GFS vs ECMWF Demand-Hub Outlook: 2026-09-16 00z

Executive Summary

The GFS and ECMWF models present a highly divergent 15-day outlook, reducing forecast confidence. The GFS is significantly colder, projecting a total of 14.9 weighted HDDs versus only 9.0 on the ECMWF, driven by a late-period cold shot in the Midwest. This run also saw the GFS trend sharply colder (+7.7 HDDs) versus its prior cycle, while the ECMWF remained stable, exacerbating the model disagreement.

Hub-Level Weather

The primary drivers of the national forecast are the key population centers in the Midwest and East, which show the most significant model disagreement.

• Midwest (Chicago, 35% weight): This is the epicenter of the forecast divergence. The GFS projects 15.8 cumulative HDDs, while the ECMWF sees only 2.6. The GFS introduces a notable shift to below-normal temperatures after September 23rd, which is not present in the milder ECMWF solution.
• East (New York City, 30% weight): The models also diverge significantly here, though differently. The GFS is much warmer, forecasting 57.8 cumulative CDDs versus just 29.4 for the ECMWF. This is primarily due to the GFS seeing a sharp warm-up at the end of the forecast period (Sept 28-30) where the ECMWF turns much colder.
• South (Houston & Atlanta, 25% combined weight): Both models keep the South firmly in cooling mode, with no HDDs projected. The ECMWF is slightly stronger with cooling demand in Houston (300.9 CDDs vs. 290.5 GFS), while the GFS is warmer in Atlanta (174.2 CDDs vs. 159.2 ECMWF).

Demand Implications (Natural Gas)

This forecast captures the critical shoulder season transition from cooling to heating demand.

• The ECMWF outlook implies a bearish scenario for Natural Gas, with a gentle decline in cooling demand and a very slow start to any meaningful heating load.
• The GFS presents a more supportive picture for NG prices. It forecasts stronger late-season cooling demand overall (95.4 weighted CDDs vs. 87.3 on ECMWF) and, more importantly, an early arrival of heating demand in the high-consumption Midwest region. The GFS's +5.9 net HDD advantage over the ECMWF represents a material difference in potential gas consumption.

Model Agreement & Caveats

Confidence in this forecast should be low, especially beyond the 7-10 day window.

• Convergence: The model agreement is poor, with a convergence_score of just 0.50. This score worsened from the prior cycle by -0.10, indicating the models are moving further apart.
• Run-over-Run Trends: The GFS showed significant instability, trending much colder (+7.7 weighted HDDs) and less hot (-21.3 weighted CDDs). In contrast, the ECMWF was very stable run-over-run. Such a large shift in a single GFS cycle warrants caution.
• Source: This forecast uses Open-Meteo's API, which provides free, non-commercial weather data. Model resolution and specific hub definitions may differ from proprietary providers.