The official National Weather Service forecast for Diamond Bar — 34.0286°N, 117.8103°W, 2.4 miles from I-10. Forecasts here are written by NWS LOX, this county’s forecast office.
| Los Angeles County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 61 | 62 | 66 | 70 | 76 | 83 | 89 | 90 | 86 | 77 | 68 | 61 | 74 |
| Avg low °F | 41 | 41 | 43 | 46 | 52 | 58 | 64 | 63 | 60 | 52 | 45 | 40 | 50 |
| Precip in | 2.8 | 3.6 | 2.0 | 0.8 | 0.3 | 0.0 | 0.0 | 0.0 | 0.1 | 0.6 | 1.1 | 2.7 | 14.0 |
1996–2025 normals for Los Angeles County — the climate division Diamond Bar sits in. All-time county record high 108.0°F (2020-09-07); record low 19.1°F (1990-12-23). Source: NOAA / National Weather Service.