| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 32°F | 47°F | 39°F | 7.3" |
| February | 32°F | 51°F | 41°F | 4.8" |
| March | 35°F | 55°F | 44°F | 7.0" |
| April | 36°F | 61°F | 48°F | 3.5" |
| May | 42°F | 68°F | 55°F | 2.7" |
| June | 47°F | 73°F | 59°F | 1.5" |
| July | 50°F | 80°F | 64°F | 0.6" |
| August | 50°F | 81°F | 64°F | 1.0" |
| September | 46°F | 74°F | 59°F | 2.5" |
| October | 41°F | 62°F | 51°F | 5.5" |
| November | 35°F | 52°F | 44°F | 9.7" |
| December | 31°F | 46°F | 38°F | 8.0" |
| Index | Chehalis | Washington | National |
| Air quality index | 20 | 36 | 42 |
| Pollution index | 4,215,410 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 343 |
| Days with moderate air quality | 22 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.005% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.030% |
| Mercury | 0.250% |
| City | Population | Air Quality Index | National Avg. |
| Centralia, WA | 17,473 | 20 | 60.0% |
| Rochester, WA | 4,854 | 20 | 60.0% |
| Fords Prairie, WA | 2,582 | 20 | 60.0% |
| Tenino, WA | 1,744 | 20 | 60.0% |
| Oakville, WA | 647 | 20 | 60.0% |
| Chehalis, WA | 7,607 | 20 | 60.0% |
| City | Population | Air Quality Index | National Avg. |
| Emerson, NJ | 7,606 | 51 | 2.0% |
| Hales Corners, WI | 7,609 | 44 | 12.0% |
| Oceano, CA | 7,601 | 46 | 8.0% |
| Homeland, CA | 7,613 | 89 | 78.0% |
| Ojai, CA | 7,613 | 46 | 8.0% |
| Chehalis, WA | 7,607 | 20 | 60.0% |
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