Articles

Outlook for Lingering Summer Heat and Typhoons After September 2025, and Their Impact on Power Demand

2025.09.02

In recent years, due to global warming and other factors, Japan has increasingly experienced prolonged high temperatures beyond September — a phenomenon commonly referred to as lingering summer heat (zansho), which has now become the norm.

When such lingering heat persists beyond September, it can disrupt the balance of electricity supply and demand. In fact, in September 2024, record-breaking lingering heat brought consecutive days of high temperatures, which drove up electricity demand, and by mid-September, regions spanning from Kanto to western Japan experienced periods of particularly tight supply–demand conditions.

Looking ahead, the lingering summer heat of 2025 is again expected to persist, with the arrival of autumn likely to be delayed. This article looks at the outlook for lingering summer heat and typhoons, their potential impact on electricity demand, and measures to address them—critical information for power generation and retail electricity businesses.

September 2025 Outlook: More Clouds and Rain, and Muggy Heat

In September 2025, the Pacific High is forecast to be strong over the southern seas. However, unlike in 2024, its ridge is expected to extend weakly toward Honshu. As a result, Japan is likely to be more susceptible to the influence of troughs of low pressure and moist air along the high’s periphery, leading to an increased number of cloudy and rainy days.

At the same time, upper-atmosphere temperatures across the Northern Hemisphere are expected to be lower than in 2024. This suggests that while short-lived spikes of hot weather may still occur, the risk of prolonged periods of extreme heat is likely to be relatively low. Nonetheless, warmer-than-average air is likely to continue flowing into Japan.

Even with limited sunshine, temperatures are expected to remain relatively high, resulting in continued hot and humid days. Nighttime cooling will also be limited, and regions from Kanto to western Japan may still experience tropical nights, with minimum temperatures remaining above 25°C.

For forecasts of temperatures, perceived comfort, and product sales from August to October 2025, please click: “Lingering Summer Heat in 2025 Delays Autumn: Comfortable Conditions in Tokyo Expected From Around Late September.” (Only available in Japanese)

Caution on Typhoon Approaches After September

The number of typhoons forming in the coming months is expected to be close to the seasonal average. In addition, from September to October, the number of typhoons approaching Honshu, Hokkaido, Kyushu, and Shikoku is expected to be around or above average.

Sea surface temperatures near Japan are projected to remain higher than seasonal average. As a result, even if the total number of typhoons forming is not particularly large, some may intensify into strong systems.

Particularly from September to October, because it is the period when the Pacific High gradually weakens, depending on the timing of formation, and northward movement, there is a likelihood of typhoons approaching Japan.

For details on the typhoon outlook, please refer to: “Japan’s Typhoon Season 2025 – Outlook from August Onward”

Impact of Lingering Summer Heat and Typhoons on Power Demand

Increased Cooling Demand from Uncomfortable Heat

During hot periods, air conditioning makes up the majority of electricity demand.

Regarding the effects of temperature changes on regional electric power consumption, according to Narumi et al. (2007), cooling demand for business applications typically begins to rise once temperatures reach around 26°C, followed shortly thereafter by increased residential demand (*1).

In addition, the “Discomfort Index,” one of the indicators of heat stress, is closely related to electricity demand, including for cooling. This Index expresses the degree of muggy or oppressive heat and is an index significantly influenced not only by temperature but also by humidity.
See: [tenki.jp] Discomfort Index (Only available in Japanese)

In September 2025, although temperatures are expected to be lower than in 2024, high humidity will likely result in many days with an elevated Discomfort index. While cooling demand is not expected to reach the levels seen last year, the persistence of muggy conditions could keep cooling demand elevated over an extended period.

Reference
*1 D. Narumi, Y. Niurao, Y. Shimoda, M. Mizuno: Effects of air temperature change upon the regional electricity consumption, Energy and Resources, Vol. 163, No. 3, 2007.

Caution: Impact of Typhoon Approaches on Fuel Procurement

The effects of typhoons on fuel procurement should not be underestimated.
While the number of typhoons forming in 2025 is expected to be roughly in line with the seasonal average, strong typhoons may approach Japan between September through October.
In particular, this could disrupt seaborne fuel shipments, warranting careful attention.

Weather Variability, Forecast Accuracy, and Supply-Demand Planning

When high pressure dominates and there is a strong tendency for clear skies, as was the case in 2024, the accuracy of weather forecasts is generally high.

By contrast, when conditions such as low pressure troughs or moist air contribute to cloud formation, even short-term forecasts, such as one week ahead, tend to be more difficult to predict accurately.

In September 2025, more factors are expected to favor cloud development compared to last year, which may increase forecast errors. As a result, it is expected that plans for electricity demand forecasting and supply adjustments will need to incorporate greater flexibility.

Such uncertainties in forecasting could also affect electricity demand outlooks, making it essential to develop and operate supply-demand plans with greater flexible than in previous years.

For information on temperature and electricity market trading prices, please click: “High Temperatures in June 2025 and Their Impact on Market Prices, and Future Forecasts” (Only available in Japanese)

Leveraging High-Precision Electricity Demand and Solar Radiation Forecasts to Prepare for Lingering Summer Heat and Typhoons

To help electricity providers respond to fluctuations in demand, Japan Weather Association provides a high-precision Electric Power Demand Forecasting Service that integrates the expertise of certified weather forecasters with artificial intelligence (AI) and machine learning technologies.

Electric power demand forecasting

Electricity demand is strongly influenced by various weather factors, including temperature, humidity, solar radiation, rainfall, and snowfall. In demand forecasting, proprietary weather forecast data—such as on temperature and solar radiation are combined with expertise in renewable energy forecasting technologies. Artificial intelligence (AI) is then employed to predict electricity demand by power area from the current day up to two days ahead.

Service Details
Prediction Target	Electric power demand by power area (unit: kWh)
Forecast Interval	Every 30 minutes
Forecast Period	From the current day up to two days ahead.
Data Used	Actual power demand (unit: kWh) Contract power amount (unit: kW) Weather forecast data for the target area(e.g. temperature, humidity, solar radiation etc.)
Forecasting Functions	Forecasts by voltage Forecasts by industry type Forecasts by contract plan
Options	Extended forecast period and other optional features available

Examples of applications

Supply–demand management for electric utilities, including major power companies and Power Producers and Suppliers (PPS)
Demand estimation for Demand Response (DR) and Virtual Power Plants (VPP) (For baseline setting)
Optimization of charge–discharge scheduling of storage battery systems and market bidding plans by grid-scale battery storage operators

For more information on this service, please click: Electric power demand forecasting

Solar Radiation and Photovoltaic Power Output Forecasting — SYNFOS-solar (Only available in Japanese)

Since photovoltaic (PV) output has a significant impact on both electricity demand and supply capacity, the use of high-precision forecasting models is essential.

JWA provides a high-precision solar radiation and PV output forecasting service using its proprietary weather prediction models. Both power demand forecasts and solar radiation/photovoltaic output forecasts are available not only through online delivery but also via API.

For details, please click: API Service ENeAPI for the energy providers.

2-Year Long-Term Weather Forecast (Only available in Japanese)

JWA has developed the industry’s first forecasting method that offers higher accuracy and longer forecast periods than conventional approaches. This service provides numerical forecasts for monthly temperature, precipitation/snowfall, and sunshine duration, as well as information on the rainy season and typhoon activity.
In addition to data provision, we also offer consulting services to help clients make effective use of long-term forecasts.