Weather in Boise in September

Prime time. If you get to pick your dates, September is the one. Highs average 27.9 °C, lows 13.3 °C, and rain falls on 3 days — nothing in the month is working against you.

September is the highest-scoring month of the twelve. The nearest rival is October, and the month at the other end of the year is December.

Rain falls on only 3 days, so an itinerary with no wet-weather fallback is a reasonable risk. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: 12 days top 30 °C — hot spells arrive, but they break. Across the twenty years measured, the hottest September day reached 40 °C and the coldest night fell to 2.4 °C.

This is the volatile part. September rainfall has ranged from 0 mm to 47 mm across the twenty years measured — a 47 mm spread around an average of 14 mm. The average is real but almost no individual September looks like it, so treat the figure as the middle of a wide range rather than a forecast.

Wind is light at 7.1 km/h, so the temperature on the page is close to the temperature on your skin, and the air is dry at 34% humidity, which makes the heat far more bearable than the same figure somewhere humid — and dehydrates you faster than you expect.

How September sits in the year

The month is on the way down: 6 °C cooler than August, with October dropping another 9.1 °C. The first week and the last are not the same trip.

Daylight in September

12.6 hours of daylight, losing 1.5 hours across the month. That is between Boise's extremes of 15.4 and 9 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 38.7 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. At 823 m the air is thin enough to matter — sun that burns faster than the temperature suggests, and nights that cool off quickly. Within September itself the daily swing is 14.6 °C, so mornings and afternoons need different clothes.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Boise versus nearby

If Boise is one option among several, the nearby comparison is worth making. Tri-Cities is close enough to be a coin toss — within a degree on temperature, the same number of rain days. Spokane is close enough to be a coin toss — 4 °C cooler, 1 day more rain.

So: September is the answer for Boise. If it is booked out or priced out, October is the next best thing at 3 points behind, and December is the month to keep off the shortlist.

Every month in Boise

See all twelve months in Boise, or where else to go in September.