1) Maybe it will rain in the evening, so you should take an umbrella with you.

Using the modal verb "may":

It may rain in the evening, so take an umbrella with you.

2) It's possible for the weather to change very quickly in England.

Using the modal verb "can":

The weather can change very quickly in England.

3) I'm sure it will be warm at the weekend, so let's have a picnic!

Using the modal verb "must":

It must be warm at the weekend, so let's have a picnic!

4) Maybe we'll have an early spring this year, but I'm not sure.

Using the modal verb "may":

It may be an early spring this year, but I'm not sure.

5) It's possible for it to drizzle all day long here.

Using the modal verb "can":

It can drizzle all day long here.

6) I'm sure there is a lot of snow in the mountains already.

Using the modal verb "must":

There must be a lot of snow in the mountains already.