The official National Weather Service forecast for Port William — 39.552°N, 83.7863°W, 3.1 miles from I-71.
| Clinton County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 37 | 42 | 52 | 64 | 73 | 81 | 84 | 83 | 78 | 66 | 53 | 42 | 63 |
| Avg low °F | 22 | 24 | 32 | 41 | 52 | 60 | 63 | 62 | 54 | 44 | 34 | 27 | 43 |
| Precip in | 3.2 | 2.9 | 3.9 | 4.5 | 4.6 | 4.8 | 4.3 | 3.2 | 3.1 | 3.1 | 2.9 | 3.5 | 44.0 |
1996–2025 normals for Clinton County — the climate division Port William sits in. All-time county record high 101.0°F (1988-06-26); record low -24.5°F (1994-01-19). Source: NOAA / National Weather Service.