Weather in Tri-Cities in September
United States · 20-year averages, September
Prime time. If you get to pick your dates, September is the one. Highs average 27.4 °C, lows 13.7 °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 May, 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: 9 days top 30 °C — hot spells arrive, but they break. Across the twenty years measured, the hottest September day reached 38.9 °C and the coldest night fell to 2.8 °C.
This is the volatile part. September rainfall has ranged from 0 mm to 51 mm across the twenty years measured — a 51 mm spread around an average of 11 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 8.3 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 47%.
How September sits in the year
The month is on the way down: 5.8 °C cooler than August, with October dropping another 8.1 °C. The first week and the last are not the same trip.
Daylight in September
12.6 hours of daylight, losing 1.6 hours across the month. That is between Tri-Cities's extremes of 15.8 and 8.6 hours, so days are long enough not to constrain the plan.
An inland climate
With no coast nearby, the year swings a fair way — 34.6 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage.
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 Tri-Cities versus nearby
If Tri-Cities is one option among several, the nearby comparison is worth making. Spokane is close enough to be a coin toss — 3.5 °C cooler, 1 day more rain. Portland is close enough to be a coin toss — within a degree on temperature, 4 days more rain.
So: September is the answer for Tri-Cities. If it is booked out or priced out, May is the next best thing at 4 points behind, and December is the month to keep off the shortlist.
Every month in Tri-Cities
| Month | High °C | Low °C | Rain days | Rain mm | Humidity % | Daylight h | Score |
|---|---|---|---|---|---|---|---|
| January | 6.9 | -0.3 | 9 | 36 | 81 | 9 | 43 |
| February | 10.2 | 0.6 | 7 | 25 | 72 | 10.3 | 57 |
| March | 14.9 | 3.6 | 8 | 27 | 64 | 11.8 | 71 |
| April | 18.9 | 6.2 | 7 | 24 | 58 | 13.5 | 85 |
| May | 24.5 | 10.9 | 7 | 27 | 52 | 14.9 | 89 |
| June | 28.5 | 14.3 | 5 | 21 | 48 | 15.8 | 85 |
| July | 34.3 | 18.3 | 1 | 4 | 41 | 15.5 | 48 |
| August | 33.2 | 18.1 | 2 | 7 | 41 | 14.2 | 69 |
| September | 27.4 | 13.7 | 3 | 11 | 47 | 12.6 | 93 |
| October | 19.3 | 8.1 | 6 | 24 | 60 | 11 | 88 |
| November | 11.8 | 3 | 8 | 29 | 71 | 9.5 | 61 |
| December | 6.7 | -0.3 | 10 | 42 | 79 | 8.6 | 40 |
See all twelve months in Tri-Cities, or where else to go in September.
Planning the rest of the trip
- Flight time to Tri-Cities how long you are actually in the air
- Tri-Cities time zone and the difference from home
- Days until your trip count down to the departure date
- Gyms & clubs in Tri-Cities keep training while you are there