GFS vs ECMWF Demand-Hub Outlook: 2026-09-30 00:00 UTC

Executive Summary

The latest 15-day model cycle shows a material divergence between weather models, with the GFS projecting a colder pattern across major population centers resulting in 65.98 weighted cumulative HDDs versus 56.90 HDDs in the ECMWF. Conversely, ECMWF favors warmer conditions across the South, generating 43.87 weighted CDDs compared to 37.80 CDDs in the GFS. Overall run-over-run adjustments were modest, with both models shedding marginal heating load (-0.26 HDD on GFS, -0.98 HDD on ECMWF) relative to the prior 00:00 UTC cycle.

Hub-Level Weather

• Midwest (Chicago, 35% weight): Cumulative totals align closely at 81.35 HDDs (GFS) versus 80.00 HDDs (ECMWF), but daily pacing differs. ECMWF projects sharper mid-period cooling on October 6 (13.93 HDDs vs. 9.92 HDDs GFS), whereas GFS maintains colder air late in the period on October 10–13.
• East (New York City, 30% weight): GFS is considerably colder, projecting 74.74 HDDs against ECMWF's 64.92 HDDs. The primary split occurs during October 10–14, where GFS introduces significant heating demand while ECMWF warms into the upper 60s°F (averaging 0.00 HDDs and 5.30 CDDs across October 12–13).
• Northeast (Boston, 10% weight): The largest regional variance appears here, with GFS accumulating 126.84 HDDs versus 92.52 HDDs on ECMWF. GFS sustains elevated HDDs above 11.00 daily through October 10–14, while ECMWF moderates sharply.
• South Central (Houston, 15% weight) & South (Atlanta, 10% weight): Houston remains entirely CDD-driven (177.11 CDDs GFS vs. 199.04 CDDs ECMWF). In Atlanta, GFS introduces unseasonal heating demand of 23.99 HDDs (notably 12.53 HDDs on October 11), while ECMWF keeps heating load minimal at 1.76 HDDs and generates 80.65 CDDs.

Demand Implications (Natural Gas)

With the market transitioning into early shoulder season, implied demand for Natural Gas contracts (NG) hinges on the timing of early heating loads across the Midwest and East versus lingering cooling demand in the South.

The GFS run presents a distinctly more bullish demand profile for heating consumption, driven by back-to-back troughing patterns into high-weight consuming hubs (Chicago, NYC, and Boston). In contrast, the ECMWF scenario presents a more typical shoulder-season lull in the East while sustaining slightly higher power-burn support in the South Central region.

Model Agreement & Caveats

• Convergence Score: Model convergence is weak at 0.297, reflecting a weighted mean absolute daily HDD difference of 2.37. Confidence in extended-range outcomes (Days 10–15) is low given substantial disagreement on the amplitude and southward progression of Canadian air masses.
• Cycle Shifts: Run deltas show minor warming across both suites, with GFS losing 0.26 weighted HDDs / 0.46 CDDs and ECMWF losing 0.98 weighted HDDs / 0.60 CDDs.
• Caveats: Forecast data is derived from Open-Meteo point extractions for metropolitan demand centers. Local weather station sampling may not fully account for broader pipeline-corridor heating degrees or intra-day extremes that drive peak hourly gas sendout. Further updates should be tracked via the natural gas weather stream.