GFS vs ECMWF Demand-Hub Outlook: 2026-04-17 00:00Z

Executive Summary

The 15-day outlook shows a classic shoulder season pattern, with lingering heating demand in the North and emerging cooling demand in the South. While the national weighted HDD totals are closely aligned between models (GFS: 144.7, ECMWF: 140.1), this masks significant regional disagreements and a notable divergence in model trends. The latest ECMWF cycle trended substantially colder (+18.9 weighted HDDs), while the GFS trended warmer (-10.2 weighted HDDs), reducing overall forecast confidence.

Hub-Level Weather

The national forecast is primarily driven by the Midwest and East, which account for a combined 75% of the demand weighting.

• Midwest (Chicago, 35% weight): This is a key area of disagreement. The GFS is materially colder, forecasting 191.8 cumulative HDDs versus 171.3 HDDs from the ECMWF. The divergence is most pronounced in the latter half of the forecast (post Day 8), where the GFS shows a stronger late-April cold pattern.
• East (New York City, 30% weight): The models are in better agreement here, though the ECMWF is slightly colder with 172.1 cumulative HDDs compared to the GFS's 160.8.
• South Central (Houston, 15% weight): Models diverge on cooling demand. The ECMWF is significantly warmer, projecting 119.9 cumulative CDDs against the GFS's 99.7. This difference of over 20 CDDs is driven by the GFS forecasting a sharp cooldown at the end of the 15-day period, which the ECMWF does not see.

Demand Implications (Natural Gas)

The forecast implies moderate, but waning, natural gas demand for residential and commercial heating, alongside early-season gas-for-power demand for cooling.

• Heating Demand: Lingering cold in the Midwest and Northeast will support heating load. The GFS's colder outlook in Chicago, if realized, would be moderately bullish for demand. However, total national HDDs are not indicative of extreme or widespread cold.
• Cooling Demand: Cooling demand is emerging as a consistent driver in the South. The ECMWF's warmer forecast in Texas and the Southeast (total weighted CDDs of 29.4 vs. 27.3 for GFS) points to a stronger call on gas-fired power generation, particularly if its warmer long-range outlook verifies.

Model Agreement & Caveats

Overall model agreement has decreased since the prior cycle, warranting caution.

• Convergence: The national weighted HDD totals are superficially close. However, the weighted_mean_abs_hdd_diff_by_day is 3.62, and the key hubs show notable disagreement.
• Run-over-Run Deltas: The most significant takeaway is the strong model divergence. The ECMWF trended sharply colder/more bullish (+18.9 weighted HDDs), while the GFS trended warmer/more bearish (-10.2 weighted HDDs). This opposite trend increases forecast uncertainty, especially for days 8-15. The delta_convergence_score of +0.0672 confirms that model agreement has worsened.
• Source Limitations: This analysis is based on raw GFS and ECMWF model output from Open-Meteo. It does not incorporate ensemble guidance or analyst adjustments, and long-range forecast accuracy diminishes significantly beyond 7-10 days.