The official National Weather Service forecast for Lake Saint Louis — 38.7976°N, 90.7857°W, 21.3 miles from I-44. Forecasts here are written by NWS LSX, this county’s forecast office.
| St. Charles County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 40 | 45 | 57 | 68 | 77 | 85 | 88 | 87 | 81 | 69 | 56 | 45 | 66 |
| Avg low °F | 22 | 26 | 35 | 44 | 55 | 64 | 68 | 66 | 58 | 46 | 36 | 27 | 46 |
| Precip in | 2.5 | 2.2 | 3.6 | 4.7 | 4.9 | 4.4 | 4.2 | 3.6 | 3.1 | 3.1 | 3.1 | 2.5 | 41.9 |
1996–2025 normals for St. Charles County — the climate division Lake Saint Louis sits in. All-time county record high 110.6°F (1954-07-14); record low -19.2°F (1989-12-23). Source: NOAA / National Weather Service.