GFS vs ECMWF Demand-Hub Outlook: 2026-08-20 00Z

Executive Summary

The GFS and ECMWF models present a divergent 15-day outlook, with the GFS forecasting a considerably hotter and higher-demand period at 174.7 total weighted CDDs versus the ECMWF's 145.4. This ~29 CDD gap is primarily driven by significant disagreement in the high-demand Midwest and Eastern hubs, particularly in the 6-15 day range. While the models have trended towards each other since the last cycle, with the GFS cooling (-13.9 CDD) and the ECMWF warming substantially (+28.8 CDD), the remaining divergence warrants caution.

Hub-Level Weather

The national demand picture is dominated by the Midwest (Chicago) and East (New York City) hubs, which have demand weights of 0.35 and 0.30, respectively. It is in these key regions that the models disagree most significantly:

• Midwest (Chicago): The GFS is substantially hotter, forecasting 123.5 cumulative CDDs compared to just 89.8 from the ECMWF. The largest deviations appear late in the forecast period.
• East (New York City): A similar story unfolds, with the GFS at 144.2 CDDs versus the ECMWF's 102.5 CDDs.
• South Central (Houston): Both models agree on persistent, strong cooling demand, with the GFS (357.6 CDDs) only slightly warmer than the ECMWF (341.9 CDDs).
• South (Atlanta): The GFS shows a much hotter outlook with 285.3 CDDs against the ECMWF's 218.4 CDDs.

Demand Implications (Natural Gas)

The competing forecasts carry different implications for Natural Gas power burn over the next two weeks.

• The GFS solution (174.7 wCDD) points to robust, above-normal cooling demand extending into early September. This scenario would be supportive for NG futures by sustaining high consumption for air conditioning.
• The ECMWF solution (145.4 wCDD) suggests a more moderate demand environment, presenting a less bullish case for consumption.
• The run-over-run changes introduce uncertainty. The GFS saw a notable bearish shift, shedding 13.9 weighted CDDs. Conversely, the ECMWF made a very large bullish jump, adding 28.8 weighted CDDs. Such a significant revision reduces confidence in the ECMWF's immediate outlook until subsequent runs can confirm the new, warmer trend.

Model Agreement & Caveats

The forecast's convergence score is 1.0, which typically indicates perfect model agreement. However, this is contradicted by the significant 29.3 CDD gap in the total weighted outlook. This discrepancy suggests the raw convergence score may be misleading for this cycle, and traders should focus on the material differences in the underlying hub forecasts.

The primary caveat is the volatility seen in the latest ECMWF run (+28.8 wCDD). Large cycle-over-cycle changes often indicate model instability in resolving the medium-range pattern, reducing forecast confidence.

Forecast data is provided by Open-Meteo and is subject to their specific data aggregation and processing methodologies.