The official National Weather Service forecast for Vandalia — 41.917°N, 85.9147°W, 14.9 miles from I-80. Forecasts here are written by NWS IWX, this county’s forecast office.
| Cass County | J | F | M | A | M | J | J | A | S | O | N | D | Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 32 | 35 | 47 | 60 | 71 | 80 | 83 | 81 | 76 | 62 | 48 | 37 | 59 |
| Avg low °F | 18 | 19 | 28 | 37 | 48 | 58 | 61 | 60 | 53 | 42 | 32 | 23 | 40 |
| Precip in | 2.6 | 2.2 | 2.5 | 3.5 | 4.2 | 4.2 | 3.8 | 3.8 | 3.1 | 3.9 | 2.5 | 2.6 | 38.9 |
1996–2025 normals for Cass County — the climate division Vandalia sits in. All-time county record high 102.0°F (2012-07-07); record low -19.5°F (1994-01-19). Source: NOAA / National Weather Service.