GFS vs ECMWF Demand-Hub Outlook: 2026-08-24T00:00Z

Executive Summary

The 15-day outlook shows a material disagreement between models, with the ECMWF forecasting significantly more cooling demand than the GFS (173.5 vs 159.6 weighted CDDs). This divergence is driven almost entirely by the high-weighted Midwest (Chicago) hub in the Day 10-15 period, where the ECMWF maintains summer heat while the GFS shows a sharp cooldown. While the GFS added demand versus its prior run, the overall model divergence has increased, lowering forecast confidence.

Hub-Level Weather

The primary drivers of the national forecast are the Midwest and East hubs, which show conflicting signals.

• Midwest (Chicago, 35% weight): This is the main point of contention. The ECMWF is dramatically hotter, forecasting 173.0 cumulative CDDs versus just 106.7 CDDs from the GFS. The split occurs after September 3rd, with the ECMWF showing mean temperatures in the 80s while the GFS drops into the 60s.
• East (New York City, 30% weight): The models are in closer alignment here, with the GFS at 112.9 CDDs and the ECMWF at 116.0 CDDs. However, the GFS shows a much cooler period around September 4th, briefly registering heating demand (3.7 HDDs) that the ECMWF does not.
• South Central (Houston, 15% weight): In contrast to the Midwest, the GFS is the hotter model here, forecasting 360.9 CDDs against the ECMWF's 307.5 CDDs.

Demand Implications (Natural Gas)

The competing forecasts present different scenarios for late-summer power burn, a key component of Natural Gas demand.

• The ECMWF's hotter solution, especially its persistence of heat in the high-demand Midwest, implies a stronger-than-normal finish to the cooling season. This would be supportive for NG futures by extending the period of elevated power generation demand.
• Conversely, the GFS forecast signals a more rapid transition into the fall shoulder season. Its projection of a significant cooldown in the Midwest and Northeast would curtail cooling demand and represents a more bearish outlook for natural gas prices. The GFS's introduction of minor heating demand (2.8 weighted HDDs) further underscores this potential seasonal shift.

Model Agreement & Caveats

Confidence in the 15-day forecast is moderate and decreasing, warranting caution.

• Convergence: The overall model agreement score is 0.844, which is reasonable but has decreased by 0.07 since the prior cycle, indicating the models are diverging. The primary source of this disagreement is the Day 10+ forecast for the Midwest.
• Run-over-Run Shifts: The GFS trended hotter versus its previous run, adding 2.9 weighted CDDs. The ECMWF forecast was stable. The GFS shift in a warmer direction may add a slight degree of confidence to the hotter ECMWF outlook, but the significant disagreement remains the key takeaway.
• Source Data: This analysis is based on raw GFS and ECMWF model output from Open-Meteo. It does not incorporate any proprietary adjustments or blending that may be used by other weather vendors.