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SunnyDays

ERA5 almanac — 8 years of real data

Portland in September

Portland in September: 28% of days see measurable rain, 71°F average high, 30% of days historically clear (n=240, ERA5 2018–2025).

Portland in September is historically a poor month for sun: only 30% of days clear across 8 years (n=240, avg high 71°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 Portland 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 Portland 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		 40% n=120 71°F
Late September
16th–end		 19% n=120 71°F
Full month average 30% n=240 71°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 Portland 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		 57% n=120
Late September
16th–end		 27% n=120
Full month average 42% n=240
11.3 h median sunshine per day 76% of days exceed 8 h sunshine 28% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 30%. Sunshine-based “warm & sunny”: 42%. The 12-point gap is from brief afternoon convective storms that stamp the whole day as “rain” in the WMO code but leave the bulk of the sunshine untouched.

Bottom line

Portland is mostly overcast in September historically: only 30% of days are clear across 8 years (n=240), avg high 71°F. The two halves differ materially (40% early, 19% late) — favour the early half of the month. Most visitors this month will see more clouds than sun.

Every month in Portland

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

Historical clear-sky % for Portland, 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 2% n=240 All destinations →
May 14% n=248 All destinations →
June 27% n=240 All destinations →
July 38% n=248 All destinations →
August 39% n=248 All destinations →
September ← this page 30% n=240 All destinations →
October 7% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Portland in September?

Sometimes. Portland sees measurable rain on 28% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Portland good for sun in September?

It's mixed. Portland in September is clear on 30% of days historically (avg high 71°F, n=240 days, ERA5 2018–2025) — the outcome varies year to year.

How Portland compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Portland 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.