Climate averages 1991–2020
Boise weather
in September.
A warm, very dry month: the 4th warmest and the 3rd driest of the year. Meets the pleasant-weather rule
Long-term climate patterns, not a forecast for a particular trip.
September is a warm month in Boise: daytime highs average 26.2°C and nights cool to about 12.6°C. By daytime temperature it is the 4th warmest of the 12 months.
Expect about 10 mm of rain on roughly 2 days, which makes September a very dry month (the 3rd driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 33 min on an average September day.
The nearest other city in this atlas is Meridian (15 km away). In September its highs average 26.1°C, about as warm as Boise, with 9.8 mm of rain.
Among the 498 cities listed in the United States, Boise ranks joint 268th of 498 for the warmest September highs and joint 112th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; Boise is 0.7°C cooler than that mean.
From September to October, average highs fall by 8.4°C to 17.7°C, and monthly rainfall rises from 10 mm to 22 mm.
The annual picture
September in context
Monthly rainfall
Average total · mmAverages from hourly ERA5 reanalysis data. Actual weather varies from year to year. How these numbers are calculated
Every month, side by side
Boise climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 3.8°C-4.6°C | -4.6°C | 45 | 9.5 | 9.39 h |
| February | 6.8°C-2.5°C | -2.5°C | 31 | 7.7 | 10.510 h |
| March | 11.9°C0.9°C | 0.9°C | 36 | 8.9 | 11.912 h |
| April | 15.9°C3.8°C | 3.8°C | 31 | 7.2 | 13.413 h |
| May | 21.6°C8.5°C | 8.5°C | 31 | 5.9 | 14.715 h |
| June | 26.5°C13.0°C | 13.0°C | 16 | 3.2 | 15.415 h |
| July | 32.7°C17.9°C | 17.9°C | 2.6 | 0.9 | 15.115 h |
| August | 31.9°C17.4°C | 17.4°C | 3.3 | 0.7 | 14.014 h |
| September, selected | 26.2°C12.6°C | 12.6°C | 10 | 2.2 | 12.613 h |
| October | 17.7°C6.3°C | 6.3°C | 22 | 4.3 | 11.111 h |
| November | 9.3°C0.2°C | 0.2°C | 34 | 7.9 | 9.710 h |
| December | 3.7°C-4.1°C | -4.1°C | 50 | 10.1 | 9.09 h |
Rainfall is the average of the 30 separate monthly totals. A rain day is a local calendar day with at least 1 mm of precipitation. Daylight is the calculated sunrise-to-sunset length, not measured sunshine.
Compare places
September in nearby cities
- Meridian in SeptemberUnited States · 15 kmHigh 26.1°C · Low 12.4°C · 9.8 mm rain
- Nampa in SeptemberUnited States · 30 kmHigh 26.5°C · Low 13.4°C · 9.8 mm rain
- Kennewick in SeptemberUnited States · 370 kmHigh 27.4°C · Low 13.9°C · 6.9 mm rain
- Tri-Cities in SeptemberUnited States · 376 kmHigh 27.5°C · Low 14.0°C · 6.9 mm rain
- Missoula in SeptemberUnited States · 401 kmHigh 22.5°C · Low 9.8°C · 30 mm rain
- Bend in SeptemberUnited States · 413 kmHigh 22.9°C · Low 7.9°C · 16 mm rain
- Ogden in SeptemberUnited States · 437 kmHigh 24.4°C · Low 11.4°C · 32 mm rain
- Layton in SeptemberUnited States · 449 kmHigh 24.2°C · Low 11.3°C · 30 mm rain
Read the numbers correctly
Climate, not a promise.
These patterns help compare months. They cannot tell you the weather on a specific day of your trip.
Sources, method and limitsHow these numbers are made (what the highs, lows and rain days mean, the sources and the model cell used for Boise) is explained once on the Boise year overview and on the Sources, method and limits page.