The official National Weather Service forecast for Janesville — 40.2966°N, 120.5241°W, 61.1 miles from I-80. Forecasts here are written by NWS REV, this county’s forecast office.
| Lassen County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 42 | 44 | 50 | 56 | 65 | 76 | 86 | 84 | 76 | 63 | 49 | 41 | 61 |
| Avg low °F | 22 | 23 | 27 | 30 | 37 | 44 | 51 | 49 | 43 | 34 | 27 | 22 | 34 |
| Precip in | 2.8 | 2.6 | 2.4 | 1.8 | 1.6 | 0.8 | 0.2 | 0.2 | 0.4 | 1.2 | 2.0 | 3.3 | 19.3 |
1996–2025 normals for Lassen County — the climate division Janesville sits in. All-time county record high 102.5°F (2002-07-12); record low -20.6°F (1972-12-09). Source: NOAA / National Weather Service.