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The Atmosphere Today
No. 0920 · Sunday, September 20, 2026 · Morning run
A typhoon is weakening as it turns onto Tokyo, and that is why the rain is the danger.
Eastern Japan takes about 350 millimetres through Monday, onto ground that has been soaking for two weeks.
AT THIS HOUR970 hPa, 10-minute sustained 126 km/h, still offshore and already transitioningThe 9 a.m. position carries 970 hPa, maximum sustained winds of 35 m/s — about 126 km/h on a 10-minute averaging period — and gusts to 50 m/s, about 180 km/h. The averaging period is part of the figure: this is not the same scale as a one-minute number and is not converted onto one here. A separate relayed figure of 130 km/h circulates on a cross-agency board and is held apart rather than merged. Reached through desk relays rather than off the bulletin page directly, and marked reported for that reason.
WHAT IS ALREADY IN THE GROUNDAbout two weeks of record rain, before a millimetre of this storm's rain has landedEastern Japan comes into this window on soil soaked by roughly two weeks of record rain. That antecedent is the reason a weakening storm is the day's lead: saturated ground converts rainfall into runoff and slope failure at a far lower threshold than dry ground does. No soil-moisture index or river stage is printed here, because none was directly read this run — the saturation is carried as it was reported, and the rainfall forecast is the sourced number this page stands on.
WHEN IT LANDSIzu and Kanto from Monday afternoon; offshore of Boso Monday morning; extratropical by TuesdayThe published sequence: the storm reaches the Izu Islands and the Kanto region from Monday afternoon into early Tuesday, passes offshore south-east of the Boso Peninsula on Monday morning, Sep 21, and is expected to be extratropical by Tuesday, Sep 22. A small change in the turn point moves how close the core comes to the Kanto coast, so the timing is firm and the proximity is not. Confidence on intensity and exact timing is medium.
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.)
HELD STEADYTYPHOON DUJUAN — THE WIND EASES AS IT TURNS ONTO TOKYO · THE AMERICAN INTERIOR — FLASH FLOODING WHERE THE SOIL HAS NOTHING LEFT TO TAKE · INDIA — THE MONSOON LIFTS OFF THE NORTH-WEST AND THE RAIN MOVES TO THE PENINSULA · WESTERN LUZON AND PALAWAN — THE SOUTH-WEST MONSOON · AMERICAN SEVERE HOLDS A SLIGHT RISK — OKLAHOMA TODAY · SOUTH-WEST OF THE AZORES — A DEPRESSION WITH ITS ENDING ALREADY IN THE FORECAST · PATAGONIA — THE LAST POLAR WAVE BEFORE SPRING DAY · THE AMERICAN FIRE LEDGER — LEVEL 3 HELD · THE SPANISH MEDITERRANEAN AND THE BALEARICS — INSTABILITY THAT RE-FIRED INSTEAD OF CLEARING · THE RHINE AT KAUB — 21.0 CM · THE SOUTH-DALMATIAN COAST — A TWO-DAY TEMPERATURE PEAK WITH A STRENGTHENING BURA BESIDE IT · CENTRAL NEW ZEALAND — A TROUGH CROSSES · BANGLADESH — A RIVER-FLOOD SIGNAL CARRIED BECAUSE THE BULLETIN WOULD NOT ANSWER · SOUTH-WESTERN AND NORTH-WESTERN CHINA — CARRIED · THE CARIBBEAN SEA AND THE GULF OF MEXICO — NO ACTIVE SYSTEM · THE NORTH INDIAN OCEAN — NO DEPRESSION IN THE BAY OR THE ARABIAN SEA · AUSTRALIA — QUIET IN AUSTRAL SPRING · SOUTH-WEST INDIAN
rain on loaded ground — Same physics, two continents — in both places the ground is the forecast, not the rain. Eastern Japan takes about 350 mm onto soil loaded by roughly two weeks of record rain, while the American interior runs flash flooding especially where soils remain saturated from previous rainfall. Neither of these is a rainfall-intensity story. Both are antecedent stories: the same depth of rain becomes a different hazard depending on what the column underneath it has already absorbed, and in both cases that antecedent was knowable before the rain arrived. It is why a weakening typhoon leads this page and a routine-looking outlook sits at rank two..
five live water emergencies with five different distances between the warning and the water
Five water emergencies, five different lead times, and the lead time is the whole job today. In eastern Japan the warning runs more than a day ahead of the rain — time to pre-position pumps on known-saturated slopes, stage shelter capacity and publish rail and ferry decisions before the holiday crowd is already moving, rather than time to watch. In Nigeria the window is dated Sep 19 to 25 and the driver is named: rainfall and river rise, not a release schedule, which means gauges and rain radar are the instruments and a dam calendar is a distraction. Across the American interior the lead time has largely collapsed to the crossing — flooding on ground already saturated gives you the length of one shift, and the decision is closing a road before a driver finds the creek. Central and southern Vietnam and Palawan are the two where the hazard is real and the coverage is thin, which is where a duty officer has to trust the forecast over the news cycle. The person at the end of every one of these chains is the same: someone deciding whether to move tonight or wait one more hour.
a holiday Monday over the Tokyo airports, a hail day on the High Plains and a bura pulse on the Adriatic coast
The expensive decision today is a Monday bank out of Tokyo, and it has to be made on Sunday. A transitioning storm reaching Izu and Kanto from Monday afternoon with about 350 mm behind it puts the risk on Sep 21, in the middle of Silver Week, when the recovery cost of a late cancellation is at its annual worst and a proactive thin-out is cheap by comparison. On the American board the SLIGHT risk over Oklahoma and eastern Colorado is a large-hail day — a ramp and parked-aircraft problem more than an en-route one — and the flooding across the Ohio Valley into the Mid-Atlantic is a ground-access and crew-positioning problem before it is a runway one. On the Adriatic, the footprint's forecast bura pulse to about 80 km/h at the Velebit crossing and 54 km/h at Dubrovnik Airport is a crosswind-limit conversation for Sep 20-21, held here as a watch signal because no official warning is in force.
a rail and road network under a holiday rain shield, a saturated American interior, and a river that will not refill
Three different ways for a corridor to stop, and only one of them is a storm. In eastern Japan the Monday rain window sits on the densest rail and road network in the region during its busiest travel weekend, and the failure mode is slope and embankment, not wind — which is why the pre-positioning decision is about drainage and cut-slope inspection rather than tie-downs. Across the American interior flooding on saturated ground closes low crossings and reroutes trucking with roughly a shift of notice. And on the Rhine at Kaub the level reads 21.0 cm, status low — a loading-draft constraint that has been grinding on for weeks and has no weather event attached to it at all, which is precisely the kind of slow constraint that gets planned around badly. Central and southern Vietnam's rain adds a port and inland-road question on the same map.
island ferries under a transitioning typhoon, a coastal cyclone in southern Brazil, and a strengthening bura on the Dalmatian coast
The sea keeps the wind story even after the land stops caring about it. A storm can be easing in the wind column and still leave the Izu Islands run unworkable — swell and the offshore Boso pass on Monday morning are the operative facts for a harbourmaster, not the pressure. Off Rio Grande do Sul an extratropical cyclone peaks today with gusts to 100 km/h and hail across RS, SC and Parana, which is a port-closure and small-craft decision now rather than a forecast. On the Dalmatian coast the footprint's forecast bura reaches about 80 km/h at the Velebit crossing and 71 km/h at the Gruz quay on Sep 20-21 — enough to matter for ferry sequencing to Brac, Hvar and Korcula, and carried as a watch signal because there is no official orange in force. The Azores depression is the one wind story that is not a maritime problem: 35 mph over open water with no land threat.
a national holiday weekend under a rain shield, hail over outdoor operations, and a shrinking but real heat index
Outdoor operations are a lead-time business, and today hands out three very different amounts of it.Silver Week in eastern Japan gets more than a day — enough to move, shorten or roof an event rather than cancel it at the gate, which is the difference between a refund and a reschedule. The SLIGHT hail risk over Oklahoma and eastern Colorado gives hours, and hail is the hazard that damages the site itself rather than just the schedule. The South-Central heat is the one easing — about 4 advisories, down from 13 — but the index still reaches about 108 °F in Middle Tennessee, and standing down a cooling plan too early is its own failure mode. In KwaZulu-Natal a Yellow Level-2 covers a single afternoon window, 13:00 to 23:59, which is about as precise as an event decision ever gets.
a heat index that is still dangerous on a smaller map, and three flood recoveries where the health risk outlives the water
A smaller warning map is not a smaller heat risk for the people still under it. Advisories contracted to about 4 from 13, but the index still reaches about 108 °F in Middle Tennessee and about 105 °F in Louisville, and outdoor workers and unhoused people sit under exactly the tier that stopped making headlines. On the water side, the health load arrives after the flood: Nigeria's 15-state riverine window through Sep 25 is a water, sanitation and disease-surveillance staging problem as much as an evacuation one, and Nepal and Pakistan are both in the long tail where the ledgers have stopped moving — at least 1,410 dead and about 6,145 missing and about 183 dead and 515 injured — while the exposure has not. This desk equips the institutions that make those calls; it does not make them. (Handled with care.)
a monsoon retreat you can plan a harvest against, a flood window over farmland, and one last freeze before spring
The most valuable forecast on this desk today is a seasonal one, not a storm. India's monsoon has withdrawn from parts of West Rajasthan with further withdrawal favourable over Rajasthan, Punjab and Saurashtra-Kutch — a retreat line is the single input a harvest and a rabi sowing window are planned around, and it is arriving on schedule while the heavy rain re-concentrates over the peninsular states that are still in season. In Nigeria, riverine flooding across about 15 states through Sep 25 is a lift-the-harvest and move-the-livestock decision keyed to rain and river rise. Over the American High Plains the SLIGHT risk carries large hail, which is a standing-crop and equipment call with a few hours on it. And in Patagonia the last polar wave before Spring Day puts plateau minima near -10 °C with isolated -18 °C — one more frost decision before the season turns.
an easing but still loaded evening peak, a distribution network under a holiday rain shield, and a river that constrains fuel movement
Load is coming down and the risk is moving from generation to distribution. The South-Central evening peak still runs under an index near 108 °F even as the advisory footprint contracts to about 4, so the planning question shifts from scarcity to when to stand down a heat posture without standing it down early. In eastern Japan, the Monday rain window is a distribution and access problem — poles, substations and the crew routes to reach them on saturated slopes — rather than a wind-loading one. In southern Brazil gusts to 100 km/h with hail are a straightforward outage-and-restoration day. And the Rhine at Kaub at 21.0 cm keeps a barge-draft constraint on fuel and coal movement that no single weather event will lift.
