。
The forecast says it'll be cloudy tomorrow.
Literal
Forecast-[according-to-では], tomorrow [topic-は] cloudy is-[polite-です].
The composite particle では here is 'according to' — the で marks a source or basis, and は adds topic framing. 予報では ('according to the forecast') is a common news/report opener. Note the double topic flavor: 予報では frames the source, and 明日は frames the day being talked about — Japanese allows stacking these context-setters without any ambiguity.