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SunnyDays

ERA5 almanac — 8 years of real data

Seattle in September

Seattle in September: 29% of days see measurable rain, 70°F average high, 30% of days historically clear (n=240, ERA5 2018–2025).

Seattle in September is historically a poor month for sun: only 30% of days clear across 8 years (n=240, avg high 70°F). Historical average, not a forecast.

What you are reading: 8-year historical averages from the ERA5 reanalysis, not a forecast. A month that is historically 90% clear can still be overcast on your specific trip. For this week’s actual forecast, see the live Seattle page. Both can be true at once: “historically strong in this month; this particular week is not.” We label which is which.

Early vs. Late September

Clear-sky percentage for Seattle in September, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early September
1st–15th		 47% n=120 70°F
Late September
16th–end		 13% n=120 70°F
Full month average 30% n=240 70°F

Sunshine & warmth in September

A second measure using actual sunshine hours (ERA5 sunshine_duration), not the daily weather code. A Lowcountry day with 12 hours of sun and a 25-minute afternoon storm is counted below as a warm, sunny day — because it was. How we define this.

Sunshine-based metrics for Seattle in September. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early September
1st–15th		 64% n=120
Late September
16th–end		 16% n=120
Full month average 40% n=240
9.9 h median sunshine per day 70% of days exceed 8 h sunshine 29% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 30%. Sunshine-based “warm & sunny”: 40%.

Bottom line

Seattle is mostly overcast in September historically: only 30% of days are clear across 8 years (n=240), avg high 70°F. The two halves differ materially (47% early, 13% late) — favour the early half of the month. Most visitors this month will see more clouds than sun.

Every month in Seattle

Compare across all months with data. Numbers are historical clear-sky %.

Historical clear-sky % for Seattle, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 0% n=248 All destinations →
April 1% n=240 All destinations →
May 13% n=248 All destinations →
June 32% n=240 All destinations →
July 70% n=248 All destinations →
August 61% n=248 All destinations →
September ← this page 30% n=240 All destinations →
October 3% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Seattle in September?

Sometimes. Seattle sees measurable rain on 29% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Seattle good for sun in September?

It's mixed. Seattle in September is clear on 30% of days historically (avg high 70°F, n=240 days, ERA5 2018–2025) — the outcome varies year to year.

How Seattle compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Seattle this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

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.