sentence structure when talking about weather
天晴 sunny (adj)

晴天 sunny day (n)

明天天晴 = (it will be) sunny tomorrow.

明天是個晴天 = Tomorrow is a sunny day.

If you wrote 明天晴天, it would be a noun (tomorrow) followed by another noun (sunny day). The verb 是 (is) and the classifier(個) are omitted.