Insights & News

How Much Will Nepartak Disrupt Tokyo 2020?

26th July 2021

In my blog last Tuesday, I flagged up the potential for a tropical cyclone to disrupt the Olympic Games during 28th-30th July.

This threat is now a reality, as tropical storm Nepartak bears down on the northern half of Japan. Fortunately, it’s not a typhoon, in fact it’s barely a tropical storm as I write this. Even so, it could become the first tropical storm (or stronger system) in recorded history to impact the Olympic Games directly.

Here’s a quick update on the likely level of disruption that this may bring to the Tokyo Olympic Games.

Wind Threat: Low Inland but High by Sea

As of 06:00 UTC 26th July, Nepartak was analysed by the Joint Typhoon Warning Centre (JTWC) to be a minimal strength tropical storm. This classification is debatable; its structure consists of one broad spiral band (grey-blue shading in snapshot below). This layout, with not much of a central mass of clouds and rainfall, is more typical of a subtropical storm.

Regardless, its highest sustained wind speed is somewhere close to 40 mph. Any lower and it would be a mere tropical depression by that measure.

Studying the JTWC forecast, only slight strengthening of winds is expected before landfall in Japan. Given the current structure of the cyclone, I can’t see any clear reason to disagree with that.

(For full resolution please view at source:

While at the low end for tropical cyclones, this is still strong enough to bring rough seas across a wide area. Already, this has forced postponement of rowing events that were scheduled for Tue 27th. Other water sports may also be disrupted, including surfing (most at risk), sailing, and canoeing (least at risk). Depending on the sea state, even the beach volleyball could be affected.

As for the inland sports, as luck would have it, only the baseball, softball and football matches are being held within or very close to the expected landfall area.

It’s possible that the baseball and softball may be interrupted or postponed by high winds. After all, it’s not much use if the ball is steered more by the airflow than the players’ skills!

Football may see some minor impact on ball motion, but probably not enough to warrant emergency actions.

Aside from these direct impacts, some travel disruption is possible where the strongest winds hit. This is relevant to both spectators and players travelling between venues.

Rain Threat: Risk of Flash-Flooding

With low-wind tropical cyclones, rainfall tends to have by far the greater impact on infrastructure and events. In fact, a weak tropical storm can feasibly put down as much rainfall as a strong typhoon.

Thankfully, Nepartak does not have such potential. Again, this comes down to its structure: With little central mass of clouds (known as a central dense overcast; CDO), there isn’t a very high concentration of moisture within the core of the system.

This could change as it approaches landfall during the next 36 hours, but at present, forecast models aren’t predicting a huge amount of rainfall from this weather event (see below).

The example shown has a peak rainfall total close to 100 mm. By local standards, this is not a vast amount. There’s also been plenty of dry weather in recent weeks. River flooding seems unlikely when considering all this.

It should be noted, however, that much of this could fall in the space of a day:

…in which case, some flash-flooding can’t be ruled out in that peak area. Depending on the lay of the land around the venue in each case, interruption, postponement or relocation of baseball, softball or football matches is not out of the question. At the very least, spectators and players may face some travel disruption due to areas of surface water and poor visibility during torrential rain spells.

Little Historical Precedent

Here’s a little ‘bonus content’ for the interested reader.

For the time of year, Nepartak is highly unusual for two reasons.

Firstly, for its location. In 75 years of reliable record, only one tropical cyclone has affected the area spanning the Ibaraki to Miyagi prefectures between 21st July and 8th August: Tropical Storm Alice on 23rd July 1958 (encircled on the map below).

This map shows 3-hourly tropical cyclone positions and intensities as observed by institutions in Japan, China and the USA (source: IBTrACS v4).

The second unusual trait is its direction of approach. Non-tropical low pressure systems usually pass near or over northern parts of Japan, while high pressure systems traverse southern parts. This occurs due to a large-scale west-east flow of air at around 10 km altitude, known as the jet stream.

This steers tropical cyclones eastward as they move north from the tropics, producing an overall north-eastward motion.

However, Nepartak is instead moving north-westward­. That’s because a recent southward meander of the jet stream (it behaves like a river) has deposited a ‘cut-off low’ centred just south of Japan. This has resulted in an unusual steering pattern for Nepartak, which is the first tropical cyclone to approach Japan from the southeast since Lionrock in early September 2016.

By some coincidence, Lionrock hit the same region of Japan that Nepartak is currently forecast to!

Anything After Nepartak?

Briefly looking ahead to the first week of August, forecast models show a tendency for low pressure areas to the south and southeast of Japan. Aided by unusually warm sea surface temperatures, some could become tropical cyclones. Concerningly, there are hints of the same unusual steering pattern that’s driving Nepartak to the northwest.

While it would be statistically extraordinary to see two systems approach Japan from the southeast in a fortnight, I’m afraid I can’t rule out such a scenario based on this information. Consider the risk low, but not as low as it should be at this time of year.


James Peacock MSc
Head Meteorologist at MetSwift

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