GFS vs ECMWF Demand-Hub Outlook: 2026-11-10 12:00 UTC

Executive Summary

The 12:00 UTC operational suite projects a demand-weighted 15-day cumulative heating degree day (HDD) total of 245.37 for the GFS and 252.25 for the ECMWF, reflecting a modestly colder national baseline in the European solution. Run-over-run, the ECMWF experienced a notable cold shift, adding +21.11 weighted HDDs relative to its prior 00:00 UTC cycle. However, overall model convergence remains poor at 0.1958, driven by severe Day 12–15 disagreements over arctic air delivery into the Midwest and South Central hubs.

Hub-Level Weather

• Midwest (Chicago, 35% weight): Cumulative totals are nearly aligned over the 15-day period (GFS 321.70 vs. ECMWF 320.40 HDDs), but daily paths diverge sharply. ECMWF holds a colder profile through days 5–10 (Nov 14–19), while GFS projects an intense late-period arctic blast on Nov 23–24, dropping Chicago to a daily mean of 27.5°F (37.5 HDDs) versus ECMWF's moderating 49.8°F (15.2 HDDs).
• East (New York City, 30% weight): ECMWF generates substantial incremental space-heating demand, totaling 284.40 HDDs versus 245.77 HDDs on the GFS (+38.63 HDD spread). While ECMWF maintains persistent chill between 40°F and 47°F through late November, GFS projects a sharp warm spike on Nov 23 (mean temp 60.4°F, yielding just 4.56 HDDs vs. ECMWF’s 17.01 HDDs).
• Northeast (Boston, 10% weight): Follows the East Coast pattern, with ECMWF delivering 306.14 cumulative HDDs compared to 273.06 HDDs on the GFS (+33.08 HDD spread), with GFS warming to 59.5°F on Nov 23.
• South Central (Houston, 15% weight) & South (Atlanta, 10% weight): GFS brings an aggressive cold front across the South on Nov 22–24, driving Houston mean temperatures down to 49.0°F (16.0 HDDs) on Nov 24, whereas ECMWF stays mild at 65.4°F (0.0 HDDs). Similarly, Atlanta falls to 34.5°F (30.5 HDDs) on Nov 24 under GFS, compared to 57.1°F (7.9 HDDs) under ECMWF. Cumulative Houston HDDs stand at 68.77 GFS vs. 19.42 ECMWF.

Demand Implications (Natural Gas)

National weather-driven load for natural gas is fully dominated by heating demand, with cooling degree days (CDDs) negligible (4.28 GFS vs. 2.61 ECMWF). The +21.11 weighted HDD gain in the ECMWF 12:00 UTC run-over-run delta indicates strengthening baseline residential and commercial consumption across the high-population East Coast corridor.

However, pipeline load expectations for late November remain bifurcated. If the GFS solution verifies, late-horizon heating load will spike sharply across ERCOT and the Southeast, coupled with freeze-off risks and strong Midwest burns. If the ECMWF verifies, East Coast utility burns will remain elevated and steady, but aggregate South Central demand will stay subdued without late-period freeze risks.

Model Agreement & Caveats

• Convergence Score: Low at 0.1958 (down -0.0169 from the prior run), with a weighted mean absolute daily difference of 4.1082 HDDs (+0.4065 delta), signaling weak forecast confidence and elevated synoptic spread past Day 8.
• Horizon Days 1–6 Spread Significance:
  • Raw Days 1–6 weighted HDD: ECMWF 108.266 vs. GFS 100.384, yielding a raw spread of +7.883 HDDs (ECMWF colder).
  • Contextual Baseline: Open-Meteo's ECMWF historically carries a systematic cold bias from October to February, with an expected seasonal spread of +13.503 HDDs.
  • Anomaly & Classification: Today’s spread anomaly is -5.621 HDDs (anomaly z-score of -1.073), earning a notable label. This indicates the ECMWF-GFS gap is narrower than usual for this time of year (ECMWF is running less cold relative to GFS than normal).
  • Verification: Historical verification indicates 31.6% of past anomalies in this window were realized (share_of_anomaly_realized = 0.316), but this relationship is not statistically significant (p-value = 0.068). This metric evaluates past model tendencies and is not an adjusted HDD forecast, nor does it indicate NG price direction.
• Caveats: Open-Meteo ECMWF raw model points exhibit recurring seasonal cool biases relative to surface observations. High-latitude blocking and cold-air discharge timing beyond Day 10 remain volatile.