先行詞は、”It should be warmer today as compared to yesterday and the day before”という「文」。

接続詞と代名詞のitを使って書き換えると、
It should be warmer today as compared to yesterday and the day before, and I am very thankful to the sun for it.
この最後のitが関係代名詞になってるので、whichは目的格。