GFS vs ECMWF Demand-Hub Outlook: 2026-03-27T00:00Z

Executive Summary

The 15-day outlook shows strong model agreement on a national level, with lingering late-season heating demand expected across the Midwest and East. The GFS has trended materially colder in this cycle (+12.6 weighted HDDs), bringing it into tight alignment with the stable ECMWF forecast. The primary source of forecast uncertainty is a significant model split in the South Central U.S. beyond Day 10, where the GFS is substantially hotter and implies greater cooling demand.

Hub-Level Weather

The national forecast is driven by heating demand in the two largest weighted hubs, the Midwest and the East, where cumulative HDD forecasts are nearly identical between models.

• Midwest (Chicago, 35% weight): Both models project significant heating demand, with GFS at 298.1 cumulative HDDs and ECMWF at 301.6. However, they disagree on the timing of cold air late in the period. The GFS is notably colder around April 4th (18.3 HDDs vs. 8.7 on ECMWF), while the ECMWF is much colder from April 6th-7th.
• East (New York City, 30% weight): Overall agreement is excellent, with cumulative HDDs of 252.3 (GFS) and 251.8 (ECMWF). Minor daily timing differences exist, particularly around April 5th where the GFS is colder (16.2 HDDs) than the ECMWF (5.8 HDDs).
• South Central (Houston, 15% weight): This hub is the primary source of model disagreement. The GFS is significantly hotter, forecasting 132.7 cumulative CDDs versus just 89.8 from the ECMWF. The divergence begins around April 6th, when the ECMWF brings in a cool front (forecasting 13.0 HDDs over the 15-day period) while the GFS remains warm and generates strong cooling demand.

Demand Implications (Natural Gas)

The forecast implies a period of stronger-than-normal heating demand for the late March/early April shoulder season, providing support for natural gas consumption.

• Heating Demand: Total weighted heating demand is robust and in close agreement, with GFS at 217.4 HDDs and ECMWF at 220.4 HDDs. The GFS added a significant 12.6 weighted HDDs versus its prior run, a bullish shift that brings it in line with the more consistent ECMWF.
• Cooling Demand: Cooling demand is emerging but concentrated in the South. The GFS projects stronger national cooling demand (23.4 weighted CDDs) than the ECMWF (18.6 weighted CDDs) due to its much hotter outlook for the Houston hub. This represents a key risk to the demand picture in the second week of the forecast.

Model Agreement & Caveats

Overall model agreement is high, but confidence decreases significantly in the Day 10-15 period due to the divergence in the southern U.S.

• Convergence: The national convergence_score of 0.24 is low, indicating strong agreement on the total demand-weighted picture. The weighted_mean_abs_hdd_diff_by_day is also a modest 3.1, further supporting high confidence in the near-term.
• Caveats: The primary caveat is the major GFS vs. ECMWF split in the South Central U.S. beginning around April 6th. This divergence on a key cooling front reduces confidence in the long-range temperature and power burn forecast for the region. Minor timing differences for cold shots in the Midwest also warrant monitoring.
• Source Limitations: This analysis is based on a simplified multi-hub view from Open-Meteo's free API. It does not incorporate other fundamental drivers such as renewable generation, power outages, or industrial demand.