rain is why this leads. Eastern Japan is forecast about 150 mm in the first 24 hours and about another 200 mm through midday Monday — roughly 350 mm onto the Izu Islands and the Kanto region, which is where Tokyo is. The depth on its own is not the story; the ground under it is. This soil has already taken about two weeks of record rain, so the column has very little left to absorb. The same rainfall that a dry September routes into drains and rivers instead sits on slopes and streets that are already at capacity. That is the entire distance between a wet holiday and a landslide, and it is a distance you can measure days in advance.

And it is a holiday. The rain window falls across Silver Week, which makes the decisions in front of people concrete rather than abstract: whether to thin a Monday bank of departures out of the Tokyo airports, whether to suspend a rail line, whether to hold a ferry to the islands, whether to move a family's travel by a day. Those calls are being made with more than twenty-four hours of lead time, which is the useful part of a forecast that has been this steady on track and timing for three days running. The intensity wording will keep moving as the storm transitions — typhoon, severe tropical storm, extratropical low are three labels for the same body of water in the air — and the honest instruction is to ignore the label and watch the total. (Handled with care: no casualty figure is carried for this system, because none has been issued.)

THE RECORD

5 turning points on record since Sep 15.

Sep 19cresting → peak

Sep 18building → cresting

Sep 17↑ firmed

Sep 16emerging → building · intensified sharply

Sep 15appeared, emerging

THE FACTS

THE NUMBER THAT IS FALLING, AND THE NUMBER THAT IS RISINGWind down — 970 hPa and a 10-minute sustained 126 km/h, easing through the transition. Water up — about 350 mm over Izu and Kanto through midday Monday. Both are from the same bulletin, and only one of them is what the holiday turns on.

AN EXPOSURE RATING IS A WIND RATINGThe cross-agency board carries this storm green, on about 9,640 people exposed to Category-1-equivalent winds. That figure is doing exactly what it says — counting wind exposure. It does not count 350 mm of rain on primed soil, which is why a green rating and a serious day are not a contradiction. The board is read as a lead here, never as an authority.

THE PHILIPPINES ARE NOT IN THIS STORYThe storm is far offshore with no effect inside the Philippine area of responsibility. What is driving flash-flood and landslide risk over Palawan and storms over Mindoro and Metro Manila is the south-west monsoon, a separate system carried separately on this board.

WHAT ELSE IS ON THE BOARD — THE TROPICSThe Atlantic has one depression 475 mi south-west of the Azores at 35 mph, due to be a tropical storm for about a day then weaken from Monday, with no other formation expected for seven days and no land threat. The eastern Pacific has two areas at 90 percent over both windows, one 400 mi south-south-west of Manzanillo with a coast-monitor note, and a third off Hawaii at 60 percent over seven days.

WHAT ELSE IS ON THE BOARD — THE WATERNigeria carries a national advisory for high riverine flooding across about 15 states, Sep 19-25, rain-driven, with the reservoir at the centre of the circulating release claim impounding at 209.82 m of a 216 m ceiling. Vietnam has flipped — the north dry, the centre and south taking 20-40 mm and locally over 100 mm. America runs flash flooding “especially where soils remain saturated” from the Rockies to the Mid-Atlantic.

WHO IT TOUCHES

Public safetyfive live water emergencies with five different distances between the warning and the water

Aviationa holiday Monday over the Tokyo airports, a hail day on the High Plains and a bura pulse on the Adriatic coast

Supply chainsa rail and road network under a holiday rain shield, a saturated American interior, and a river that will not refill

Maritimeisland ferries under a transitioning typhoon, a coastal cyclone in southern Brazil, and a strengthening bura on the Dalmatian coast

Eventsa national holiday weekend under a rain shield, hail over outdoor operations, and a shrinking but real heat index

Energyan easing but still loaded evening peak, a distribution network under a holiday rain shield, and a river that constrains fuel movement

CONNECTED

Same physics, two continents — in both places the ground is the forecast, not the rain.