The official National Weather Service forecast for Arnold — 38.4328°N, 90.3776°W, 1.4 miles from I-55. Forecasts here are written by NWS LSX, this county’s forecast office.
| Jefferson County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 42 | 47 | 58 | 69 | 77 | 85 | 89 | 88 | 81 | 70 | 57 | 46 | 67 |
| Avg low °F | 23 | 26 | 35 | 45 | 55 | 64 | 68 | 66 | 58 | 46 | 36 | 27 | 46 |
| Precip in | 2.7 | 2.7 | 3.8 | 4.8 | 5.2 | 4.1 | 4.3 | 3.5 | 3.1 | 3.2 | 3.3 | 2.6 | 43.3 |
1996–2025 normals for Jefferson County — the climate division Arnold sits in. All-time county record high 109.9°F (1954-07-14); record low -17.8°F (1977-01-17). Source: NOAA / National Weather Service.