GFS vs ECMWF Demand-Hub Outlook: 2026-06-27 00Z

Executive Summary

Both GFS and ECMWF models forecast strong cooling demand over the next 15 days, though the ECMWF is materially hotter, projecting 30.8 more weighted CDDs nationally. The primary divergence stems from the Midwest and East, where the ECMWF is significantly warmer. The latest cycle showed a notable split in trends: the GFS shifted substantially cooler (-29.3 wCDDs), while the ECMWF trended slightly warmer (+6.1 wCDDs), reducing confidence despite a high headline agreement score.

Hub-Level Weather

The key driver of the national forecast difference is a significant temperature disagreement across the highest-weighted northern demand centers.

• Midwest (Chicago, 35% weight): The ECMWF is substantially hotter, forecasting 243.5 CDDs compared to the GFS's 190.7 CDDs—a surplus of 52.8 CDDs. The largest daily differences appear in the Day 8-10 period (July 4-6).
• East (New York City, 30% weight): A similar pattern exists here, with the ECMWF projecting 258.2 CDDs versus the GFS at 223.7 CDDs (+34.5 CDDs). The ECMWF shows a much stronger heat event around July 5th-6th.
• South/South Central (Atlanta/Houston, 25% combined weight): In contrast to the north, the GFS is the warmer model in the southern hubs. It projects 42.3 more CDDs in Atlanta and 22.1 more in Houston than the ECMWF. This highlights a key difference in the forecast placement of the strongest heat.

Demand Implications (Natural Gas)

The ECMWF solution implies a significantly stronger call on natural gas for power generation over the 15-day outlook.

• National Demand: The ECMWF's forecast of 249.7 weighted CDDs is considerably more supportive for gas demand than the GFS's 219.0 wCDDs.
• Regional Demand: The ECMWF's heat concentration in the high-population Midwest and East would drive robust power burn in those key markets. The GFS outlook suggests more moderate demand in the north, partially offset by stronger demand in the South.
• Run-over-Run Trend: The GFS underwent a major bearish correction, losing 29.3 weighted CDDs from the prior cycle. This suggests instability in its solution. The ECMWF's modest addition of 6.1 wCDDs presents a more stable and consistently hot forecast. The market will likely place more weight on the ECMWF until the GFS shows more run-to-run consistency.

Model Agreement & Caveats

The headline forecast agreement is high, but run-over-run trends introduce significant uncertainty.

• Convergence: The 15-day convergence score is strong at 0.95, indicating the models agree on the broad synoptic pattern, particularly in the first week.
• Caveats: The primary caveat is the starkly divergent run-over-run trend. A large shift like the one seen in the GFS reduces confidence in its specific temperature outcomes, especially in the Day 10-15 range. The market should be cautious of the GFS's cooler trend until it is confirmed by subsequent runs or the ECMWF model.
• Source Limitations: This analysis is based on raw GFS and ECMWF model output from Open-Meteo for five specific demand hubs. It does not represent a full CONUS-wide grid or a bias-corrected forecast.