GFS vs ECMWF Demand-Hub Outlook: 2026-05-08T00:00Z

Executive Summary

The 15-day outlook shows moderate disagreement between the GFS and ECMWF models, with the GFS forecasting slightly higher overall weather-driven demand. The GFS is warmer, projecting higher cooling demand (57.8 weighted CDDs vs. ECMWF's 51.7), while the ECMWF is colder, retaining more heating demand (81.9 weighted HDDs vs. GFS's 78.6). Both models trended significantly warmer compared to their prior runs, reducing HDD forecasts and adding CDDs, though the GFS made a much more aggressive shift.

Hub-Level Weather

The primary drivers of the national forecast are the high-weighted Midwest and East hubs, which also exhibit the most significant model disagreement, particularly in the latter half of the forecast period.

• Midwest (Chicago, 35% weight): Models diverge significantly after Day 8. The ECMWF projects a strong warm-up around May 16-18, while the GFS remains cool. This pattern then flips, with the GFS turning warm on May 19-20 as the ECMWF sees a cold shot. Cumulatively, the GFS is slightly colder (121.0 HDD) than the ECMWF (114.2 HDD).
• East (New York City, 30% weight): The ECMWF is materially colder over the 15-day period, forecasting 93.1 cumulative HDDs versus just 74.0 HDDs from the GFS. The disagreement is most pronounced from May 17-21, where the models show opposing temperature trends on multiple days.
• South Central (Houston, 15% weight): The GFS is consistently and significantly hotter than the ECMWF in this key cooling market, projecting 194.0 cumulative CDDs compared to the ECMWF's 147.8 CDDs. This difference is a primary contributor to the GFS's higher national cooling demand forecast.

Demand Implications (Natural Gas)

The forecast reflects a typical shoulder season transition, with waning heating demand being replaced by nascent cooling load.

• Heating Demand: The ECMWF's outlook implies a more prolonged period of late-season heating demand, especially in the East. However, both models showed a strong bearish trend versus the prior cycle, with the GFS shedding 17.4 weighted HDDs and the ECMWF shedding 10.0 weighted HDDs.
• Cooling Demand: The GFS projects a stronger start to the cooling season, driven by sustained warmth in the South and late-period heat in the Midwest/East. This bullish cooling signal was amplified in the latest run, with the GFS adding 15.7 weighted CDDs compared to a more modest 7.4 CDD addition from the ECMWF.

Model Agreement & Caveats

Overall confidence in this forecast is low-to-moderate, with significant uncertainty in the Day 8-15 period.

• Convergence: The national convergence score of 0.345 indicates moderate model disagreement. This is primarily due to the opposing temperature solutions for the Midwest and East in the extended range.
• Forecast Instability: The model agreement worsened from the prior run (delta_convergence_score: +0.05). This, combined with the large run-over-run temperature changes, suggests the forecast is unstable and subject to further significant revisions in subsequent cycles.
• Source Limitations: This analysis is based on deterministic GFS and ECMWF runs provided by Open-Meteo. It does not incorporate ensemble guidance, which would provide a more comprehensive view of the range of potential outcomes and overall forecast uncertainty.