Japan Weather Outlook for September and October 2026: How Long Will the Heat Last?

2026.08.28

In 2026, intense heat is expected to continue through the second half of August, with periods of hot weather also likely in September. However, unlike in 2025, exceptionally high temperatures are not expected to persist for an extended period. Instead, the effects of typhoons and fronts may bring alternating periods of more severe and milder heat.

October is not considered part of the lingering summer heat period, but temperatures are expected to be above normal, with the progression of autumn delayed.

This article explains how long the heat is expected to last in 2026, the temperature outlook for the second half of August, September, and October, and the points businesses should keep in mind when transitioning from summer to autumn operations.

*For more details on summer in Japan, see: Summer Weather in Japan: What Global Businesses Need to Know (Beyond Travel Tips)

Table of Contents

Weather Outlook for Japan in August, September, and October 2026

According to forecasts by Japan Weather Association (JWA), intense heat is expected to continue through the second half of August 2026, with periods of lingering summer heat also likely in September. However, exceptionally high temperatures like those seen in 2025 are unlikely to persist for an extended period, and temperature fluctuations are expected to be greater due to the effects of typhoons and fronts.

Temperatures themselves will decline in October, but are expected to remain above normal for the month, with the progression of autumn delayed.

Accordingly, hot days are expected to continue into September in 2026, while temperatures in October are also expected to remain above normal.

*The Japan Meteorological Agency defines “lingering summer heat” as heat occurring from around August 8, the beginning of autumn on the traditional calendar, to around September 23, the autumnal equinox.
Accordingly, this article discusses August and September as the period of “lingering summer heat,” and October as a period of “warmer-than-normal autumn temperatures following the lingering summer heat.”

Will the Heat in Japan Continue Through the Second Half of August?

According to forecasts by JWA, temperatures are expected to temporarily decline around mid-August 2026 before rising again in late August, with lingering summer heat becoming more intense.

In August, there are expected to be periods when the extent of Pacific High is weaker than in the previous year, making Japan more susceptible to the effects of typhoons and moist air. In addition to precautions against intense heat, attention will also be needed for heavy rain, strong winds, and reduced sunshine.

*Related articles: August 2026 Weather Forecast (only available in Japanese), Japan’s 2026 Typhoon Outlook

Will September Be Hot in Japan?

September is expected to include days of severe lingering summer heat, but especially in western Japan, temperatures are not expected to be as exceptionally high as they were in 2025. With typhoons, fronts, and low-pressure systems affecting the country, weather conditions are expected to change periodically, with large swings in temperature as well.

For this reason, heat stress measures and the rollout of summer products will likely need to be reviewed in stages based on the latest forecasts for each region.

Will October Be Hot in Japan?

October is expected to have many days with temperatures above normal for the month, and the progression of autumn is likely to be gradual.

When considering the transition from summer to autumn, it is advisable to monitor not only the lingering summer heat through September, but also average temperatures in October and trends in morning and evening temperatures.

How Will the Heat in 2026 Differ from 2025?

In 2025, temperatures were exceptionally high nationwide from summer into autumn. In October, however, temperatures began to fall from northern Japan, and autumn set in rapidly.

Temperatures from August through October 2026 are also expected to trend above normal. However, from August through September, exceptionally high temperatures are less likely to persist as they did in 2025. In contrast, October is expected to be warmer than normal nationwide, with the progression of autumn delayed.

One point to note here is the difference between the expressions “above normal” and “lower than the previous year.”
In this article, “normal” refers to the 30-year average for 1991–2020 as defined by the Japan Meteorological Agency.
Because 2025 experienced record-breaking heat, temperatures in 2026 can be above normal while still being lower than those recorded in 2025.

What Factors Will Influence Heat and Temperature Changes in Japan in 2026?

Temperatures from August through September 2026 are expected to be near normal to slightly above normal, but exceptionally high temperatures like those seen in 2025 are not expected to persist for an extended period. This is partly because the extent of the Pacific High is likely to fluctuate and because Japan is expected to be more susceptible to the effects of typhoons and fronts.

The Extent of the Pacific High Is Likely to Fluctuate

From summer into autumn 2026, the extent of the Pacific High is expected to fluctuate, partly in association with factors such as the development of El Niño conditions.

Periods when Japan is covered by the Pacific High will bring intense heat. On the other hand, when its extent weakens, the influence of moist air and low-pressure systems may increase the number of cloudy and rainy days, temporarily easing the heat.

As a result, rather than the exceptionally high temperatures persisting as they did in 2025, periods of severe heat are expected to alternate with periods when the heat eases.

*Related article: What Is the Pacific High? (only available in Japanese)

Temperatures Can Change Easily Before and After Typhoons and Fronts Pass

According to the JWA’s typhoon outlook, more typhoons are expected to approach Japan in August 2026, and well-developed typhoons may approach Japan from September onward.

Before a typhoon or front approaches, warm, moist air may flow in from the south, causing temperatures to rise. By contrast, temperatures may fall after the typhoon or front passes if winds shift to a northerly direction or if rain continues.

For this reason, it is important to review not only the monthly average temperature outlook, but also two-week temperature forecasts, weekly forecasts, typhoon information, and other updates.

*Related articles: Japan’s 2026 Typhoon Outlook

How Late-Summer Heat in Japan Affects Businesses

How long the heat lasts also affects when businesses can end summer product sales, heat stress measures, air-conditioning operations, and other seasonal measures.
Businesses also need to prepare for typhoons and heavy rain while responding to lingering summer heat. Rather than ending all summer measures at once, businesses will need to transition in stages by region and type of operation.

1. Transitioning from Summer Products to Autumn Products

In retail, apparel, food and beverage manufacturing, and other sectors, changes in temperature affect product demand.
If sales and inventories of summer products are reduced too early, shortages may occur if the heat returns.
In addition, because temperatures in October are expected to be above normal, demand for autumn and winter products such as heavier clothing, hot beverages, and hot-pot meals may pick up later than usual.

*Related article: [For Apparel Businesses] Autumn/Winter 2026 Forecast (only available in Japanese)

2. Scaling Back Heat Stress Measures and Air-Conditioning Operations

In manufacturing, construction, logistics, agriculture, facility management, and other sectors, precautions against heat-related illness will still be necessary from September onward.
A monthly average temperature outlook alone cannot determine day-to-day heat stress risk. Even from September onward, there may be individual days when temperatures and the Wet Bulb Globe Temperature (WBGT) are high.
Decisions on heat stress measures should be made by monitoring temperatures and WBGT for each site.

The severity of lingering summer heat also affects cooling demand and electricity demand.
Because temperature fluctuations are expected to be large in 2026, businesses need to check short- and medium-range temperature forecasts rather than relying only on demand results from the same period in the previous year.

*Related article: Autumn 2026 Weather and Its Impact on Electricity Demand and Electricity Market Prices: Supply-Demand Operations for Sporadic Heat and Temperature Fluctuations (only available in Japanese)

3. Preparing for Typhoons and Heavy Rain at the Same Time

During the lingering summer heat period in 2026, businesses will need to prepare not only for heat, but also for heavy rain and strong winds caused by typhoons and fronts.
Lingering summer heat and typhoons or heavy rain are not separate issues.
Before and after a typhoon approaches or passes, temperatures can rise or fall sharply depending on its track, so multiple weather factors need to be considered together.

Which Weather Data Services Should Businesses Use from Summer to Autumn?

Different forecast periods and types of information are needed for transitioning from summer to autumn products, reviewing heat stress measures, and responding to typhoons and heavy rain.
It is important to use long-range forecasts to understand trends for the season as a whole, adjust plans with one-month forecasts and two-week temperature forecasts, and then use weekly forecasts and typhoon information for day-to-day operational decisions.

Services can be selected not only by forecast period, but also by how the information will be used.

• Review long-term trends through quantitative data and report-based information: 2-Year Long-Term Weather Forecasts
• Receive data tailored to your company’s analyses: Weather Data Distribution
• Integrate weather data into your company’s systems: Weather Data API, ENeAPI, an API Service for the Energy Industry
• Review risks at worksites and locations through web dashboards and alerts: Weather Risk Information for Businesses, GoStop Management System for Logistics

The forecasts and delivery methods you need can be combined according to your business activities and the timing of your decisions.

Conclusion

In 2026, intense heat is expected to continue through the second half of August, with periods of hot weather also likely in September.
However, exceptionally high temperatures like those seen in 2025 are unlikely to persist for an extended period, and the effects of typhoons and fronts are expected to bring alternating periods of more severe and milder heat.
October is not considered part of the lingering summer heat period, but temperatures may be above normal, with the progression of autumn delayed.
In other words, hot days are expected to continue into September in 2026, while temperatures are also likely to remain above normal for the month.

For businesses, the following points are important:

• Do not end summer product sales or heat precautions all at once
• Do not base sales, production, or electricity demand decisions solely on 2025 results
• Use long-range forecasts to understand trends for the season as a whole
• Revise plans in stages using one-month forecasts, two-week forecasts, and short-range forecasts
• Monitor the risks of lingering summer heat and typhoons and heavy rain in parallel

The forecast period and delivery method best suited to a business will vary by industry and type of operation. Combining information that matches your decision-making timeline, operations, and system environment makes it easier to review summer-to-autumn plans in stages.
Please feel free to contact us to inquire about the forecast periods and delivery methods best suited to your company.

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