It's been unseasonably mild/cold.

Meaning

The temperature is unusually warm or cold for the time of year.

Examples

• John: 'It's almost December, and yet it feels like spring.'
• Alice: '<b>It's been unseasonably mild.</b> I'm not complaining, though!'
• John: '<b>It's been unseasonably cold.</b> Make sure you bundle up.'
• Alice: 'I'm freezing! It's much colder than usual for this time of year.'