ERA5 almanac — 8 years of real data

Portland in September

Portland in September: rain frequency data not yet backfilled for this month, 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

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

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.

→ Portland live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in September — all destinations ranked by September clear-sky %
→ All destinations
→ Raw data (JSON) — buckets, n, source

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.