It's getting colder today, 12 degrees. It's still only autumn, so I'm worried that the winter will be harder than last year.
- 'It's getting' is better than 'it's', it implies that the weather is changing - i.e. colder today than yesterday, probably colder again tomorrow.
- Nobody outside the US uses units other than Celsius, so it is uncommon now to bother specifying them.
- There is no need to state explicitly that you are referring to an outdoor temperature: weather is always outdoors.
- I think you are taking the cold autumn weather as a sign of a cold winter coming, the logic is made clearer here.
- We would usually describe an intense winter as cold or hard.