GFS vs ECMWF Demand-Hub Outlook: 2026-09-03 00Z

Executive Summary

The 15-day outlook presents a significant divergence between models, with the GFS forecasting a much hotter and more demand-intensive period than the ECMWF. The GFS projects 182.5 weighted CDDs versus the ECMWF's 131.5, a material difference driven by a very warm GFS solution in the 8-15 day range across the Midwest and South. This GFS forecast has trended significantly hotter (+14.8 CDDs) since the prior cycle, while the ECMWF trended slightly cooler, widening the model disagreement.

Hub-Level Weather

The primary drivers of the national forecast are the high-weight hubs in the Midwest and East, where the models show considerable disagreement in the extended range.

• Midwest (Chicago, 35% weight): The GFS is notably hotter, accumulating 171.7 CDDs compared to 138.2 for the ECMWF. The divergence is most pronounced after day 7, with the GFS showing a strong resurgence of heat that the ECMWF does not.
• East (New York City, 30% weight): A major split exists here, with the GFS forecasting 122.4 CDDs against the ECMWF's much cooler 69.0 CDDs. The ECMWF sees a seasonal cooling trend take hold, even generating 9.1 HDDs, while the GFS brings back significant late-season heat.
• South (Houston & Atlanta): Both southern hubs show a consistently hotter GFS. The cumulative difference in Houston is nearly 94 CDDs (GFS 334.7 vs ECMWF 240.9), and in Atlanta, it is over 100 CDDs (GFS 319.6 vs ECMWF 219.0).

Demand Implications (Natural Gas)

The conflicting model outlooks carry starkly different implications for late-season Natural Gas demand.

• The GFS scenario, with 51 more weighted CDDs than the ECMWF, implies a robust extension of summer-like power burns for cooling demand. This outcome would be supportive for NG futures by slowing the pace of storage injections. The significant +14.8 CDD addition in this GFS run amplifies this bullish signal.
• Conversely, the ECMWF forecast points to a more rapid seasonal decline in demand, particularly across the northern US. This would weigh on gas prices as power sector consumption falls off more quickly than anticipated by the GFS. The ECMWF's slight cooler trend (-4.1 CDDs run-over-run) reinforces this outlook.

Model Agreement & Caveats

The national convergence score is 0.80, which indicates moderate agreement but masks the significant directional divergence in the cumulative outlook. Confidence in either forecast should be tempered.

• Divergence: The primary caveat is the substantial disagreement in the 8-15 day period. Forecasts this far out carry inherent uncertainty, and the current model split means traders may await further runs for confirmation before strongly pricing in either scenario.
• Trends: The fact that the models moved in opposite directions since the last cycle (GFS hotter, ECMWF cooler) increases uncertainty and suggests instability in the long-range pattern.
• Source: This analysis is based on Open-Meteo's implementation of the GFS and ECMWF models. Specific degree-day calculations may vary from other weather service providers.