Tomorrow morning will be the coldest day we've had this year.
Looks like tomorrow morning will be the lowest temperature we've had so far this winter.
ーTomorrow morning will be the coldest day we've had this year.
「明日の朝は今年一番の寒さになります。」
coldest day we've had this year で「今年一番の寒い日」
ーLooks like tomorrow morning will be the lowest temperature we've had so far this winter.
「明日の朝はこの冬一番の寒さになります。」
the lowest temperature we've had so far this winter で「この冬もっとも気温が低い」
ご参考まで！
The temperature for tomorrow morning will be the coldest we've had this year.
こんにちは。
様々な言い方ができると思いますが、例えば以下のような英語表現はいかがでしょうか：
The temperature for tomorrow morning will be the coldest we've had this year.
明日の朝の気温は、今年で最も寒くなります。
ぜひ参考にしてください。
カナダ 
アメリカ合衆国 
