GFS vs ECMWF Demand-Hub Outlook: 2026-07-26 00Z

Executive Summary

The 15-day outlook shows a significant divergence between models, with the ECMWF forecasting a nationally stronger cooling demand profile (+9.3 weighted CDDs) compared to the GFS. This difference is driven by a much hotter ECMWF outlook for the high-demand-weight Midwest and Northeast, which more than offsets a hotter GFS forecast for the South. Confidence in the forecast is low, as the models show sharp regional disagreements and are trending in opposite directions cycle-over-cycle.

Hub-Level Weather

The primary drivers of the national forecast are the major population centers, with significant model disagreement outside of the near term.

• Midwest (Chicago, 35% weight): This is the largest point of contention. The ECMWF is substantially hotter, forecasting 188.1 CDDs versus 157.2 CDDs for the GFS. The disagreement is most pronounced in the 8-15 day period (e.g., on Aug 8, ECMWF has 20.8 CDDs vs. 9.8 on the GFS).
• Northeast (Boston, 10% weight): A similar pattern exists here, with the ECMWF forecasting 113.3 CDDs compared to a much cooler 44.8 CDDs from the GFS.
• South (Atlanta, 10% weight) & South Central (Houston, 15% weight): The roles are reversed in the South. The GFS is significantly hotter, forecasting 272.8 CDDs in Atlanta (vs. 224.4 ECMWF) and 328.1 CDDs in Houston (vs. 312.4 ECMWF).
• East (New York City, 30% weight): The models are in closer alignment, with the GFS at 168.5 CDDs and the ECMWF at 164.5 CDDs.

Demand Implications (Natural Gas)

The competing forecasts imply different regional distributions for natural gas power burn over the next two weeks.

• National Demand: The ECMWF's hotter solution (195.8 weighted CDDs) points to a more robust national cooling demand load than the GFS (186.5 weighted CDDs).
• Regional Demand: The ECMWF scenario would place higher demand and potential stress on power grids in the Midwest and Northeast. The GFS scenario concentrates the most intense cooling demand in the South and South Central regions.
• Run-over-Run Trends: Confidence is further weakened by diverging model trends. The ECMWF trended significantly hotter versus its prior run (+4.5 weighted CDDs), while the GFS trended cooler (-2.1 weighted CDDs).

Model Agreement & Caveats

Overall model agreement is low, and the provided convergence metric should be viewed with caution.

• Convergence: Despite significant regional disagreements, the convergence_score is a high 0.9927. This appears to be driven by a metric based on Heating Degree Days (weighted_mean_abs_hdd_diff_by_day), which are negligible in this summer forecast. The score does not reflect the substantial CDD divergence between the models and is therefore misleading.
• Confidence: True forecast confidence, especially beyond Day 7, is low due to the sharp regional disagreements and the opposing run-over-run model trends.
• Source Limitations: This analysis is based on a limited set of demand hubs and deterministic model runs. It does not incorporate ensemble guidance, which would provide a better measure of forecast uncertainty.