Weather Forecast Accuracy in Japan: JWA’s Accuracy Verification and Forecasting Technology

2025.09.05

For those seeking reliable weather forecasts in Japan, this article introduces the accuracy and advanced forecasting technology behind JWA’s weather reports.
With Japan’s unique topographical features causing rapid weather changes, accurate and up-to-date information is essential for both travel and business activities.
Use this resource to find the most reliable weather forecasts that best suit your needs.

1. Why Are High-Accuracy Weather Forecasts Essential in Japan?

Japan’s Weather

Japan stretches from Hokkaido in the north to Okinawa in the south, with dramatic differences in climate and topography across its islands. The country experiences four distinct seasons, and during seasonal transitions, atmospheric fronts often cause unstable weather. In addition, Japan faces a wide range of weather events throughout the year, including the rainy season, typhoons, and heavy snowfall, making forecasting particularly complex.
For more information on this topic, follow the link: Why Is Japan’s Weather So Difficult to Forecast?

Natural Disaster Risks

Weather in Japan is closely tied to natural hazards. Weather phenomena, such as seasonal rain fronts, typhoons, extreme heat (heatstroke), localized heavy rain, thunderstorms, and heavy snowfall have increasingly been leading to natural disasters. Particularly from June to October, torrential rains can cause river overflows and flooding, while typhoons bring destructive winds and heavy rainfall, resulting in human and economic losses almost every year.
In recent years, the intensification and rising frequency of hydrometeorological disasters caused by climate change have underscored the need for accurate and easy-to-understand weather information for disaster prevention.
Global warming has also amplified the danger of extreme summer heat. For example, in 2024, prolonged extreme heat with temperatures exceeding 35°C (95°F) across many parts of Japan resulted in 1,257 casualties from heatstroke in workplaces. Increasingly, summer heat itself is also being recognized as a “disaster”.

For more information about Japan’s summer weather, click here: Summer Weather in Japan

How to Understand Typical Weather Forecasts in Japan

• From how many days in advance are weather forecasts reliable?
  It depends on the season. In winter, when weather is generally more stable, forecasts issued up to two weeks in advance can often be accurate. In contrast, during spring and autumn, when migratory highs are frequent, during summer and the rainy season, or when typhoons form, weather forecasts are more likely to change and may become less reliable.
• How should precipitation probability be interpreted?
  Precipitation probability indicates the likelihood that 1 mm or more of rain will fall within the forecasted area and time period. This means that even an 80% chance of rain may result in very light rain, while a 10% chance could still bring heavy rain. It also does not specify exactly which parts of the area will be affected, only that somewhere within the forecasted area, at least 1 mm of rain is expected.
• Why do weather forecasts change?
  The Earth’s atmosphere is a single, interconnected system where all parts influence each other. To forecast the weather in a specific region (such as Japan), it is necessary to calculate the future state of the global atmosphere using physical laws—a process known as numerical weather prediction. Numerical prediction uses data (observed data) that capture, as accurately as possible, the state of the entire Earth’s atmosphere at a given point in time, including pressure, temperature, amount of water vapor, and wind, to calculate future atmospheric conditions. However, because no measurement or calculation is perfect, small errors are always present, and these grow larger the further into the future the forecast extends. As a result, new forecasts may differ from earlier ones.
• Why do forecasts differ between apps and news sites?
  Weather forecasts are produced by applying observed data to computational programs (numerical prediction models) based on physical equations.
  These models are developed not only by the Japan Meteorological Agency but also by institutions in Europe, the United States, and private weather companies, each using its own methods.
  Because the calculation results from numerical prediction models include errors, and because providers apply different ways of correcting errors, forecasts can vary.
  Thus, the choice of which numerical prediction model’s results to utilize and how to apply corrections results in different weather forecasts being published across various apps and websites.

In countries like Japan, where weather changes rapidly, the use of highly accurate weather forecasts has a significant impact on daily activities. Therefore, it is important to choose reliable sources of information and make proper use of them.

2. How Accurate Are JWA’s Weather Forecasts?

In 2024, the Japan Weather Association’s forecasts outperformed those of the Japan Meteorological Agency (JMA) in overall accuracy.

• The “JWA Blend of Models” achieved an 89% accuracy rate for same-day precipitation forecasts throughout 2024. Compared with JMA’s monthly results for the same metric, the accuracy rate of the “JWA Blend of Models” was equal to or higher than JMA’s in 11 of 12 months, with the exception of August.
• For hourly weather forecasts for the following day, the “JWA Blend of Models” achieved an accuracy rate of 70%. When compared to the JMA’s monthly accuracy rates, it outperformed JMA forecasts in every month.
• For maximum and minimum temperature forecasts up to seven days ahead, the forecasts from “JWA Blend of Models” consistently showed smaller errors and higher accuracy compared with JMA’s forecasts.

Details: 2024 Japan Weather Association’s Forecast Accuracy Verification Results

JWA continually refines its forecasting technology to address Japan’s unique and diverse weather conditions, delivering its own forecasts that are both highly accurate and reliable.

3. How Is JWA’s Weather Forecast Technology Evolving?

• Use of Numerical Weather Prediction Models and Proprietary Technology (e.g., JWA Blend of Models)
  At JWA, the proprietary numerical weather prediction system SYNFOS, developed and operated in-house, forms the core of its forecasting. Building on this foundation, JWA created the Integrated Weather Forecast “JWA Blend of Models”, which combines several world-leading numerical weather prediction models.
  While meteorological agencies worldwide—including the Japan Meteorological Agency—operate global numerical weather prediction models, each model differs in grid resolution, forecast range, and regional strengths.
  “JWA Blend of Models” applies tailored corrections and integration based on each model’s forecast accuracy and performance characteristics. In Japan, localized short-term heavy rainfall events, such as linear rainbands (quasi-stationary band-shaped precipitation systems, named ‘senjo-kousuitai’) can occur. During such heavy rainfall events, JWA provides specialized forecast information tailored to these phenomena. As a result, JWA delivers weather forecast information that is highly accurate, reliable, and does not fluctuate significantly from day to day.
  The factors used for these corrections and integrations are updated daily, utilizing JWA’s accumulated meteorological expertise and AI to reflect the latest observation data.
  For more information about JWA’s technology, click here: Technology

4. Conclusion: How to Choose Reliable Weather Information？

• In Japan, where weather patterns are highly volatile, the accuracy of forecasts is a critical factor influencing the quality of decisions in both travel and business. Leveraging high-precision forecasts and up-to-date meteorological data is essential for risk reduction and successful planning.
• When selecting a weather forecast source, it is important to consider the accuracy of the information, update frequency, and the format in which it is delivered.
• JWA offers “tenki.jp”, a free service accessible via smartphone or PC (only available in Japanese). It delivers frequently updated forecasts nationwide, including highly accurate local forecasts and weekly outlooks that are especially useful for travel and tourism planning.
• For businesses, JWA forecasts offer reliability and advanced technical capability. By using precise local forecasts, companies can reduce costs, minimize risks, avoid missed opportunities, and ultimately enhance overall performance.
  • Weather Data API:
    Provides observed and forecast data in JSON format.
  • Logistics-oriented GoStop Management System:
    Provides transport impact risk information caused by weather conditions, supporting decisions that help protect lives and cargo in logistics operations.
  • Probabilistic Scenario-Based Typhoon Forecasts (Only Available in Japanese):
    Unlike traditional typhoon forecasts that present a single forecast track, this service offers “probabilistic scenario-based forecasts” with multiple possible tracks.
  • Product Demand Forecasting:
    Analyze product sales and weather data to forecast product demand and customer traffic to adjust production volume and optimize inventory management.
  • 2-Year Long-Term Weather Forecasts:
    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.
    For more details, please see:
    What are Long-Term Forecasts? JWA’s Proprietary “2-Year Long-Term Weather Forecast” and Its Business Applications Part 1 / Part 2

For more information about an outlook for typhoon activity from August onward, click here: Japan’s Typhoon Season 2025 – Outlook from August Onward

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