▲句子成分分析：

句首省略了主语 I 或 We之类的词，即原句可补充为：

I / We expect today to be partly sunny and partly cloudy.

主语：I / We

谓语：expect

宾语：today

宾补：to be partly sunny and partly cloudy

▲省略主语的补充:

省略主语的用法主要见于口语。如：

(I) Hope to hear from you soon.

(I) Haven’t seen you for ages.

(You) Had a good time?

(He / She) Doesn’t look very well today.

(It) Must be very hot in Africa.

(It) Sounds fine to me.