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

Executive Summary

The GFS model forecasts a significantly hotter 15-day period than the ECMWF, projecting an additional +21.2 demand-weighted Cooling Degree Days (CDDs). This bullish divergence is driven by a much warmer GFS outlook in the Eastern and Southern U.S. Importantly, this GFS run trended substantially hotter versus its prior cycle (+20.7 CDDs), while the ECMWF remained stable, reducing confidence in the GFS's more extreme solution.

Hub-Level Weather

The national-level forecast difference is a composite of significant regional disagreements between the two models. The Midwest (Chicago) and East (New York City) are the largest demand hubs, weighted at 35% and 30% respectively.

• GFS Hotter: The GFS is materially warmer in the East (NYC: 244.5 CDD vs 189.4 ECMWF), South Central (Houston: 347.7 CDD vs 290.1 ECMWF), and South (Atlanta: 274.8 CDD vs 225.9 ECMWF). The divergence is most pronounced in the 6-10 day period.
• ECMWF Hotter: Conversely, the ECMWF projects higher cooling demand in the most heavily-weighted hub, the Midwest (Chicago: 149.3 CDD vs 130.4 GFS), and the Northeast (Boston: 170.5 CDD vs 147.6 GFS).

The GFS's intense heat across the South and parts of the East outweighs the ECMWF's relative warmth in the Midwest, driving the GFS's hotter national total.

Demand Implications (Natural Gas)

Both models indicate strong and sustained cooling demand consistent with early August, implying a robust call on Natural Gas for power generation.

• The GFS solution (213.4 weighted CDD) represents a more bullish scenario for Natural Gas futures (NG) than the ECMWF (192.2 weighted CDD).
• The significant run-over-run trend in the GFS (+20.7 CDD) suggests a meaningful bullish shift in market weather expectations if this model proves correct. The stability of the ECMWF (-1.1 CDD) provides a countervailing, more neutral signal.

Model Agreement & Caveats

Confidence in the 15-day forecast is low due to material disagreement between the GFS and ECMWF models and significant volatility in the GFS run-over-run.

• Model Divergence: The GFS added +20.7 weighted CDDs compared to its previous cycle, a large bullish shift. The ECMWF remained very stable, losing just -1.1 CDDs. This growing divergence suggests instability in the forecast pattern, particularly in the GFS model.
• Convergence Score: The provided convergence score is 1.0, which typically indicates perfect agreement. This is inconsistent with the ~21 weighted CDD difference between the models and likely reflects a calculation based only on Heating Degree Days (HDDs), which are zero in this forecast.
• Source Data: This analysis is based on raw, non-bias-corrected model outputs from Open-Meteo. It does not represent a final, human-adjusted meteorological forecast.