The Global Forecast System (GFS)

By: admin Published: November 25, 2024 | About a 4 minute read

Sknowed.com uses data from the Global Forecast System (GFS) to create its 16 days forecasts. So what is GFS and how well does it predict weather? The Global Forecast System (GFS) is a numerical weather prediction model developed and maintained by the National Oceanic and Atmospheric Administration (NOAA) in the United States. It provides weather forecasts and climate analysis on a global scale. Here’s an overview of its key features and how it works:

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Core Features

1. Global Coverage: The GFS model provides weather forecasts for the entire planet, making it a critical tool for global meteorological analysis.
2. High Resolution:
   • Horizontal Resolution: The GFS operates at a resolution of approximately 13 km (8 miles / ≈0.125°) for the near-term forecast period (up to 10 days). Beyond 10 days, it switches to a coarser resolution to focus on longer-term trends about 25km (15.5 miles / ≈0.25°).
   • Vertical Levels: The model divides the atmosphere into 127 vertical layers, providing detailed insights into atmospheric dynamics from the surface to the stratosphere.
3. Forecast Range:
   • Short to Medium Term: It produces forecasts up to 16 days into the future, though its accuracy decreases over these longer periods.
   • Hourly and Daily Outputs: GFS generates forecasts at hourly intervals for the first five days, transitioning to every three hours for the remaining period.
4. Data Assimilation:
   • The GFS incorporates vast amounts of observational data, including satellite measurements, weather balloons, surface stations, and aircraft observations. This process, called data assimilation, improves its initial conditions and enhances forecast accuracy.
5. Comprehensive Variables:
   • The GFS predicts a wide range of meteorological parameters, such as temperature, precipitation, wind, humidity, and pressure. It also simulates ocean and land-surface interactions.

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How GFS Works

1. Initial Conditions:
   • The model starts with a snapshot of the atmosphere’s current state, which is created by blending observational data with prior model forecasts (data assimilation).
2. Physics and Equations:
   • GFS uses complex mathematical equations based on the principles of atmospheric physics to simulate how weather systems evolve over time. These equations account for dynamics like air movement, heat transfer, and moisture processes.
3. Computational Power:
   • Running the GFS requires immense computational resources. It operates on NOAA’s high-performance supercomputers to process trillions of calculations for each forecast cycle.
4. Forecast Cycles:
   • The model runs four times daily (at 00Z, 06Z, 12Z, and 18Z), providing updated forecasts with each cycle. Don’t get confused with Forecast Cycles vs Forecast outputs. The GFS produces forecast outputs at hourly intervals for the first 120 hours (5 days). Beyond 120 hours, outputs are available at 3-hour intervals up to 10 days and 12-hour intervals beyond 10 days and up to 16 days.

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Applications

1. Operational Meteorology:
   • GFS is widely used by weather agencies worldwide for short- and medium-term forecasts.
   • It supports severe weather prediction, including hurricanes, winter storms, and heavy rainfall.
2. Aviation and Maritime Navigation:
   • The model is crucial for planning flight paths and maritime routes, ensuring safety and efficiency.
3. Climate Research:
   • GFS data contributes to long-term climate studies and seasonal forecasting.
4. Ski and Snow Forecasting:
   • In mountainous regions like Hakuba, the GFS provides critical inputs for predicting snowfall and temperature patterns.

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Strengths and Limitations

Strengths:

• Global Scale: Covers the entire globe consistently.
• Timely Updates: Produces forecasts every six hours.
• Rich Data Integration: Assimilates diverse observational data sources for accuracy.

Limitations:

• Resolution Constraints: Despite high resolution, it may not capture small-scale phenomena, such as localized storms in mountainous areas.
• Long-Term Accuracy: Forecast reliability decreases significantly after 7–10 days.
• Competition: Regional models (e.g., ECMWF) might outperform the GFS for specific regions or parameters due to finer resolutions.

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Recent Updates

In recent years, the GFS has undergone significant upgrades:

• GFSv15 (2019): The 2019 update to the Global Forecast System (GFS), known as version 15 (or FV3-GFS), marked a significant advancement after decades of reliance on a spectral dynamical core. This update transitioned to the Finite Volume Cubed-Sphere (FV3) dynamical core, which enhanced global weather forecasting by improving numerical stability and physical accuracy. Key improvements included a refined microphysics scheme, better representation of ozone and water vapor processes, and a higher resolution maintained throughout the forecast period.
• GFSv16 (2021): Improved physics, data assimilation, and resolution.
• GFSv17 (Planned): Enhanced cloud and precipitation processes, better hurricane prediction, and finer resolution.

This are some visual references to show you how FV3 improved some forecasts. On the left you will see GFS before FV3 while the middle graphic shows the forecast with FV3 and the far right shows the actual observed wearther.

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Now the same thing but for a snow forecast:

Above images are from NOAA.gov

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So now you know a bit of the background as to how to read Sknowed.com’s weather reports.