The official National Weather Service forecast for Brentwood — 38.6176°N, 90.3493°W, 2.1 miles from I-44. Forecasts here are written by NWS LSX, this county’s forecast office.
| St. Louis County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 40 | 46 | 57 | 68 | 77 | 85 | 88 | 87 | 81 | 69 | 56 | 45 | 67 |
| Avg low °F | 23 | 27 | 36 | 45 | 56 | 65 | 69 | 67 | 59 | 47 | 36 | 28 | 46 |
| Precip in | 2.5 | 2.2 | 3.7 | 4.7 | 4.9 | 4.4 | 4.3 | 3.4 | 3.1 | 3.1 | 3.1 | 2.6 | 42.0 |
1996–2025 normals for St. Louis County — the climate division Brentwood sits in. All-time county record high 110.4°F (1954-07-14); record low -17.5°F (1989-12-23). Source: NOAA / National Weather Service.