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Models disagree by different amounts on different days. Does the disagreement tell you anything about how wrong the forecast ends up? 258 verified days, 21 cities, ERA5 truth. Seven models: ECMWF, GFS, UKMO, ICON, GEM, and the two AI runs, AIFS and AIGFS.

The Spread

@spreadwx.com

10 Aug

Sort the days by how much those seven disagreed one day out. The third that agreed most: the consensus missed by 1.27°F on average. The third that argued most: 2.38°. Same direction at 3, 5 and 7 days — 1.35x to 1.69x.

The Spread

@spreadwx.com

11 Aug

Before believing it: r = 0.37. Real, and loose. But it isn't one maverick — drop any single model and it holds, 1.48x without AIFS through 2.07x without UKMO. Spread is informative, not decisive.

The Spread

@spreadwx.com

The edges do have regulars. AIFS was the coolest of the seven on 71 days and the warmest on none; UKMO warmest on 47, coolest on 6. Bias runs AIFS −1.5°F to UKMO +1.1°, ECMWF nearest zero — and the median beat all seven, 1.78° against ECMWF's 1.83.

11/8/26, 12:03 am0 reposts0 likes

The Spread

@spreadwx.com

11 Aug

Caveat that carries the weight: 20 August days, 21 US cities, one lead. That's this sample's bias, not a skill ranking, and season or region should flip the sign.