GFS vs ECMWF Demand-Hub Outlook: 2026-09-18 00Z

Executive Summary

A significant divergence marks the 15-day weather outlook, with the GFS model forecasting a much warmer and higher-demand scenario than the ECMWF. The GFS projects lingering cooling demand, while the ECMWF shows a decisive shift toward early-season heating load. Overall forecast confidence is low, as reflected by a convergence score of just 0.33, with the models trending further apart in the latest cycle.

Hub-Level Weather

The primary model disagreement is concentrated in the two largest demand centers, the Midwest and the East, which together account for 65% of the weighted total.

• Midwest (Chicago, 35% weight): The ECMWF is substantially colder, projecting 47.0 HDDs over the next 15 days, compared to just 28.4 HDDs from the GFS. The GFS sees a late-period warmup, while the ECMWF maintains cooler temperatures.
• East (New York City, 30% weight): The split is even more pronounced here. The ECMWF forecasts 57.0 HDDs, more than double the GFS's forecast of 24.7 HDDs.
• South Central (Houston, 15% weight): The roles are reversed in the south, where the GFS is significantly hotter. It calls for a robust 319.9 CDDs in Houston, far exceeding the ECMWF's milder forecast of 263.4 CDDs.

Demand Implications (Natural Gas)

The competing model forecasts imply starkly different outcomes for Natural Gas demand during this critical shoulder season.

• ECMWF Scenario (Colder/Bearish): With a total of 42.8 demand-weighted HDDs, the ECMWF suggests a firm start to the heating season. This would accelerate the transition away from cooling demand and begin drawing on gas for residential/commercial heating, supporting NG futures.
• GFS Scenario (Warmer/Bullish): The GFS projects a much stronger 79.3 weighted CDDs against only 27.0 weighted HDDs. This outlook would extend power burn demand for air conditioning well into late September, delaying the typical seasonal demand lull.
• Run-over-Run Shift: The GFS trended significantly warmer versus its prior run, shedding 8.4 weighted HDDs and adding 14.4 weighted CDDs. The ECMWF remained largely stable. This growing model split increases market uncertainty.

Model Agreement & Caveats

Forecast confidence is low due to the substantial disagreement between the two leading weather models.

• Convergence Score: The current score is 0.33, indicating a high degree of model divergence, particularly in the 8-15 day period.
• Trend: The models moved further apart in this cycle, with the convergence score decreasing by 0.04 from the prior run. The weighted mean absolute HDD difference per day is a notable 2.04.
• Source: This analysis is based on raw model output from Open-Meteo, using GFS and ECMWF data. It is a snapshot of probabilistic guidance and not an official government forecast. The hub weightings are simplified and may not capture all regional demand nuances.