The official National Weather Service forecast for Lewistown — 47.0625°N, 109.4282°W, population 6,024, 87.9 miles from I-90.
| Fergus County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 35 | 36 | 46 | 54 | 64 | 73 | 84 | 83 | 72 | 58 | 45 | 35 | 57 |
| Avg low °F | 13 | 13 | 22 | 30 | 39 | 47 | 54 | 52 | 43 | 32 | 22 | 14 | 32 |
| Precip in | 0.6 | 0.6 | 0.8 | 1.7 | 3.0 | 3.1 | 1.5 | 1.5 | 1.5 | 1.3 | 0.6 | 0.6 | 16.8 |
1996–2025 normals for Fergus County — the climate division Lewistown sits in. All-time county record high 104.6°F (2026-07-13); record low -38.5°F (1954-01-20). Source: NOAA / National Weather Service.