GFS vs ECMWF Demand-Hub Outlook: 2026-06-18T00:00Z

Executive Summary

The 00Z GFS model is significantly hotter than the ECMWF, projecting 33.3 more weighted CDDs over the 15-day forecast. This bullish divergence is driven by a major GFS run-over-run addition of demand (+20.2 wCDD), concentrated in the Midwest and South during the Day 10-15 period. The ECMWF solution remained largely unchanged from its prior run and presents a much more moderate demand outlook.

Hub-Level Weather

The primary model disagreement stems from key high-weight demand centers in the eastern two-thirds of the country.

• Midwest (Chicago, 35% weight): This is a key area of divergence. The GFS forecasts 103.5 cumulative CDDs, while the ECMWF is much cooler at 67.5 CDDs. The difference is most pronounced from June 28th onward, where the GFS shows mean daily temperatures climbing into the mid-80s, while the ECMWF remains in the mid-70s.
• East (New York City, 30% weight): The GFS is also hotter in the East, forecasting 155.0 CDDs versus the ECMWF's 135.1 CDDs. This ~20 CDD difference emerges primarily in early July.
• South (Atlanta, 10% weight): This hub shows an extreme GFS solution, with 252.9 CDDs compared to just 167.4 CDDs from the ECMWF. The GFS model calls for mean temperatures near 90°F to start July, a dramatically hotter scenario than the ECMWF.
• South Central (Houston, 15% weight): Both models show persistent strong heat, but the GFS is hotter still, with 316.2 CDDs versus the ECMWF's 273.6 CDDs.

Demand Implications (Natural Gas)

The large discrepancy between models presents a conflicting signal for natural gas power burn.

• GFS Scenario: The GFS outlook implies a period of intense, above-normal cooling demand to begin July, which would be strongly bullish for natural gas prices by tightening supply/demand balances. The significant addition of +20.2 weighted CDDs versus its prior run signals a strong trend toward this hotter outcome within the GFS model suite.
• ECMWF Scenario: The ECMWF forecast suggests a more moderate, closer-to-normal demand profile. This outlook would be less impactful on natural gas markets.
• Overall: The market faces considerable forecast risk, particularly for the early July period. The stability of the cooler ECMWF contrasts sharply with the increasingly hot GFS, a divergence that will be a key focus in coming model cycles.

Model Agreement & Caveats

• Convergence: The overall convergence score is relatively high at 0.85, indicating good model agreement in the near term. However, this masks the significant divergence in the Day 10-15 range, where forecast confidence is inherently lower. The score has decreased slightly (-0.014) from the prior cycle, reflecting the GFS's move away from the ECMWF solution.
• Run-over-Run Shift: The primary caveat is the large bullish shift in the GFS (+20.2 wCDD), which makes its solution both higher-risk and potentially less stable. In contrast, the ECMWF was nearly unchanged (+0.4 wCDD).
• Source Limitations: This analysis is based on deterministic GFS and ECMWF runs from Open-Meteo. It does not incorporate ensemble guidance (e.g., GEFS or EPS), which is critical for assessing the probability and full range of potential temperature outcomes.