GFS vs ECMWF Demand-Hub Outlook: 2026-10-04 00:00 UTC

Executive Summary

The latest 15-day weather outlook reflects a significant divergence between the GFS and ECMWF models, resulting in a low convergence score of 0.231. The ECMWF presents a distinctly cooler overall profile across key population centers, projecting a national demand-weighted cumulative 70.56 Heating Degree Days (HDDs) compared to the GFS at 61.39 HDDs. In run-over-run changes, the GFS shed 17.72 weighted HDDs while adding 8.10 Cooling Degree Days (CDDs), whereas the ECMWF remained comparatively stable (+2.66 HDDs), creating an acute split in late shoulder-season Natural Gas demand expectations.

Weighted HDDs:  GFS 61.39  |  ECMWF 70.56  |  Spread: -9.17 HDDs
Weighted CDDs:  GFS 44.33  |  ECMWF 33.31  |  Spread: +11.02 CDDs
Convergence Score: 0.231 (Low Agreement)

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Hub-Level Weather

+--------------------------+--------+----------------+----------------+----------------+----------------+
| Hub / Region             | Weight | GFS Cum. HDD   | ECMWF Cum. HDD | GFS Cum. CDD   | ECMWF Cum. CDD |
+--------------------------+--------+----------------+----------------+----------------+----------------+
| Midwest (Chicago)        | 0.35   | 87.60          | 88.26          | 27.75          | 0.14           |
| East (New York City)     | 0.30   | 51.68          | 87.51          | 5.96           | 1.55           |
| Northeast (Boston)       | 0.10   | 147.33         | 133.26         | 0.00           | 0.00           |
| South Central (Houston)  | 0.15   | 0.00           | 0.00           | 171.88         | 182.95         |
| South (Atlanta)          | 0.10   | 4.89           | 0.89           | 70.49          | 53.60          |
+--------------------------+--------+----------------+----------------+----------------+----------------+

Key Drivers and Disagreements:

• East (New York City - 30% weight): The primary source of structural divergence. ECMWF projects 87.51 HDDs versus just 51.68 HDDs on the GFS—a spread of 35.83 HDDs. ECMWF projects sustained sub-60°F mean temperatures across October 7–11 and October 14–16, whereas GFS advertises a prolonged mid-period warm spell (e.g., 67.71°F on Oct 8).
• Midwest (Chicago - 35% weight): Cumulative HDDs appear closely matched (GFS 87.60 vs. ECMWF 88.26), but the progression differs sharply. GFS predicts late-summer warmth on October 10–13 (averaging 70–73°F and generating 27.75 CDDs) followed by a sharp cold front (16.70 HDDs on Oct 15). In contrast, ECMWF moves the cold air in earlier (14.40 HDDs on Oct 13) and moderates toward the end of the 15-day window.
• Northeast (Boston - 10% weight): GFS runs colder than ECMWF (147.33 HDDs vs. 133.26 HDDs), driven by an aggressive late-period cold plunge delivering 21.10 HDDs on October 16.
• South Central & South (Houston & Atlanta - 25% combined weight): Lingering late-season cooling demand remains steady in Houston (171.88 GFS CDDs vs. 182.95 ECMWF CDDs). In Atlanta, GFS yields higher cooling load (70.49 CDDs vs. 53.60 CDDs).

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Demand Implications (Natural Gas)

• Shoulder Season Transition: Total degree days are split between residual power burn in the South and emerging space-heating load in the Midwest and East. The ECMWF’s colder eastern regime would accelerate early residential/commercial (res/comm) withdrawals or curb late-season storage injections faster than the GFS path.
• Storage & Balance: If ECMWF materializes, the incremental ~9.2 weighted HDDs over GFS across high-density consuming hubs will tighten the near-term supply-demand balance and weigh on weekly EIA storage build projections.
• Trading Context: For NG futures, the sharp GFS run-over-run warm shift (-17.72 HDDs) introduces headline downside risk, but the sticky cold solution on the ECMWF suggests that downside price momentum may face resistance if European guidance holds in subsequent cycles.

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Model Agreement & Caveats

• Convergence Metric: The model convergence score is 0.231, with a weighted daily absolute HDD divergence of 3.33 HDDs/day. This indicates poor alignment and elevated forecast uncertainty beyond Day 5 (October 9).
• High-Risk Windows: Days 8–12 (October 11–15) show complete out-of-phase pattern timing in Chicago and New York, where the models disagree by more than 10°F on daily mean temperatures.
• Limitations: Raw model output via Open-Meteo does not account for ensemble spread clustering, convective feedback biases in transitional seasons, or localized urban heat island effects. Analysts tracking weather drivers should avoid taking directional exposure purely on GFS operational runs until ECMWF ensemble (EPS) and GEFS means establish a clearer consensus.