GFS vs ECMWF Demand-Hub Outlook: 2026-09-24 00:00 UTC

Executive Summary

The 15-day outlook presents sharp divergence between operational models, reflected in a low convergence score of 0.2958. ECMWF projects a cooler, heating-dominated pattern with 50.79 demand-weighted HDDs compared to 36.73 HDDs on the GFS, while GFS leans significantly warmer late in the period with 78.89 weighted CDDs versus ECMWF's 47.18 CDDs. Run-over-run shifts since September 21 added +10.26 HDDs to GFS and +12.90 HDDs to ECMWF, confirming an incremental seasonal shift toward heating load despite large inter-model spread.

Hub-Level Weather

The national demand picture is predominantly driven by high-weight northern hubs:

• Midwest (Chicago, 35% weight): ECMWF forecasts higher cumulative heating load at 60.89 HDDs (3.41 CDDs) versus GFS at 49.39 HDDs (24.30 CDDs). GFS introduces an unseasonably warm spike on October 4–5 (mean temps reaching 76.88°F on Oct 5), whereas ECMWF holds autumn chill (mean temp 56.88°F, generating 8.13 HDDs). GFS then shows a sharp cold front on October 7 (12.64 HDDs vs. ECMWF’s 3.70 HDDs).
• East (New York City, 30% weight): Marked model conflict appears in the 6–15 day window. ECMWF projects 65.89 HDDs and 8.84 CDDs, while GFS projects 35.69 HDDs and 52.43 CDDs. GFS brings a prolonged warm spell from October 1–4 (peaking at 79.58°F on Oct 2), while ECMWF shifts into steady heating demand (e.g., 9.48 HDDs on Oct 5 and 9.63 HDDs on Oct 8).
• Northeast (Boston, 10% weight): Both models resolve the highest absolute heating requirements here, though ECMWF remains colder with 97.08 HDDs vs. GFS at 87.38 HDDs.
• South Central & South (Houston 15%, Atlanta 10%): Cooling demand remains active. In Houston, GFS yields 259.85 CDDs against ECMWF’s 230.26 CDDs. In Atlanta, GFS projects 141.78 CDDs versus ECMWF’s 87.97 CDDs, as ECMWF cools down substantially after October 3.

Demand Implications (Natural Gas)

For natural gas (NG) balance modeling: * Shoulder Season Transition: Total degree days are split between lingering Southern power-burn cooling load and emerging Northern residential/commercial heating load. * ECMWF Scenario (Bullish Heating): The ECMWF solution accelerates early-season heating consumption across the Midwest and Northeast (50.79 weighted HDDs), pulling natural gas burn into local distribution companies (LDCs) ahead of normal seasonal ramps. * GFS Scenario (Extended Power Burn): The GFS solution limits early space heating in major population centers while extending electric cooling burn across the Mid-Atlantic, Southeast, and Texas (78.89 weighted CDDs).

Model Agreement & Caveats

• Forecast Convergence: The low convergence score (0.2958) and high weighted mean absolute daily HDD difference (2.3811 HDDs/day, widening by +0.4163 HDDs vs. prior run) signal high forecast uncertainty, particularly for days 7–15. Market participants should avoid heavily leaning on single-model extremes.
• Caveats: Run-over-run delta indicates both suites are adjusting colder on net (+10.26 GFS / +12.90 ECMWF HDDs), but spatial timing of ridge/trough progression across the Great Lakes and Northeast differs by 48–72 hours. Open-Meteo point extractions do not capture sub-grid microclimates or real-time pipeline aggregation weighting.