ERA5 almanac comparison — 8 years of real data
Napa vs Phoenix: weather in September
Essentially a tie in September: Napa is 86% clear historically, Phoenix is 82% clear (8-year ERA5 average) — within the noise. Avg high 86°F vs 101°F. Pick based on flight time or price, not weather — the weather won't decide this one for you.
What you are reading: 8-year historical averages, not a forecast.
Either destination can still surprise you this specific September. For this
week’s actual forecast, each destination links to its live page below.
Napa
CA
- Clear days, September
- 86%
- Sample size
- n=240 days
- Avg high
- 86°F
- Early September (1st–15th)
- 93%
- Late September (16th–end)
- 79%
- Rain frequency
- 4% of days
Phoenix
AZ
- Clear days, September
- 82%
- Sample size
- n=240 days
- Avg high
- 101°F
- Early September (1st–15th)
- 82%
- Late September (16th–end)
- 81%
- Rain frequency
- 10% of days
Compare Napa vs Phoenix in another month
All percentages are historical averages from the ERA5 reanalysis via Open-Meteo, 8 full years (2018–2025). “Clear” means avg high ≥ 72 °F, precipitation < 1 mm, and a non-precipitation weather code. This is history, not a forecast. The live 7-day outlook for any destination is on its own page and may differ from the long-run average. Data from Open-Meteo.