GFS vs ECMWF Demand-Hub Outlook: 2026-06-30 00Z

Executive Summary

The GFS model forecasts a significantly hotter and more demand-intensive 15-day period than the ECMWF, projecting a total of 264.3 weighted Cooling Degree Days (CDDs) versus the ECMWF's 232.1. This ~32 CDD national difference is driven by a major model disagreement in the 8-15 day outlook for the Eastern and Southern U.S. Run-over-run trends show the models diverging, with the GFS adding a significant +19.0 CDDs while the ECMWF trended slightly cooler (-2.1 CDDs), lowering overall forecast confidence.

Hub-Level Weather

The national forecast is primarily driven by the Midwest (Chicago, 35% weight) and East (NYC, 30% weight) hubs. While both models are in strong agreement on the outlook for the Midwest, they diverge substantially elsewhere:

• East (New York City & Boston): This region is the epicenter of the model disagreement. The GFS is dramatically hotter than the ECMWF, particularly in the 8-15 day range. Over the 15-day period, the GFS forecasts 48.0 more CDDs for NYC and 33.3 more CDDs for Boston.
• South (Atlanta & Houston): A similar pattern of significant divergence exists in the South. The GFS is much hotter, forecasting +93.0 CDDs in Atlanta and +38.0 CDDs in Houston compared to the ECMWF, almost entirely due to differences in the second week.
• Midwest (Chicago): The models show excellent agreement in this key hub. The GFS and ECMWF cumulative CDD forecasts are nearly identical (225.0 vs. 226.4), providing high confidence in a hot outlook for the region.

Demand Implications (Natural Gas)

The significant difference in forecasted CDDs implies two very different scenarios for natural gas demand for power generation ("power burn").

• The GFS scenario, with its +32.3 weighted CDD premium over the ECMWF, points to a much more robust call on gas-fired power plants to meet air conditioning load, especially in the eastern half of the country during the second week of July.
• The prior GFS model cycle added a substantial +19.0 weighted CDDs, a materially bullish shift. In contrast, the ECMWF's minor revision of -2.1 CDDs suggests stability in its cooler outlook. This growing divergence between models presents significant uncertainty for market participants.

Model Agreement & Caveats

Confidence in the 15-day outlook is high for the first week but low for the 8-15 day period due to the stark disagreement between the GFS and ECMWF models for the Eastern and Southern U.S.

• Convergence: The provided convergence_score is 1.0, however, the absolute difference of over 32 weighted CDDs is substantial and indicates poor model agreement. The diverging run-over-run trends further underscore the forecast uncertainty.
• Caveats: This analysis is based on a specific set of demand-weighted hubs from the Open-Meteo API. It is a proxy for national demand and may not capture all regional nuances. The 65°F base is a standard but may not perfectly reflect power demand trigger points in all regions.