I can't believe it's this sunny today, but then it's going to rain tomorrow.
「今日はこんなに晴れてるのに明日は雨なんだね」は英語で I can't believe it's this sunny today, but then it's going to rain (all day) tomorrow. になると思います。I can't believe the weather is this nice today, but then it's meant to rain tomorrow. もいいと思います。
I can't believe - 信じられない (「驚き」の信じられないより「意外感」の信じられないです)
晴れる - sunny, nice/good weather
私ならこう言います。
The weather is so weird. I can't believe it's so/this sunny today, but tomorrow it's going to be raining all day.
I can't believe it's going to rain tomorrow when the weather is so nice today.
こんにちは。
さまざまな言い方ができると思いますが、下記のような英語表現はいかがでしょうか:
I can't believe it's going to rain tomorrow when the weather is so nice today.
今日はこんなに天気が良いのに明日は雨なんて信じられない。
believe は「信じる」という意味になります。
can't believe = 信じられない
ぜひ参考にしてください。