September 30 in Portland
Weather forecasts run out after about two weeks. For any date further out than that, the honest answer isn't a single number — it's the range Portland actually delivers on this date, and how often.
On September 30, Portland, OR typically reaches 70°F, but has run anywhere from 61°F to 82°F in eight years out of ten. About 69% of September 30s see no measurable precipitation.
The odds on September 30
Odds computed from the last 30 years (1997–2026).
Highs around September 30 have cooled about 0.1°F per decade since 1973 — roughly 0.3°F across the record. The figures above are shifted along that trend to 2026, so they describe today's climate rather than the average of the last 53 years.
Where this comes from: 792 daily observations from PORTLAND KGW-TV, covering 1973–2026. We pool every day within 7 days of September 30 — a single calendar date has only about 53 readings, far too few to place a 10th percentile you'd want to trust.
Why it's not a plain average: A climatology computed straight across 1973–2026 describes the middle of that period, not today, so as a statement about the present it runs cold. We fit a trend on year and shift each observation to 2026 before taking the range. The probabilities use the last 30 years instead — shifting a temperature along a trend is well defined, shifting “did it rain” is not.
What this is not: Not a forecast. It is what Portland, OR has actually done on this date, which is the only defensible answer once you're past the two-week horizon where real forecasts stop. Data: NOAA NCEI GHCN-Daily.